A method for detecting chloride ions in cement

Abstract

This invention discloses a method for detecting chloride ions in cement. The method specifically includes the following steps: Weighing a cement sample and adding distilled or deionized water, while stirring, adding nitric acid (1+2) to a beaker, then heating and simmering for 1-2 minutes to obtain a sample solution; adding sodium chloride solution to the beaker, then adding silver nitrate solution and stirring until homogeneous, then boiling for 1-2 minutes; filtering; adding ferric ammonium sulfate indicator solution to an Erlenmeyer flask, and titrating the excess silver ions in the Erlenmeyer flask with ammonium thiocyanate standard solution until the solution turns reddish-brown and the color does not disappear when the Erlenmeyer flask is shaken, and recording the volume V of ammonium thiocyanate consumed at this point. NH4SCN Without adding cement samples, perform a blank test and record the volume V of ammonium thiocyanate used in the blank test titration. NH4SCN空白 The chloride ion content of cement samples is calculated. This invention controls the detection range of chloride ions in cement to be within the range of added Ag. + By designing the concentration of added ions to be an integer multiple of 0.0141, ranging from 20% to 80% of the molar number, the detection accuracy is improved.

Description

Technical Field

[0001] This invention belongs to the field of chemical analysis technology, specifically relating to a method for detecting chloride ions in cement. Background Technology

[0002] In recent years, river sand mining has been gradually banned in various parts of the Yangtze River basin, leading to a large influx of sea sand into the building materials market as a substitute. Sea sand has a very high chloride ion content, and the chloride ion content in desalinated sea sand is also unstable. Cement, due to the addition of admixtures and grinding aids during production, also has a significant chloride ion content. While the chloride ion content of other raw materials besides sand has not been a routine testing item for concrete, with the entry of sea sand into the building materials market, testing the chloride ion content of all raw materials and concrete has become an essential part of controlling concrete durability.

[0003] In most national standards, chloride ion detection methods are broadly classified into: silver nitrate titration (Mohr's method, suitable for neutral and weakly alkaline environments), potentiometric titration (suitable for neutral and acidic environments), and ammonium thiocyanate volumetric method (Volhard's method, suitable for acidic environments and environments with many interfering ions). Because cement contains complex ions, the ammonium thiocyanate volumetric method is used as the standard method for chloride ion detection. Section 6.13 of the national standard "Chemical Analysis Methods for Cement" (GB / T 176-2017) describes this method in detail. Its detection principle involves first adding an excess of Ag... + This allows it to react with Cl in the sample. - After the precipitate forms, it is filtered off, and then the remaining Ag is titrated with ammonium thiocyanate (NH4SCN). + Using ammonium ferric sulfate (NH4Fe3(SO4)2) as an indicator, under acidic conditions, thiocyanate (SCN) - Priority with Ag + Precipitation forms in Ag + In the instant of exhaustion, SCN - With indicator Fe 3+ They combine to form a reddish-brown complex ion, Fe(SCN). 2+ It indicates the titration endpoint. However, the ammonium thiocyanate volumetric method has a relatively large error when testing trace amounts of chloride ions.

[0004] The specific reasons for the error are as follows: Firstly, in GB / T 176-2017, 5 mL of silver nitrate (0.05 mol / L) is added and titrated with 0.05 mol / L ammonium thiocyanate solution. The author tested ordinary silicate cement, Class C II fly ash, and S95 grade mineral powder supplied to mixing plants over the past year and found that in 99% of cases, 5 g of sample contained 0.3–4 mg of chloride ions, within the detection range (Cl...). -The maximum range (Ag) is 0.0085 mmol - 0.1128 mmol. + The range of 3.40%-45.12% for a molar concentration of 0.2500 mmol is too large, causing significant errors in the measurement of trace chloride ions; secondly, the condition of n0 << n is not met. Ag+ At that time, the formula for calculating chloride ion content in GB / T 176-2017 is invalid, where n0 is the number of moles of interfering ions in the titration system.

[0005] n Ag + indicates added Ag + Thirdly, the concentration of ammonium thiocyanate titrant (0.05 mol / L) is relatively high. A conventional burette can only be read to 0.05 mL. The error (0.02 mL) between the measured volume of the burette (e.g., 11.35 mL) and the actual volume of titrant consumed (e.g., 11.33 mL) will cause a large error in the number of moles of thiocyanate ions (0.05 mol / L * 0.02 mL = 0.001 mmol), thus causing a large error in the mass of chloride ions (0.001 mmol * 35). The detection of 45 g / mol (0.0355 mg) produces a large error, while 5 g of cement generally contains 2 mg of chloride ions (chloride ion content 0.04%). This means that even if the operation is accurate, there is still a relative error of 0.0355 mg / 2 mg = 1.775%. If the concentration of ammonium cyanate is changed to 0.01 mol g / L, the relative error is 1.775% / 5 = 0.355%, but the concentration of ammonium thiocyanate titrant is less than 0.05 mol / L, and the color change at the stoichiometric point is not obvious. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for detecting chloride ions in cement. This method is designed to detect chloride ions in cement within a range that includes the addition of Ag. + The number of moles is 20% to 80%, and the increase is n0 << n 4g To improve detection accuracy, the concentration of added ions is designed to be an integer multiple of 0.0141.

[0007] To achieve the above-mentioned objectives, the technical solution provided by this invention patent is as follows:

[0008] A method for detecting chloride ions in cement, the method specifically includes the following steps:

[0009] S1. Weigh 5g of cement sample, accurate to 0.0001g, and place the weighed sample in a beaker.

[0010] S2, add 50mL of distilled water or deionized water to the beaker and stir well. While stirring, add 50mL of nitric acid (1+2) to the beaker, then heat to a boil and simmer for 1-2 minutes to obtain the sample solution.

[0011] S3. Add a certain volume of sodium chloride solution to the beaker, then add a certain volume of silver nitrate solution and stir well, then boil for 1-2 minutes.

[0012] S4. To prepare the filtrate, add a small amount of filter paper pulp to the beaker. Take an Erlenmeyer flask, a funnel, a glass rod, and a rapid filter paper (previously washed with nitric acid (1+100)). Attach the rapid filter paper to the funnel, attach the bottom of the glass rod to the filter paper, tilt the beaker, and attach the beaker mouth to the top of the glass rod. Pour the solution from the beaker into the Erlenmeyer flask. The solution flows along the glass rod, filter paper, and funnel neck into the Erlenmeyer flask. Wash the beaker and stirring equipment with nitric acid (1+100) until the solution in the Erlenmeyer flask reaches 200 mL. Place the beaker in a dim light or dark place and cool the solution to below 25°C.

[0013] S5. After cooling, add 5 mL of ferric ammonium sulfate indicator solution to the conical flask. Titrate the excess silver ions in the conical flask with ammonium thiocyanate standard solution until the solution turns reddish-brown and the color does not disappear when the conical flask is shaken. Stop the titration and record the volume V of ammonium thiocyanate consumed at this time. NH4SCN ;

[0014] S6, without adding cement sample, repeat S2-S5 to perform blank tests, and record the volume V of ammonium thiocyanate used in the blank test titration. NH4SCN空白 The chloride ion content of the cement sample was calculated.

[0015] Furthermore, the determination of the volume and concentration of the sodium chloride solution and silver nitrate solution added to the beaker in step S3 is as follows:

[0016] S31. Add 2 mL of 0.0141 mol / L sodium chloride standard solution and 13 mL of 0.0141 mol / L silver nitrate standard solution to a beaker.

[0017] S32, perform the titration of ammonium thiocyanate according to steps S4 and S5, and measure V. NH4SCN ;

[0018] S33, when the measured V NH4SCN If the value is less than 0.75 mL, it indicates that the chloride ion content in the cement sample is greater than 4 mg. Weigh the cement sample again and repeat steps S1 and S2 to obtain a sample solution. Add 4 mL of 0.141 mol / L silver nitrate standard solution to the sample solution, without adding sodium chloride solution. Repeat steps S4 and S5 to measure the chloride ion content of the cement sample.

[0019] S35, when the measured V NH4SCN If the concentration is greater than 2.60 mL, it indicates that the chloride ion content in the cement sample is less than 0.3 mg. Weigh the cement sample again and repeat steps S1 and S2 to obtain a sample solution. Add 2 mL of 0.00141 mol / L sodium chloride standard solution and 1 mL of 0.141 mol / L silver nitrate standard solution to the sample solution, repeat steps S4 and S5, and measure the chloride ion content of the cement sample.

[0020] S36, when the measured V NH4SCN If the chloride ion content in the cement sample is greater than or equal to 0.75 mL and less than or equal to 2.60 mL, it indicates that the chloride ion content in the cement sample is between 0.3 mg and 4 mg. The cement sample is not weighed again, and the chloride ion content of the cement sample is measured.

[0021] Furthermore, the specific method for measuring the chloride ion content of the cement sample is as follows:

[0022] When the chloride ion content of a cement sample is between 0-4 mg, the chloride ion content of the cement sample is:

[0023]

[0024] When the chloride ion content of the cement sample is between 4 and 15.9525 mg, and |n AgNO3 -n NH4SCN空白 | / n AgNO3 ≤2%, C AgNO3 V AgNO3 -C NH4SCN V NH4SCN空白 / (C AgNO3 V AgNO3 When the chloride ion content is ≤2%, the chloride ion content of the cement sample is:

[0025]

[0026] When the chloride ion content of the cement sample is between 4 and 15.9525 mg, and |n AgNO3 -n NH4SCN空白 | / n AgNO3 >2%, |C AgNO3 V AgNO3 -C NH4SCN V NH4SCN空白 | / (C AgNO3 V AgNO3 When the chloride ion content is greater than 2%, the chloride ion content of the cement sample is:

[0027] Among them, W Cl- This indicates the percentage of chloride ion mass in the cement sample mass, expressed as %. (C)NH4SCN This indicates the concentration of the ammonium thiocyanate standard solution, expressed in mol / L, V. NH4SCN空白 V represents the volume of ammonium thiocyanate solution consumed in the blank experiment, in mL. NH4SCN This indicates the volume of ammonium thiocyanate solution consumed by the cement sample, in mL and m. 试样 The mass of the cement sample is expressed in grams (g) and cubic meters (C). AgNO3 This indicates the concentration of the added silver nitrate standard solution, in mol / L, V. AgNO3 This indicates the volume of added silver nitrate standard solution, in mL.

[0028] Furthermore, the concentration of the ammonium thiocyanate standard solution used in S5 is 0.0564 mol / L. If the chloride ion content of the cement sample is between 4 and 15.9525 mg and |n AgNO3 -n NH4SCN空白 | / n AgNO3 If the chloride ion content of the cement sample is >2%, or the chloride ion content is between 0-4 mg, a standardization measurement should be performed before titration with ammonium thiocyanate standard solution. Specifically: Measure 25 mL of the standardized silver nitrate standard solution (concentration 0.0564 mol / L) into a 250 mL Erlenmeyer flask. Add 50 mL of distilled or deionized water to the Erlenmeyer flask and stir well. Then add 2 mL of nitric acid (1+2) and 1 mL of ferric ammonium sulfate solution to the Erlenmeyer flask. Titrate the silver nitrate solution with the ammonium thiocyanate standard solution to be standardized. Titrate until the solution in the Erlenmeyer flask turns reddish-brown. After shaking the Erlenmeyer flask, if the color does not fade, the titration is complete. Record the volume of ammonium thiocyanate consumed during the titration, V. NH4SCN And calculate the concentration of the ammonium thiocyanate solution, specifically: Among them, C NH4SCN C represents the concentration of the ammonium thiocyanate solution. AgNO3 V represents the concentration of the silver nitrate solution. AgNO3 V is the volume of silver nitrate solution added to the conical flask. NH4SCN This represents the volume of ammonium thiocyanate solution consumed.

[0029] Furthermore, the preparation of filter paper pulp in S4 is specifically as follows: tear a quantitative amount of filter paper into pieces and put it into a beaker, add water to submerge it, and heat it to a boil while stirring for 10 minutes, then cool it to room temperature for later use.

[0030] Further, the specific steps for preparing the ferric ammonium sulfate indicator solution are as follows: add 100 mL of saturated aqueous solution of ferric ammonium sulfate dodecahydrate [NH4Fe(SO4)2·12H2O] to 10 mL of nitric acid (1+2), stir evenly, and then obtain the ferric ammonium sulfate indicator solution.

[0031] Furthermore, the concentration of the ammonium thiocyanate standard solution is 0.0564 mol / L, and the preparation method of the ammonium thiocyanate standard solution is as follows: weigh 4.2932 g of ammonium thiocyanate (NH4SCN), dissolve it in distilled water or deionized water, and dilute it to 1 L to obtain the ammonium thiocyanate standard solution.

[0032] Further, the specific steps for preparing the sodium chloride solution are as follows: sodium chloride is placed in a high-temperature furnace and ignited for 40-50 minutes until constant weight is achieved. 0.8429 g of the ignited sodium chloride is weighed, and the weighed sodium chloride is dissolved in water and transferred to a 1 L volumetric flask. Diluent is added according to the required concentration of the prepared sodium chloride solution to complete the preparation of the sodium chloride solution. The temperature in the high-temperature furnace is 550±50℃.

[0033] Furthermore, the sodium chloride is heated to constant weight in a high-temperature furnace specifically as follows:

[0034] The first step is to place sodium chloride in a high-temperature furnace and ignite it for 15 minutes, then cool and weigh the ignited sodium chloride.

[0035] The second step is to repeat the first step, subjecting the sodium chloride to multiple high-temperature incinerations, cooling, and weighing. When the mass difference between two consecutive weighings is less than 0.0005g, the sodium chloride reaches a constant weight.

[0036] Furthermore, the calculated chloride ion content should be rounded. If a burette with an accuracy of 0.05 mL is used, it should be rounded to 0.002%; if a burette with an accuracy of 0.01 mL is used, it should be rounded to 0.0004%.

[0037] Based on the above technical solution, the method for detecting chloride ions in cement, as described in this invention patent, has achieved the following technical advantages through practical application:

[0038] 1. This invention provides a method for detecting chloride ions in cement, which is designed to detect chloride ions in cement within a range equal to the amount of added Ag. + The number of moles is 20% to 80%, and the increase is n0 << n Ag+ To improve detection accuracy, the concentration of added ions is designed to be an integer multiple of 0.0141.

[0039] 2. This invention provides a method for detecting chloride ions in cement by controlling the detection range of chloride ion content (the chloride ion to be measured). - The number of moles) is the detection range (additional Ag). + By reducing the amount of silver ions to 20%-80%, the detection error can be reduced from 11.98%-100% to 11.98%-21.43%, ensuring an appropriate excess of silver ions, reducing the adsorption of silver ions by the precipitate, avoiding premature titration, and reducing titration error.

[0040] 3. The present invention provides a method for detecting chloride ions in cement by adding |n0 / n Ag+ The method of determining chloride ion content by a value ≤2% involves revising the chloride ion content calculation formula and proposing to directly calculate the chloride ion content using the ammonium thiocyanate concentration. By calibrating the ammonium thiocyanate concentration, the detection error is reduced from 11.98%–100% to 0.53%–46.82%, thus reducing the influence of impurity ions on chloride ion detection.

[0041] 4. This invention provides a method for detecting chloride ions in cement. By controlling the concentration of the added ions to be an integer multiple of 0.0141, adjusting the concentration of the ammonium thiocyanate titrant to 0.0564 mol / L, and controlling the titrant volume to be an integer multiple of 0.05 mL as much as possible, the method reduces the measurement error caused by the volume consumption of the titrant. This solves the contradiction that low-concentration titrants result in indistinct indicator color development, while high-concentration titrants lead to large titration errors. Simultaneously, the concentrations of both added chloride and silver ions are controlled to be integer multiples of 0.0141. This reduces the detection error from 11.98%–100% to 11.78%–33.84%.

[0042] 5. The present invention provides a method for detecting chloride ions in cement by controlling the content (moles) of the chloride ions to be detected to be 20% to 80% of the detection range (moles of added silver ions), controlling the concentration of the added ions to be an integer multiple of 0.0141, and adding |n0 / n Ag+ The method involves determining the chloride ion content based on a value ≤2%, revising the chloride ion content calculation formula, and adding ammonium thiocyanate calibration. Using these three techniques simultaneously reduces the detection error from the existing 11.98%–100% to 0.02%–1.86%. Using any one of the three techniques individually does not significantly improve detection accuracy. Combining the three techniques in pairs slightly increases detection accuracy. Using all three techniques simultaneously yields the highest detection accuracy, with a measurement precision of 0.1 mg chloride ions and a 98.14% guarantee rate, thus improving the accuracy of determining trace chloride ion content. Detailed Implementation

[0043] To make the objectives, technical solutions, and advantages of this invention clearer, the invention is described below through specific examples. However, it should be understood that these descriptions are merely exemplary and not intended to limit the scope of the invention. Furthermore, descriptions of well-known structures and techniques are omitted in the following description to avoid unnecessarily obscuring the concept of the invention.

[0044] The chloride ion detection method of this invention is also applicable to the chloride ion content in mineral powder and fly ash, assuming the chloride ion content in the sample is... - For doses of 0.3–4 mg, the concentration and volume of the NaCl solution used were 0.0141 mol / L and 2 mL, respectively, and the concentration and volume of the AgNO3 solution used were 0.0141 mol / L and 13 mL, respectively.

[0045] S1, Weigh approximately 5g of the sample, accurate to 0.0001g, and place it in a 400mL beaker;

[0046] S2, add 50mL of distilled water or deionized water to the beaker and stir with a glass rod to completely disperse the sample;

[0047] S3, add 50 mL of nitric acid (1+2) while stirring, heat to boiling, and simmer for 1-2 minutes;

[0048] S4, remove, add 2 mL of sodium chloride standard solution (0.0141 mol / L), then add 13 mL of silver nitrate standard solution (0.0141 mol / L), stir well, and boil for 1-2 minutes;

[0049] S5, add a small amount of filter paper pulp, filter with fast filter paper that has been washed with nitric acid (1+100) beforehand, collect the filtrate in a 250mL Erlenmeyer flask, wash the beaker, glass rod and filter paper with nitric acid (1+100) until the total volume of filtrate and washings reaches about 200mL, and cool the solution to below 25°C in a dim light or dark place.

[0050] S6. Add 5 mL of ferric ammonium sulfate indicator solution to the conical flask. Titrate the excess silver ions in the conical flask with ammonium thiocyanate standard solution (0.0564 mol / L) until a reddish-brown color is formed and does not disappear upon shaking. Stop the titration and record the volume V of ammonium thiocyanate consumed. NH4SCN ;

[0051] Scenario 1:

[0052] When V NH4SCN If the concentration is between 0.75 and 2.60 mL, it indicates that Cl... - The chloride ion content is between 0.3 and 4 mg; the formula for calculating the chloride ion content is:

[0053]

[0054] The calculation formula requires the use of C. NH4SCN NH4SCN needs to be labeled; perform a blank experiment, adding 2 mL of sodium chloride (0.0141 mol / L) and 13 mL of silver nitrate (0.0141 mol / L) in step S4, without adding the sample, and the remaining steps are the same as the sample detection steps.

[0055] Scenario 2:

[0056] When V NH4SCN >2.60mL indicates Cl - <0.3mg, i.e. Cl -The concentration is between 0 and 0.3 mg; repeat step S4, adding NaCl solution with a concentration of 0.00141 mol / L and a volume of 2 mL, and AgNO3 solution with a concentration of 0.0141 mol / L and a volume of 1 mL; The formula for calculating chloride ion content is:

[0057]

[0058] The calculation formula requires the use of C. NH4SCN NH4SCN needs to be labeled; perform a blank experiment, adding 2 mL of sodium chloride (0.01141 mol / L) and 1 mL of silver nitrate (0.0141 mol / L) in step S4, without adding the sample, and the remaining steps are the same as the sample detection steps.

[0059] Scenario 3:

[0060] When V NH4SCN <0.75mL indicates Cl - >4mg, default Cl - The concentration is between 4 and 15.9525 mg; repeat step S4, without adding NaCl, and add AgNO3 solution with a concentration of 0.141 mol / L and a volume of 4 mL; the formula for calculating chloride ion content has two cases:

[0061] Case 3-1: When |C AgNO3 V AgNO3 -C NH4SCN V NH4SCN空白 | / (C AgNO3 V AgNO3 )≤2%, that is, |n0 / n Ag+ ≤2%, formula for calculating chloride ion content:

[0062]

[0063] Case 3-2: When |C AgNO3 V AgNO3 -C NH4SCN V NH4SCN空白 | / (C AgNO3 V AgNO3 >2%, that is, |n0 / n Ag+ Formula for calculating chloride ion content >2%:

[0064]

[0065] The calculation formula in case 3-1 does not use C. NH4SCN No NH4SCN calibration is required; perform a blank experiment, without adding sodium chloride in step S4, add 4 mL of silver nitrate (0.141 mol / L), without adding the sample, and the remaining steps are the same as the sample detection steps.

[0066] The calculation formula for case 3-2 uses C. NH4SCN NH4SCN needs to be labeled; perform a blank experiment, in step S4, do not add sodium chloride, add 4 mL of silver nitrate (0.141 mol / L), do not add the sample, and the remaining steps are the same as the sample detection steps.

[0067] The chloride ion content should be rounded after calculation. If a burette with an accuracy of 0.05 mL is used, round to 0.002%; if a burette with an accuracy of 0.01 mL is used, round to 0.0004%.

[0068] Example 1

[0069] This embodiment provides a method for preparing sodium chloride standard solutions of various concentrations.

[0070] The mass of chloride ion solute in the sodium chloride standard solution is 0.3–4 mg, because the chloride ion content in cement is 0.006%–0.080% when the chloride content is 99% (i.e., 0.3–4 mg of chloride ions in 5g of sample); the volume of the sodium chloride standard solution is 50 mL, because GB / T 176-2017 uses the ammonium thiocyanate volumetric method to determine the chloride ion content in cement by dissolving 5g of cement sample in 50 mL of distilled or deionized water. Therefore, to prepare a series of sodium chloride standard solutions, as shown in Table 2, if you want 1 mg of chloride ions in 50 mL of solution, you need 1 mg of sodium chloride ÷ 35.453 mg / mol × 58.442561 mg / mol ÷ 1000 = 0.0016484517812314900 g. This mass cannot be accurately measured. Multiplying the solute mass by 1000 while keeping the volume unchanged, to prepare a 1.6485 g / 50 mL sodium chloride solution, you need to dilute it 1000 times. Therefore, to prepare a 1.6485 g / 500 mL sodium chloride solution, you only need to dilute it 100 times. Therefore, the steps for preparing sodium chloride are as follows:

[0071] Step 1, Ignition: Take approximately 250g of analytical grade sodium chloride solid and ignite it in a high-temperature furnace at 550±50℃ for 40-50 minutes until constant weight is reached. Constant weight is checked by repeatedly igniting for 15 minutes each time, followed by cooling and weighing after the first ignition. Constant weight is achieved when the difference between two consecutive weighings is less than 0.0005g. Cool to room temperature in a desiccator.

[0072] Step 2, Preparation: Weigh 1.6485g of sodium chloride using an analytical balance, and dilute it to 50mL with distilled or deionized water in a volumetric flask.

[0073] Step 3, dilution: Take 10 mL, dilute to 1000 mL, and keep 50 mL for later use;

[0074] Weigh sodium chloride according to the data in column 4 of Table 2 for a chloride ion solution of 2-15 mg / 50 mL. The unit is g. Dilute it with distilled water or deionized water to 50 mL. Take 10 mL and dilute it to 1000 mL. Take 200 mL for later use.

[0075] Prepare chloride ion solutions of 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, and 0.9 mg / 50 mL. Take 20 mL of each chloride ion solution of 1, 2, 3, 4, 5, 6, 7, 8, and 9 mg / 50 mL and dilute to 200 mL.

[0076] Prepare chloride ion solutions of 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, and 0.09 mg / 50 mL. Take 10 mL of each chloride ion solution of 1, 2, 3, 4, 5, 6, 7, 8, and 9 mg / 50 mL, dilute to 1000 mL, and keep 200 mL for later use.

[0077] Weigh sodium chloride according to the data in the fourth column of Table 1 to obtain chloride ion solutions of 1.0000 mg / 50 mL, 0.1000 mg / 50 mL, and 0.0100 mg / 50 mL, accurate to four decimal places.

[0078] Table 1. Preparation of Sodium Chloride Standard Solution

[0079]

[0080]

[0081] Example 2

[0082] This embodiment verifies the accuracy of the detection method: the relative error of the detection method is verified using a sodium chloride standard solution of known concentration. The detection method is as follows:

[0083] 0.3–4 mg / 50 mL chloride ion solution (taking 1 mg / 50 mL as an example): Take 50 mL of 1 mg / 50 mL sodium chloride solution, add 50 mL of nitric acid (1+2) while stirring, heat to boiling, and simmer for 1–2 minutes. Remove from heat, add 2.00 mL of sodium chloride standard solution (0.0141 mol / L), add 13.00 mL of silver nitrate standard solution (0.0141 mol / L), stir well, boil for 1–2 minutes, add a small amount of filter paper pulp, and filter with rapid filter paper that has been pre-washed with nitric acid (1+100). Collect the filtrate in a 250 mL flask, wash the beaker, glass rod, and filter paper with nitric acid (1+100) until the total volume of the filtrate and washings reaches about 200 mL, and cool the solution to below 25°C in a dim light or dark place. Add 5 mL of ferric ammonium sulfate indicator solution, and titrate with ammonium thiocyanate standard titration solution (0.0564 mol / L) until the resulting reddish-brown color does not disappear upon shaking. Record the volume V of ammonium thiocyanate consumed. NH4SCN Perform a blank experiment without adding a sample, following the steps described above, and record the volume V of the ammonium thiocyanate standard titration solution used in the blank titration. NH4SCN空白 Determine the concentration of ammonium thiocyanate and calculate the chloride ion content. The detection of chloride ion solutions of other concentrations is the same except that the chloride ion standard solution to be tested is different.

[0084] 0–0.3 mg / 50 mL chloride ion solution (taking 0.1 mg / 50 mL as an example): Take 50 mL of 0.1 mg / 50 mL sodium chloride solution, add 50 mL of nitric acid (1+2) while stirring, heat to boiling, and simmer for 1–2 minutes. Remove from heat, add 2.00 mL of sodium chloride standard solution (0.00141 mol / L), add 1.00 mL of silver nitrate standard solution (0.0141 mol / L), stir well, boil for 1–2 minutes, add a small amount of filter paper pulp, and filter with fast filter paper that has been pre-washed with nitric acid (1+100). Collect the filtrate in a 250 mL flask, wash the beaker, glass rod, and filter paper with nitric acid (1+100) until the total volume of the filtrate and washings reaches about 200 mL, and cool the solution to below 25°C in a dim light or dark place. Add 5 mL of ferric ammonium sulfate indicator solution, and titrate with ammonium thiocyanate standard titration solution (0.0564 mol / L) until the resulting reddish-brown color does not disappear upon shaking. Record the volume V of ammonium thiocyanate consumed. NH4SCN Perform a blank experiment without adding a sample, following the steps described above, and record the volume V of the ammonium thiocyanate standard titration solution used in the blank titration. NH4SCN空白 Determine the concentration of ammonium thiocyanate and calculate the chloride ion content. The detection of chloride ion solutions of other concentrations is the same except that the chloride ion standard solution to be tested is different.

[0085] 4–15.9525 mg / 50 mL chloride ion solution (taking 15 mg / 50 mL as an example): Take 50 mL of 15 mg / 50 mL sodium chloride solution, add 50 mL of nitric acid (1+2) while stirring, heat to boiling, and simmer for 1–2 minutes. Remove from heat, add 4.00 mL of silver nitrate standard solution (0.141 mol / L), stir well, boil for 1–2 minutes, add a small amount of filter paper pulp, and filter with fast filter paper that has been pre-washed with nitric acid (1+100). Collect the filtrate in a 250 mL flask, wash the beaker, glass rod, and filter paper with nitric acid (1+100) until the total volume of the filtrate and washings reaches about 200 mL, and cool the solution to below 25°C in a dim light or dark place. Add 5 mL of ferric ammonium sulfate indicator solution, and titrate with ammonium thiocyanate standard titration solution (0.0564 mol / L) until the resulting reddish-brown color does not disappear upon shaking. Record the volume V of ammonium thiocyanate consumed. NH4SCN Perform a blank experiment without adding a sample, following the steps described above, and record the volume V of the ammonium thiocyanate standard titration solution used in the blank titration. NH4SCN空白 If |C AgNO3 V AgNO3 -C NH4SCN V NH4SCN空白 | / (C AgNO3 V AgNO3 )≤2%, that is, |n0 / n Ag+ ≤2%, formula for calculating chloride ion content: If |C AgNO3 V AgNO3 -C NH4SCN V NH4SCN空白 | / (C AgNO3 V AgNO3 >2%, that is, |n0 / n Ag+ Formula for calculating chloride ion content >2%:

[0086] For the detection of chloride ion solutions of other concentrations, only the chloride ion standard solution to be tested is different; all other conditions are the same. Ammonium thiocyanate needs to be calibrated.

[0087] The test results are shown in Tables 2 to 5.

[0088] Table 2. Results of the improved method for detecting chloride ion standard solutions (1–15 mg, 0.05 mL precision burette). )

[0089]

[0090] Table 3. Results of the improved method for detecting chloride ion standard solutions (0.1–0.9 mg, 0.05 mL precision burette). )

[0091]

[0092]

[0093] Table 4. Results of the improved method for detecting chloride ion standard solutions (0.01–0.09 mg, 0.05 mL precision burette). )

[0094]

[0095] Table 5. Results of the improved method for detecting chloride ion standard solutions (0.01–0.09 mg, 0.01 mL precision burette). )

[0096]

[0097] Based on Tables 2, 3, and 4, it can be seen that the improved method, using a 0.05 mL precision burette to measure chloride ions, achieves a relative error of less than 2.02% with an accuracy of 0.1 mg. Based on Tables 2, 3, and 5, it can be seen that the improved method, using a 0.01 mL precision burette to measure chloride ions, achieves a relative error of less than 2.02% with an accuracy of 0.02 mg. This means that the improved method, using a 0.05 mL burette to measure chloride ions, has an accuracy of 0.1 mg (for a 5 g cement sample, the accuracy of chloride ion content is 0.1 / 5000*100% = 0.002%), and using a 0.01 mL burette to measure chloride ions, has an accuracy of 0.02 mg (for a 5 g cement sample, the accuracy of chloride ion content is 0.02 / 5000*100% = 0.0004%). This implies that when using a 0.05 mL burette to measure chloride ions, the chloride ion content should be rounded to 0.002%; and when using a 0.01 mL burette, the chloride ion content should be rounded to 0.0004%. For example, using a 0.05 mL precision burette, the mass of chloride ions in a 5 g mineral powder sample was measured to be 4.2679 mg. Rounding this to the nearest multiple of 0.1 mg (0.002%), the result is 4.3 mg (0.086%), with a relative error within 2.02%. Similarly, using a 0.05 mL precision burette, the mass of chloride ions in a 5 g cement sample was measured to be 13.1527 mg. Rounding this to the nearest multiple of 0.02 mg (0.0004%), the result is 13.16 mg (0.2632%), with a relative error within 2.02%. In other words, the improved method using a 0.05 mL precision burette has a guarantee rate of ≥97.98% for measurements up to 0.002% (5 g sample), and the improved method using a 0.01 mL precision burette has a guarantee rate of ≥97.98% for measurements up to 0.0004% (5 g sample).

[0098] Example 3

[0099] The difference between this embodiment and embodiment 2 is that... All other conditions were the same as in Example 2, and the test results are shown in Table 6.

[0100] Table 6. Results of the improved method for detecting chloride ion standard solutions (0.1–15 mg, 0.05 mL precision burette). )

[0101]

[0102] Example 4

[0103] The difference between this embodiment and embodiment 2 is that... All other conditions were the same as in Example 2, and the test results are shown in Table 7.

[0104] Table 7 Results of the improved method for detecting chloride ion standard solutions (0.1–15 mg, 0.05 mL precision burette). )

[0105]

[0106]

[0107] As shown in Table 7, At that time, the relative error of detection was within 0.64%.

[0108] Example 5

[0109] Verification Scenario 1 in this embodiment, Improved Method: Weigh 5.1027g of fly ash and measure V NH4SCN =2.50mL, which belongs to case 1. Perform a blank experiment and measure V. NH4SCN空白 = 2.70 mL. To standardize ammonium thiocyanate, accurately pipette 25 mL of the standardized silver nitrate standard solution (0.0564 mol / L) into a 250 mL Erlenmeyer flask, add 50 mL of distilled or deionized water, mix well, add 2 mL of nitric acid (1+2) and 1 mL of ferric ammonium sulfate solution, and titrate the silver nitrate solution with the ammonium thiocyanate solution until a reddish-brown color appears that does not fade after vigorous shaking. Record the volume of ammonium thiocyanate consumed, V. NH4SCN =25.00 mL, calculate the concentration of ammonium thiocyanate solution.

[0110] Calculate according to case 1

[0111]

[0112] Ordinary method: Weigh 5.1027g of fly ash and measure V. NH4SCN =4.65mL, V NH4SCN空白 =4.95mL, calculate

[0113]

[0114] The fly ash samples measured by the improved method and the ordinary method were taken from the same sample and divided into two parts. The relative error of the ordinary method compared to the improved method was (0.011-0.008) / 0.008 = 37.50%, which is relatively large.

[0115] Example 6

[0116] Verification Scenario 2 in this embodiment: Improved method: Weigh 5.0767g of mineral powder and measure B. NH4SCN = 2.70 mL, belonging to case 2, repeat, add 0.00141 mol / L NaCl × 2 mL, add 0.0141 mol / L AgNO3 × 1 mL, and measure V. NH4SCN =0.10mL, perform a blank experiment, and measure V NH4SCN空白 =0.20 mL; Standardize ammonium thiocyanate, using the same method as in Example 5, calculated according to Case 2.

[0117]

[0118] Ordinary method: Weigh 5.0767g of mineral powder and measure V. NH4SCN =4.95mL, V NH4SCN空白 =5.10mL, calculate

[0119]

[0120] The relative error of the ordinary method compared to the improved method is (0.005-0.004) / 0.004 = +25.00%, which is relatively large.

[0121] Example 7

[0122] Verification Scenario 31 in this embodiment: Improved method: Weigh 5.0490g of cement and measure V NH4SCN =0.70mL, belonging to case 3, repeat, without NaCl, add AgNO3 0.141mol / L×4mL, and measure V. NH4SCN =8.10mL, perform a blank experiment, and measure V NH4SCN空白 =10.20mL.

[0123] but

[0124] This falls under case 31, where ammonium thiocyanate does not need to be standardized; calculations should be performed according to case 3-1.

[0125]

[0126] Ordinary method: Weigh 5.0490g of mineral powder and measure V. NH4SCN =2.70mL, V NH4SCN空白=5.10mL, calculate

[0127]

[0128] The relative error of the ordinary method compared to the improved method is (0.083-0.082) / 0.082=+1.22%, which is smaller.

[0129] Example 8

[0130] Verification of this embodiment 3-2: Improved method: Weigh 5.1082g of cement and measure V NH4SCN =0.70mL, belonging to case 3, repeat, without NaCl, add AgNO3 0.141mL. l / L×4mL, measure V NH4SCN =8.90mL, perform a blank experiment, and measure V NH4SCN空白 = 11.40 mL.

[0131] but

[0132] This falls under case 3-2. The ammonium thiocyanate is calibrated using the same method as in Example 5, calculated according to case 3-2.

[0133]

[0134] Ordinary method: Weigh 5.1082g of mineral powder and measure V. NH4SCN =2.85mL, V NH4SCN空白 =5.70mL, calculate

[0135]

[0136] The relative error between the conventional method and the improved method is (0.087-0.098) / 0.098 = -11.22%. Chloride ions are calculated using the corrected formula.

[0137] The relative error between the modified ordinary method and the improved method is (0.098-0.098) / 0.098 = 0%. This indicates that when the chloride ion content is around 0.10%, in the ordinary method, adding |n0| / n... Ag+ A judgment of ≤2% can eliminate a relative error of 11.22%.

[0138] Example 9

[0139] This embodiment verifies the accuracy of the testing method: the relative error of the testing method is verified using a cement standard reference material with a known chloride ion content (GSB 08-2047-F03-2021, ordinary Portland cement, standard value 0.022%). The testing method is as follows:

[0140] 5g of standard cement contains 5 * 0.022% * 1000 = 1.1mg of chloride ions. This falls under case 1.

[0141] 0.3–4 mg / 50 mL chloride ion solution: Weigh 5.0000 g of standard cement and place it in a 400 mL beaker. Add 50 mL of distilled or deionized water, and while stirring, add 50 mL of nitric acid (1+2). Heat to a boil and simmer for 1–2 minutes. Remove from heat, add 2.00 mL of sodium chloride standard solution (0.0141 mol / L), and 13.00 mL of silver nitrate standard solution (0.0141 mol / L). Stir well and boil for 1–2 minutes. Add a small amount of filter paper pulp and filter using fast filter paper that has been pre-washed with nitric acid (1+100). Collect the filtrate in a 250 mL flask. Wash the beaker, glass rod, and filter paper with nitric acid (1+100) until the total volume of the filtrate and washings reaches approximately 200 mL. Cool the solution to below 25°C in a dim light or dark place. Add 5 mL of ferric ammonium sulfate indicator solution, and titrate with ammonium thiocyanate standard titration solution (0.0564 mol / L) until the resulting reddish-brown color does not disappear upon shaking. Record the volume V of ammonium thiocyanate consumed. NH4SCN Perform a blank experiment without adding a sample, following the steps described above, and record the volume V of the ammonium thiocyanate standard titration solution used in the blank titration. NH4SCN空白 Determine the concentration of ammonium thiocyanate and calculate the chloride ion content.

[0142] V was measured NH4SCN =2.20mL, which belongs to case 1. Perform a blank experiment and measure V. NH4SCN空白 = 2.75 mL. The ammonium thiocyanate concentration was determined to be 0.0564 mol / L. Then...

[0143] The relative error of the detection is (0.022-0.022) / 0.022=0%.

[0144] Comparative Example 1

[0145] The difference between this comparative example and Example 2 is that the detection range (the number of moles of chloride ions to be measured) was not controlled to be 20% to 80% of the range (the number of moles of added silver ions). All other conditions were the same as in Example 2. The detection results are shown in Table 8.

[0146] Table 8 shows the detection results for 20%–80% of samples without control (0.1–15 mg, 0.05 mL precision burette). )

[0147]

[0148]

[0149] Note: In GB / T 176-2017, the ammonium thiocyanate titration method for the determination of chloride ions requires the addition of 0.25 mmol of Ag. + When V NH4SCN If the sample volume is less than 0.5 mL, reduce the sample mass by half and repeat the test; in this control group, add 0.2538 mmol of Ag. + When V NH4SCN When the concentration is less than 0.65 mL, reduce the mass of the solute by half and repeat the test. That is, when testing a 10 mg / 50 mL sodium chloride standard solution, you are actually testing a 5 mg / 50 mL sodium chloride standard solution. The same principle applies to the testing of 11–15 mg / 50 mL sodium chloride solutions.

[0150] As shown in Table 8, when the detection range (number of moles of chloride ions to be measured) is not controlled to be 20% to 80% of the range (number of moles of added silver ions), the relative error of the detection is within 25.02%.

[0151] Comparative Example 2

[0152] The difference between this comparative example and Example 2 is that the concentration of the added ions is not an integer multiple of 0.0141. All other conditions are the same as in Example 2. The detection results are shown in Table 9.

[0153] Table 9 shows the detection results for added ion concentrations that are not integer multiples of 0.0141 (0.1–15 mg, 0.05 mL precision burette). )

[0154]

[0155]

[0156] Note: The concentration of the added ions is not an integer multiple of 0.0141, but the added Cl- and Ag+ should make the molar ratio of the Cl- to Ag+ to be measured 20% to 80%. When chloride ions are 0.3 to 4 mg, add 2 mL of chloride ions (0.0141 mol / L) and 5 mL of silver ions (0.035 mol / L); when chloride ions are <0.3 mg, add 2 mL of chloride ions (0.00141 mol / L) and 1 mL of silver ions (0.0141 mol / L); when chloride ions are >4 mg, add 5.5 mL of silver ions (0.01 mol / L).

[0157] As shown in Table 9, when the concentration of the added ions is not an integer multiple of 0.0141, the relative error of the detection is within 11.37%.

[0158] Comparative Example 3

[0159] The difference between this comparative example and Example 2 is that it does not determine |n0| / n(Ag+)≤2%, and the detection results are shown in Table 10.

[0160] Table 10 shows the detection results without judging |n0| / n(Ag+)≤2% (0.1~15mg, 0.05mL precision burette). )

[0161]

[0162] As shown in Table 10, without judging |n0| / n(Ag+)≤2%, the relative detection error is within 16.29%.

[0163] Comparative Example 4

[0164] This comparative example uses the ammonium thiocyanate titration method from GB / T 176-2017 to detect chloride ions, referred to as the ordinary method. The difference from Example 2 is that the method does not control the chloride ion content. - The number of moles is the amount of Ag added. + For molars ranging from 20% to 80%, and where the concentration of added ions is not an integer multiple of 0.0141, it is not considered that |n0| / n(Ag+)≤2%. The detection results are shown in Table 11.

[0165] The specific steps of the ordinary method are as follows: Take 50 mL of any concentration of sodium chloride standard solution prepared in Example 1, add 50 mL of nitric acid (1+2) while stirring, heat to boiling, and simmer for 1 to 2 minutes. Remove from heat, add 5.00 mL of silver nitrate standard solution (0.05 mol / L), stir well, boil for 1 to 2 minutes, add a small amount of filter paper pulp, and filter with fast filter paper that has been pre-washed with nitric acid (1+100). Collect the filtrate in a 250 mL flask, wash the beaker, glass rod, and filter paper with nitric acid (1+100) until the total volume of the filtrate and washings reaches about 200 mL, and cool the solution to below 25°C in a dim light or dark place; add 5 mL of ferric ammonium sulfate indicator solution, and titrate with ammonium thiocyanate standard titration solution (0.05 mol / L) until the resulting reddish-brown color does not disappear when shaken, and record the volume of ammonium thiocyanate consumed, V. NH4SCN If the amount of ammonium thiocyanate consumed is V NH4SCN If the volume is less than 0.5 mL, repeat the test with half the sample mass. Perform a blank test without adding the sample, following the steps above, and record the volume V of the ammonium thiocyanate standard titration solution used in the blank titration. NH4SCN空白 The calculation formula is:

[0166]

[0167] Table 11 shows the results of the ordinary method (0.1–15 mg, 0.05 mL precision burette). )

[0168]

[0169] Note: In GB / T 176-2017, the ammonium thiocyanate titration method for the determination of chloride ions requires the addition of 0.25 mmol of Ag. + When V NH4SCN If the sample volume is less than 0.5 mL, reduce the sample mass by half and repeat the test; in this control group, add 0.25 mmol of Ag. + When V NH4SCN When the concentration is less than 0.5 mL, reduce the mass of the solute by half and repeat the test. That is, when testing a 10 mg / 50 mL sodium chloride standard solution, you are actually testing a 5 mg / 50 mL sodium chloride standard solution. The same principle applies to the testing of 11–15 mg / 50 mL sodium chloride solutions.

[0170] As shown in Table 11, the relative error of the detection using the ordinary method in the existing GB / T 176-2017 is within 100%.

[0171] Comparative Example 5

[0172] This comparative example uses the ordinary method (GB / T 176-2017), but a burette with an accuracy of 0.01 mL is selected. The difference between this comparative example and Comparative Example 4 is that the accuracy of the burette is changed from 0.05 mL to 0.01 mL. All other conditions are the same. The test results are shown in Table 12.

[0173] Table 12 Results of ordinary method test (0.01~15mg, 0.01mL precision burette) )

[0174]

[0175] Note: In GB / T 176-2017, the ammonium thiocyanate titration method for the determination of chloride ions requires the addition of 0.25 mmol of Ag. + When V NH4SCN If the sample volume is less than 0.5 mL, reduce the sample mass by half and repeat the test; in this control group, add 0.25 mmol of Ag. + When V NH4SCN When the concentration is less than 0.5 mL, reduce the mass of the solute by half and repeat the test. That is, when testing a 10 mg / 50 mL sodium chloride standard solution, you are actually testing a 5 mg / 50 mL sodium chloride standard solution. The same principle applies to the testing of 11–15 mg / 50 mL sodium chloride solutions.

[0176] As shown in Table 12, using the ordinary method in the existing GB / T 176-2017 and the most precise 0.01mL burette, the relative error for detecting chloride ions accurate to 0.1mg is within 53%, and it is impossible to detect 0.02mg chloride ions.

[0177] Comparative Example 6

[0178] This comparative example uses the ordinary method (GB / T 176-2017), but modifies the chloride ion calculation formula (i.e., determining |n0| / n(Ag+)≤2%). A burette with an accuracy of 0.01 mL is used to calibrate the ammonium thiocyanate concentration. The difference between this comparative example and Comparative Example 4 is that the burette accuracy is changed from 0.05 mL to 0.01 mL. The chloride ion calculation formula is modified from...

[0179]

[0180] Modified to

[0181]

[0182] All other conditions were the same, and the test results are shown in Table 13.

[0183] Table 13 shows the test results using the modified calculation formula of the ordinary method (0.1–15 mg, 0.01 mL precision burette). )

[0184]

[0185] Note: In GB / T 176-2017, the ammonium thiocyanate titration method for the determination of chloride ions requires the addition of 0.25 mmol of Ag. + When V NH4SCN If the sample volume is less than 0.5 mL, reduce the sample mass by half and repeat the test; in this control group, add 0.25 mmol of Ag. + When V NH4SCN When the concentration is less than 0.5 mL, reduce the mass of the solute by half and repeat the test. That is, when testing a 10 mg / 50 mL sodium chloride standard solution, you are actually testing a 5 mg / 50 mL sodium chloride standard solution. The same principle applies to the testing of 11–15 mg / 50 mL sodium chloride solutions.

[0186] As shown in Table 13, using the ordinary method in the existing GB / T 176-2017, the chloride ion calculation formula is modified, and the most precise 0.01mL burette is used, the relative error of the test results is within 46.82%.

[0187] Comparative Example 7

[0188] Based on the ordinary method, this comparative example only controls the concentration of the added ions to be an integer multiple of 0.0141. The difference between this comparative example and Comparative Example 4 is that the concentration of the added ions is controlled to be an integer multiple of 0.0141, while all other conditions are the same. The detection results are shown in Table 14.

[0189] Table 14 shows the detection results using only integer multiples of 0.0141 in the ordinary method (0.1–15 mg, 0.05 mL precision burette). )

[0190]

[0191]

[0192] Note: In GB / T 176-2017, the ammonium thiocyanate titration method for the determination of chloride ions requires the addition of 0.25 mmol of Ag. + When V NH4SCN If the sample volume is less than 0.5 mL, reduce the sample mass by half and repeat the test; in this control group, add 0.25 mmol of Ag. + When V NH4SCN When the concentration is less than 0.5 mL, reduce the mass of the solute by half and repeat the test. That is, when testing a 10 mg / 50 mL sodium chloride standard solution, you are actually testing a 5 mg / 50 mL sodium chloride standard solution. The same principle applies to the testing of 11–15 mg / 50 mL sodium chloride solutions.

[0193] As shown in Table 14, by using the ordinary method in the existing GB / T 176-2017 and only adjusting the concentration of the added ions to an integer multiple of 0.0141, the relative error of the detection results is within 33.84%.

[0194] Comparative Example 8

[0195] This comparative example, based on the ordinary method, only controls the molar number of chloride ions to be measured to be 20%–80% of the molar number of added silver ions. The difference between this comparative example and Comparative Example 4 is that the added chloride ions are controlled, and the molar number of chloride ions to be measured is controlled to be 20%–80% of the molar number of added silver ions. All other conditions are the same, and the detection results are shown in Table 15.

[0196] Table 15 shows the results of detection using the conventional method, which only controls 20%–80% of the results (0.1–15 mg, 0.05 mL precision burette). )

[0197]

[0198]

[0199] Note: In GB / T 176-2017, the ammonium thiocyanate titration method for the determination of chloride ions requires the addition of 0.25 mmol of Ag. + When V NH4SCN If the sample volume is less than 0.5 mL, reduce the sample mass by half and repeat the test; in this control group, add 0.25 mmol of Ag. + When V NH4SCNWhen the concentration is less than 0.5 mL, reduce the solute by half and repeat the test. That is, when testing a 10 mg / 50 mL sodium chloride standard solution, you are actually testing a 5 mg / 50 mL sodium chloride standard solution. The same principle applies to the testing of 11–15 mg / 50 mL sodium chloride solutions. The concentration of the added ions is not an integer multiple of 0.0141, but the added Cl- and Ag+ should make the molar ratio of the Cl- to Ag+ to be tested 20%–80%. When chloride ions are 0.3–4 mg, add 2 mL of chloride ions (0.0141 mol / L) and 5 mL of silver ions (0.035 mol / L); when chloride ions are <0.3 mg, add 2 mL of chloride ions (0.00141 mol / L) and 1 mL of silver ions (0.0141 mol / L); when chloride ions are >4 mg, add 5.5 mL of silver ions (0.01 mol / L).

[0200] As shown in Table 15, using the conventional method in the existing GB / T 176-2017, if the number of moles of chloride ions to be tested is controlled to be 20% to 80% of the number of moles of added silver ions, the relative error of the detection results is within 21.43%.

[0201] The relative errors of chloride ion detection in Examples 2 to 4 and Comparative Examples 1 to 8 are shown in Table 16.

[0202]

[0203] As shown in Table 16:

[0204] 1. The relative error of the ordinary method (GB / T 176-2017, Section 6.13) is 11.98% to 100% (see Comparative Example 4);

[0205] 2. Based on the ordinary method, the burette with a precision of 0.05 mL was replaced with a burette with a precision of 0.01 mL, and the relative error of the test was 12.29% to 53.1% (see Comparative Example 5);

[0206] 3. Based on the conventional method, the number of chloride ions and silver ions added to control the molar number of chloride ions to be measured is 20% to 80% of the number of added silver ions, and the relative error of detection is 11.98% to 21.43% (see Comparative Example 8);

[0207] 4. Based on the conventional method, the concentration of the added ions is controlled to be an integer multiple of 0.0141, and the relative error of detection is 11.78% to 33.84% (see Comparative Example 7);

[0208] 5. Determine |n0| / n based on the common method. Ag+ ≤2%, with a relative detection error of 0.53% to 46.82% (see Comparative Example 6);

[0209] 6. Based on the ordinary method, control the concentration of the added ions to be an integer multiple of 0.0141, and determine |n0| / n Ag+ ≤2%, with a relative detection error of 0.02% to 25.02% (see Comparative Example 1);

[0210] 7. Based on the standard method, control the number of chloride ions to be measured to be 20%–80% of the number of added silver ions, and determine |n0| / n. Ag+ ≤2%, with a relative detection error of 0.14% to 11.37% (see Comparative Example 2);

[0211] 8. Based on the conventional method, the molar number of chloride ions to be measured is controlled to be 20% to 80% of the number of added silver ions, and the concentration of added ions is controlled to be an integer multiple of 0.0141. The relative error of detection is 11.85% to 16.29% (see Comparative Example 3).

[0212] 9. Improved method (based on the ordinary method, the number of moles of chloride ions to be measured is controlled to be 20% to 80% of the number of added silver ions, and the concentration of added ions is controlled to be an integer multiple of 0.0141. Determine |n0| / n) Ag+ ≤2%) in n0 / n Ag+ >2%, n0 / n Ag+ =2%, n0 / n Ag+ The relative detection error was between 0.02% and 1.86% under the three conditions of <2%S (see Examples 2, 3, and 4).

[0213] Control the concentration to be 20%–80%, an integer multiple of 0.0141, and determine |n0| / n. Ag+ The accuracy is ≤2%. Using all three technologies simultaneously reduces the detection error from the existing 11.98%–100% to 0.02%–1.86%. Using the three technologies individually results in the weakest improvement in detection accuracy. Combining the three technologies in pairs results in slightly higher detection accuracy.

[0214] It should be noted that the detection errors in the examples and comparative examples are mostly negative, because the formula for calculating chloride ion content is:

[0215] or

[0216] All have (V) NH4SCN空白 -V NH4SCN In the blank experiment, the silver ion concentration was higher, resulting in more silver ions being adsorbed by the silver thiocyanate precipitate, and the volume V of ammonium thiocyanate consumed was [missing value]. NH4SCN空白 The value is too small; the silver ion concentration is low in the sample detection, the silver thiocyanate precipitate adsorbs fewer silver ions, and the volume V of ammonium thiocyanate consumed is relatively small. NH4SCN Too large, resulting in (V) NH4SCN空白 -V NH4SCNThe chloride ion value is too small, so the detected value is smaller than the true value.

[0217] The principle behind this invention's improvement in detection accuracy is as follows:

[0218] 1. The concentration of the added ions should be designed to be an integer multiple of 0.0141 or an integer multiple of 0.00141;

[0219] (1) Fe(SCN) used to indicate the titration endpoint 2+ The complex ion concentration needs to reach 6 × 10 -6 A red color can only be clearly observed at a concentration of 0.05 mol / L. To meet this condition, the ammonium thiocyanate concentration must be greater than or equal to 0.05 mol / L. The reason is as follows:

[0220] (a)Fe 3+ and SCN - The reaction equilibrium constant is: Fe 3+ and SCN - The chemical reaction equation is: Fe 3+ +SCN - =Fe(SCN) 2+ The formula for calculating the reaction equilibrium constant is: In the formula, K c This is the equilibrium constant for the reaction of a metal complex at a given temperature. The larger this value, the more stable the complex. [Fe(SCN)] 2+ [Fe] represents the concentration of the generated complex; 3+ [This represents the concentration of iron ions that did not participate in the coordination reaction;] This represents the concentration of the ligand SCN that did not participate in the coordination reaction. According to the chemical data handbook, at a temperature of 298 K (25°C, 273 K = 0°C), Fe... 3+ The equilibrium constant K for the reaction with SCN c It is 140.

[0221] (b)Fe 3+ The concentration of Fe is: 3+ The solution is derived from ferric ammonium sulfate indicator solution (prepared by adding 100 mL of saturated aqueous solution of ferric ammonium sulfate dodecahydrate [NH4Fe(SO4)2·12H2O] to 10 mL of nitric acid (1+2). According to the chemical data handbook, the solubility of NH4Fe(SO4)2·12H2O is 124 g (25℃), and its molecular weight is 482.192 g / mol. Assuming that the volume of the mixture of 10 mL (1+2) and 100 mL of saturated aqueous solution of ferric ammonium sulfate dodecahydrate [NH4Fe(SO4)2·12H2O] is 110 mL, then the concentration of the ferric ammonium sulfate indicator solution is 124 g ÷ 482.192 g / mol ÷ (110 × 10⁻⁶). -3L = 2.3378 mol / L. Since the total volume of the filtrate and washings in the conical flask is approximately 200 mL, and 5 mL of ferric ammonium sulfate indicator solution is added to the conical flask, the Fe concentration in the conical flask will increase. 3+ The concentration is 2.3378 mol / L × 5 mL ÷ 200 mL = 0.058 mol / L.

[0222] (c)Fe(SCN) 2+ The concentration is: when [Fe 3+ >250[SCN] - [A] represents the concentration of A ions, which can be considered as SCN. - Total consumption, if the concentration of NH4SCN is 0.05 mol / L, according to Fe 3+ and SCN - The chemical reaction coefficient is 1:1, and the generated Fe(SCN) 2+ If the number of moles of Fe(SCN) is equal to the number of moles of SCN consumed, then adding one drop of 0.05 mol / L NH4SCN will produce Fe(SCN). 2+ The concentration is 0.05 mol / L × 0.05 mL ÷ 200 mL = 1.25 × 10⁻⁶ -5 mol / L, verify that after adding 1 drop of NH4SCN and before the reaction of NH4SCN with NH4Fe(SO4)2, [Fe 3+ ]=0.058mol / L>250[SCN]=250×1.25×10 -5 mol / L = 0.003125 mol / L, while Fe(SCN) 2+ The concentration = 1.25 × 10 -5 mol / L > 6 × 10 -6 At mol / L, a red color can be clearly observed.

[0223] (d)Fe(SCN) 2+ The concentration is: when [Fe 3+ >250[SCN] - [A] represents the concentration of A ions, which can be considered as SCN. - Total consumption, if the concentration of NH4SCN is 0.01 mol / L, according to Fe 3+ and SCN - The chemical reaction equation ① shows the generated Fe(SCN) 2+ The number of moles equals the amount of SCN consumed. - If the molar number is such that 1 drop of 0.01 mol / L NH4SCN is added, the generated Fe(SCN) 2+ The concentration is 0.01 mol / L × 0.05 mL ÷ 200 mL = 2.5 × 10⁻⁶ -6mol / L, verify that after adding 1 drop of NH4SCN and before the reaction of NH4SCN with NH4Fe(SO4)2, [Fe 3+ ] = 0.058 mol / L > 250 [SCN] - ] = 250 × 2.5 × 10 -5 mol / L = 0.000625 mol / L, while Fe(SCN) 2+ The concentration = 2.5 × 10 -6 mol / L < 6 × 10 -6 At mol / L, no obvious red color could be observed.

[0224] (e)SCN - The concentration is: In fact, when [Fe 3+ >250[SCN] + [A] represents the concentration of A ions, SCN - It was not completely consumed. If the concentration of NH4SCN is 0.05 mol / L, after adding 1 drop of NH4SCN and reacting with NH4Fe(SO4)2, according to the reaction equilibrium constant... K c＝140 [Fe(SCN)] 2+ ] = 125 × 10 -5 mol / L, [Fe 3+ Since ]≈0.058mol / L,

[0225] (2) In the classic Mohr's method for determining chloride ion content, the concentration of the silver nitrate titrant is adjusted to 0.0141 mol / L, making it a standard concentration where 1 mL corresponds to 0.5 mg of chloride ions. This adjustment is not for ease of calculation, but because conventional burettes can only be read down to 0.05 mL. The principle of Mohr's method for determining chloride ions is that chloride ions in the sample are titrated with silver nitrate. At the instant the chloride ions are completely consumed, Ag... + It reacts with chromate ions to form a brick-red precipitate of silver chromate, indicating the endpoint; the n in the system... Ag+ =n Cl- ①, where n is the number of moles, and the unit is millimoles (mmol); and because In the formula, m is the mass of the ion, in milligrams (mg), and M is the molar mass (or molecular weight) of the ion, in milligrams per millimol (mg / mol); moreover, n = Cv③, where C is the molar concentration of the ion, in millimol per milliliter (mmol / mL), and V is the volume of the ion solution, in milliliters (mL); according to formulas ①②③, And M Cl- = 35.45 mg / mol. To measure 1 mg of chloride ions, if C Ag+=0.01mol / L, then according to equation ④ The burette can only read 2.80 mL, which is a large error; if C 土g+ =0.0141 mol / L, then The burette can be read to 2.00 mL with minimal error. Similarly, the Volha-Volhard method for determining chloride ion content can be adjusted by setting the ammonium thiocyanate titrant concentration to 0.0564 mol / L, making it a standard concentration where 1 mL corresponds to a reduction of 2 mg of chloride ions. The principle of the Volha-Volhard method for chloride ion determination is to titrate the chloride ions in the sample with excess silver nitrate, then titrate the remaining silver ions with ammonium thiocyanate. At the instant the silver ions are completely consumed, SCN... - with Fe 3+ A reddish-brown thiocyanate iron complex ion (Fe(SCN)) is formed. 2+ This indicates the endpoint because Ag is present during titration. + Both with Cl - The reaction also involves SCN, so n in the system... Cl- =n Ag+ -n SCN- ⑤, according to equations ②, ③, and ⑤, To measure 1 mg of chloride ions, if C SCN- =0.05mol / L, C Ag+ =0.05mol / L, VA g+ =5mL, then according to equation ⑥ The burette reading was 4.45 mL, which is a large error; if C SCN- =0.0564 mol / L, C Ag+ =0.0564mol / L, V Ag+ =5mL, then according to equation ⑥ The burette reading was 4.50 mL, which is a small error. Therefore, this invention suggests that it is more reasonable to change the concentration of ammonium thiocyanate titrant in the determination of chloride ion content in standard GB / T 1762017 from 0.05 mol / L to 0.0564 mol / L, and the concentration of added silver nitrate from 0.05 mol / L to 0.0564 mol / L.

[0226] Therefore, the concentration of ammonium thiocyanate titrant was modified from 0.05 mol / L in GB / T 176-2017 to 0.0564 mol / L (4 times 0.0141) instead of 0.01 mol / L. This satisfies both the requirement of a distinct reddish-brown color at the titration endpoint and ensures accurate burette readings. When the concentration of added ions is on the order of 0.001 mol / L, the concentration of added ions must be an integer multiple of 0.00141 to ensure that the burette reading is closer to the true value of the volume of titrant consumed.

[0227] Second, ensure that the minimum and maximum values ​​of the chloride ion content to be measured are within 20% to 80% of the measurement range;

[0228] (1) The chloride ion content range of common cement, mineral powder, and fly ash in the batching plant was determined to be 0.006% to 0.080% (i.e., 5g of sample contains Cl). - (Metal content: 0.3–4 mg; Molar content: 0.008463–0.112835 mmol);

[0229] (2) To ensure that the minimum and maximum values ​​of the chloride ion content to be measured account for 20% and 80% of the measurement range, respectively, assuming an external Cl... - If the number of moles is X, then X = 0.026328 mmol. Based on the standard pipette sizes (1, 2, 5, 10, 15, 25, 50 mL), the standard concentrations of NaCl standard solutions (0.1, 0.01, 0.0141, 0.02, 0.05 mol / L), and ensuring that the chloride ion concentration is an integer multiple of 0.0141, the added Cl... - If the volume is 2 mL and the concentration is 0.0141 mol / L, then add Cl... - Afterwards, the Cl to be tested - The minimum and maximum values ​​are 0.008463 + 0.0282 = 0.036663 mmol and 0.112835 + 0.0282 = 0.141035 mmol, respectively. Therefore, the maximum range (plus Ag) is... + The molar number is 0.141035 / 80% = 0.176294 mmol. Based on the standard pipette specifications (1, 2, 5, 10, 15, 25, 50 mL), standard volumetric pipette specifications (range 2, 5, 10, 15, 25, 50 mL, accuracy 1 mL), standard concentrations of silver nitrate standard solution (0.1, 0.01, 0.0141, 0.02, 0.05 mol / L), and ensuring that the silver ion concentration is an integer multiple of 0.0141, Ag is selected for external addition. + If the concentration is 0.0141 mol / L, then the volume is 13 mL, and Ag is added externally. + Then, the maximum range (plus Ag) + The molar number is 0.0141 × 13 = 0.1833 mmol, verifying the detection range (molar number of Cl- to be measured) as a percentage of the range (added Ag). + The percentages of 0.036663 / 0.1833 (in moles) are 20.00% and 76.94% respectively.

[0230] (3) Although in 99% of cases, the chloride ion content in cement, mineral powder, and fly ash ranges from 0.006% to 0.080% (i.e., 5g of sample contains Cl...). -The mass range is 0.3–4 mg, and the molar number ranges from 0.008463–0.112835 mmol. However, to ensure full coverage of chloride ion detection, two additional cases were added: less than 0.3 mg and more than 4 mg. The case of less than 0.3 mg was designed as 0–0.3 mg, and the case of more than 4 mg was designed as 4–Max. Max refers to the maximum chloride ion content determined by the ammonium thiocyanate titration method in GB / T 176-2017. The method for obtaining Max is as follows: GB / T 176-2017 stipulates that "add 5 mL of AgNO3 (0.05 mol / L) to 5 g of sample, and titrate the remaining Ag with ammonium thiocyanate (0.05 mol / L)". + When V NH4SCN "If the sample mass is less than 0.5 mL, the test should be repeated with half the sample mass." Therefore, the maximum mass of Cl- to be tested is 0.05 × 4.5 × 35.45 × 2 = 15.9525 mg.

[0231] (4) 0–0.3 mg (0–0.008463 mmol), so that the minimum and maximum values ​​of the chloride ion content to be measured account for 20% and 80% of the range, respectively. Assume that an external Cl is added. - If the number of moles is X, then Given X = 0.002821 mmol, and based on the standard pipette sizes (1, 2, 5, 10, 15, 25, 50 mL), the standard concentrations of NaCl standard solutions (0.1, 0.01, 0.0141, 0.02, 0.05 mol / L), and ensuring that the chloride ion concentration is an integer multiple of 0.00141, the added Cl... - If the volume is 2 mL and the concentration is 0.00141 mol / L, then add Cl... - Afterwards, the Cl to be tested - The minimum and maximum values ​​are 0 + 0.00282 = 0.00282 mmol and 0.008463 + 0.00282 = 0.011283 mmol, respectively. Therefore, the maximum range (plus Ag) is... + The molar number is 0.011283 / 80% = 0.014104 mmol. Based on the standard pipette specifications (1, 2, 5, 10, 15, 25, 50 mL), standard volumetric pipette specifications (range 2, 5, 10, 15, 25, 50 mL, accuracy 1 mL), standard concentrations of silver nitrate standard solution (0.1, 0.01, 0.0141, 0.02, 0.05 mol / L), and ensuring that the silver ion concentration is an integer multiple of 0.00141, the added Ag is selected. + If the volume is 1 mL and the concentration is 0.0141 mol / L, then add Ag. + Then, the maximum range (plus Ag) +The molar number is 0.0141 × 1 = 0.0141 mmol, verifying the detection range (molar number of Cl- to be measured) as a percentage of the range (added Ag). + The percentages of 0.00282 / 0.0141 (based on the number of moles) are 20.00% and 80.02% respectively.

[0232] (5) 4~15.9525mg (0.112835~0.450000mmol), because 15.9525mg is 3.988 times (less than 4 times) of 4mg, the minimum and maximum values ​​of the chloride ion content to be measured are within 20% to 80% of the range, so no additional NaCl is needed, while the maximum range (addition of Ag) + The molar number is 0.450000 / 80% = 0.562500 mmol. Based on the standard pipette specifications (1, 2, 5, 10, 15, 25, 50 mL), standard volumetric pipette specifications (range 2, 5, 10, 15, 25, 50 mL, accuracy 1 mL), standard concentrations of silver nitrate standard solution (0.1, 0.01, 0.0141, 0.02, 0.05 mol / L), and ensuring that the silver ion concentration is an integer multiple of 0.0141, Ag is selected for external addition. + If the concentration is 0.141 mol / L and the volume is 4 mL, then Ag is added externally. + Then, the maximum range (plus Ag) + The molar number is 0.141 × 4 = 0.564 mmol, verifying the detection range (molar number of Cl- to be measured) as a percentage of the range (added Ag). + The percentages of 0.112835 / 0.564 (in terms of moles) are 20.01% and 79.79% respectively.

[0233] (6) Determine Cl - Mass method: Since the concentration is 0.3–4 mg in 99% of cases, the default is 0.3–4 mg. Add 2 mL of NaCl (0.0141 mol / L) and 13 mL of AgNO3 (0.0141 mol / L). If the Cl to be measured is… - If it is 0.3mg, then

[0234]

[0235] If the Cl to be tested - If it is 4mg, then

[0236]

[0237] Therefore, there are three possible scenarios:

[0238] Case 1: If V NH4SCNIf the concentration is between 0.75 and 2.60 mL, it indicates that the Cl- concentration is between 0.3 and 4 mg.

[0239] Case 2: If V NH4SCN >2.60mL, then Cl - <0.3mg, i.e. Cl - If the concentration is between 0 and 0.3 mg, repeat the process, adding 2 mL of NaCl (0.00141 mol / L) and 1 mL of AgNO3 (0.0141 mol / L).

[0240] Case 3: If V NH4SCN If the concentration is less than 0.75 mL, it indicates that Cl... - >4mg, default Cl - For doses between 4 and 15.9525 mg, repeat the process, but without adding NaCl, and instead add 0.141 mol / L AgNO3 × 4 mL.

[0241] The concentration of chloride ions is controlled to be 20%–80% of the molar concentration of silver ions because a lower residual silver ion concentration reduces the adsorption of silver ions by silver thiocyanate precipitate, thus improving titration accuracy. Conversely, if the silver ion concentration is too low, there will be insufficient silver ions to react with ammonium thiocyanate after the chloride ion titration. Controlling the chloride ion concentration to 20%–80% of the molar concentration of silver ions has been experimentally verified to be the most suitable. - / Ag + The ratio. Controlling the molar number of chloride ions to 20%–80% of the molar number of silver ions can improve detection accuracy because the surface adsorption of the precipitate is caused by the imbalance of the force field of the crystal-forming ions on the precipitate surface. Taking AgCl precipitate as an example, each Ag ion inside the crystal... + Six Cl atoms are arranged around the ion. - Ions, each Cl - Six Ag atoms are also arranged around the ion. + The ions are in a state of equilibrium with the force field. Each Ag atom on the crystal surface... + (or Cl) - The ions are adjacent to only five opposing charged constituent ions, resulting in a force field imbalance. This imbalance is even more pronounced at crystal edges and corners. Constituent ions with imbalanced force fields have the ability to adsorb oppositely charged particles. For example, the surface of AgCl precipitate can adsorb excess Cl- from the precipitant (NaCl) brought into the solution. - Ions form an adsorption layer.

[0242] AgSCN precipitation titration uses SCN - Titration of Ag + In Ag + The moment it was exhausted, SCN - with Fe 3+Red complex ions are formed. AgSCN precipitate surfaces adsorb Ag from solution. + These Ag + Waiting for SCN - The arrival of [a substance] causes new AgSCN precipitate to form on the AgSCN surface. The AgSCN precipitate will fall off like snowflakes, one by one. Vigorously shaking the conical flask is to allow the Ag on the surface of the AgSCN precipitate to [become more precipitated]. + Don't be affected by the newly generated Ag + The precipitate covers it, and the Ag on the surface of the AgSCN precipitate is... + The positive charge is partially neutralized by the AgSCN precipitate, therefore there are no free Ag in the solution. + It can attract SCN, so AgSCN precipitates Ag on the surface. * Some Ag were encapsulated before they could react to the SCN. + This will cause titration errors.

[0243] If the entire process of adding SCN is followed by Ag + If the concentration is low, fewer Ag+ ions will be adsorbed on the surface of the AgSCN precipitate, and these Ag+ ions will be trapped and unable to react with SCN. + Therefore, the amount of titration is less, and the titration error will also be smaller.

[0244] III. Cl - Correction of content calculation formula

[0245] (1) Section 6.13.3 of GB / T 1762017, Ammonium Thiocyanate Titration Method for the Determination of Cl - The formula for calculating the content is:

[0246]

[0247] Where 5.00 represents the volume of silver nitrate in mL; 1.773 represents the titer of the silver nitrate standard solution for chloride ions in milligrams per milliliter (mg / mL), indicating that 1 mL of silver nitrate is equivalent to 1.773 mg of chloride ions. The formula for calculating the titer is as follows: In the formula, B is the titrant, A is the analyte, T is the titer (mg / mL), m is the mass (mg), and V is the volume (mL). Therefore... And because n = CV, and n Ag+ =n Cl- (Where n is the number of moles, m is the mass, M is the molar mass, C is the molar concentration, and V is the volume), therefore That is, 1.773 = C Ag+ ×M Cl- 5.00 = V AgNO3,so

[0248]

[0249] Multiplying both numerator and denominator by C NH4SCN ×10 -3 ,have to

[0250]

[0251] Because n = CV, where n is the number of moles (mol), C is the concentration (mol / L), and V is the volume (L), therefore

[0252]

[0253] GB / T 1762017 Ammonium Thiocyanate Titration Method for the Determination of Cl - The reaction equation is:

[0254] Cl - +Ag + =AgCl↓

[0255] Remaining Ag + +SCN=AgSCN↓

[0256] have

[0257] n Ag+ =n SCN +n Cl- +n0 ⑧

[0258] Where, n Ag+ For Ag + The number of moles, n SCN- For SCN - The number of moles, n Cl- For Cl - The number of moles, n0 is the number of moles of impurity ions, when the impurity ion is an anion such as Cl. - When n0 is positive, and the impurity ion is a cation such as Ag, n0 is positive. + When n0 is negative, n SCN-空白 This represents the number of moles of SCN in the blank experiment.

[0259] The blank experiment has the following reaction equation:

[0260] Remaining Ag + +SCN=AgSCN↓

[0261] have

[0262] n Ag+ =n SCN空白 +n0 ⑨

[0263] n in equation ⑧0+ With n in equation ⑨ 0+ They are equal because the impurity ions in the titration system are the same. The impurity ions in the sample to be tested are those that need to be tested and have been removed through steps such as dissolving in nitric acid, boiling, filtering, and washing. Therefore, from equations ⑦ and ⑧, it can be seen that...

[0264] n SCN-空白 -n SCN- =n Cl- ⑩

[0265] Substituting equation ⑩ into equation ⑦, we get...

[0266]

[0267] Because M = m / n, where m is the mass (g), n is the number of moles (mol), and M is the molar mass (g / mol). Therefore

[0268] The formula Enter so

[0269]

[0270] (2) Because Therefore, GB / T 176-2017 believes that but It is required that n0 << n Ag+ This is because, as can be seen from equation 9, n Ag+ =n SCN-空白 +n0, where n0 is the number of moles of impurity ions in the titration system. To determine the molar number of impurity ions in the titration system... Ag+ ≈n SCN空白 It is necessary that n0 << n Ag+ The definition of "much less than" is that if |b±a| / b≈1, then it means a<<b. The error is determined by specific requirements. For example, in physics, the experimental error is generally required to be less than 0.1%. Therefore, when a<0.1%b, it can be considered that a<<b, that is, b±a≈b.

[0271] (3) However, an error of 0.1% is quite stringent. The Cl- content in cement was determined using the method of GB / T 1762017 (ammonium thiocyanate titration method), i.e., AgNO3 (5 mL × 0.05 mol / L) was added, and titrated with NH4SCN (0.05 mol / L). 5.0827 g of cement was weighed. Ag+ The effect of the ratio on the relative error of chloride ion content is shown in Table 17.

[0272] Table 17 n0 / n Ag+ Effect of ratio on relative error of chloride ion content

[0273]

[0274]

[0275] Note: n0 is the number of moles of impurity ions in the titration system, n Ag+ Adding Ag + The number of moles, V NH4SCN空白 V represents the volume of NH4SCN consumed in the blank experiment. NH4SCN W represents the volume of NH4SCN consumed in the experiment. Cl- This represents the mass percentage of chloride ions in the sample.

[0276] As shown in Table 17, |n0 / n Ag+ When the chloride ion concentration is ≤2%, the relative error of the chloride ion content is within 2%, and the chloride ion content (%) is 0.0785±0.0016, indicating a small measurement error. Therefore, the calculation formula for determining chloride ions by the ammonium thiocyanate titration method in GB / T 176-2017 should be revised to...

[0277] When |n0 / n Ag+ When the value is ≤2%,

[0278]

[0279] The formula does not require the concentration of ammonium thiocyanate, and there is no need to calibrate NH4SCN;

[0280] When |n0 / n Ag+ When the value is greater than 2%,

[0281]

[0282] The concentration of ammonium thiocyanate used in the formula needs to be specified using NH4SCN;

[0283] (4) This invention adds Cl - ,

[0284] When |n0 / n Ag+ When the value is ≤2%,

[0285]

[0286] Still using C NH4SCN If NH4SCN still needs to be calibrated, then the formula can be used directly.

[0287]

[0288] (5) Therefore, the formula for calculating chloride ion content is revised as follows:

[0289] The test detects the chloride ion content in cement, mineral powder, and fly ash. The default value is Cl. - At a concentration of 0.3–4 mg, add 2 mL of 0.0141 mol / L NaCl and 13 mL of 0.0141 mol / L AgNO3, and measure V. NH4SCN There are three situations:

[0290] Case 1: If V NH4SCN If the concentration is between 0.75 and 2.60 mL, it indicates that Cl... - The chloride ion content is between 0.3 and 4 mg; the formula for calculating chloride ion content is:

[0291]

[0292] Case 2: If V NH4SCN >2.60mL indicates Cl - <0.3mg, i.e., Cl is between 0 and 0.3mg, repeat the test, adding 0.00141mol / L NaCl × 2mL and 0.0141mol / L AgNO3 × 1mL; Chloride ion content calculation formula:

[0293]

[0294] Case 3: If V NH4SCN <0.75mL indicates Cl - >4mg, default Cl - For concentrations between 4 and 15.9525 mg, repeat the test, without adding NaCl, but with 0.141 mol / L AgNO3 × 4 mL; the formula for calculating chloride ion content has two cases:

[0295] Case 3-1: If |C AgNO3 V AgNO3 -C NH4SCN V NH4SCN空白 / (C AgNO3 V AgNO3 )≤2%, that is, |n0 / n Ag+ ≤2%, formula for calculating chloride ion content:

[0296]

[0297] Case 3-2: If |C AgNO3 V AgNO3 -C NH4SCN V NH4SCN空白 / (C AgNO3 V AgNO3 >2%, that is, |n0 / n Ag+ Formula for calculating chloride ion content >2%:

[0298]

[0299] Of the four scenarios mentioned above (Scenario 1, Scenario 2, Scenario 3-1, Scenario 3-2), only Scenario 3-1 does not require calibration of the concentration of the ammonium thiocyanate indicator solution; the other three scenarios require calibration of the concentration of the ammonium thiocyanate indicator solution.

[0300] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them; although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications can still be made to the specific implementation of the invention or equivalent substitutions can be made to some technical features without departing from the spirit of the technical solutions of the present invention, and all such modifications and substitutions should be covered within the scope of the technical solutions claimed in the present invention.

Claims

1. A method for detecting chloride ions in cement, characterized in that, The method specifically includes the following steps: S1. Weigh 5g of cement sample, accurate to 0.0001g, and place the weighed sample in a beaker. S2, add 50mL of distilled water or deionized water to the beaker and stir well. While stirring, add 50mL of nitric acid (1+2) to the beaker, then heat to a boil and simmer for 1-2 minutes to obtain the sample solution. S3. Add a certain volume of sodium chloride solution to the beaker, then add a certain volume of silver nitrate solution and stir well, then boil for 1-2 minutes. The determination of the volume and concentration of the sodium chloride solution and silver nitrate solution added to the beaker in step S3 is as follows: S31. Add 2 mL of 0.0141 mol / L sodium chloride standard solution and 13 mL of 0.0141 mol / L silver nitrate standard solution to a beaker. S32, perform the titration of ammonium thiocyanate according to steps S4 and S5, and measure V. NH4SCN ; S33, when the measured V NH4SCN If the value is less than 0.75 mL, it indicates that the chloride ion content in the cement sample is greater than 4 mg. Weigh the cement sample again and repeat steps S1 and S2 to obtain the sample solution. Add 4 mL of 0.141 mol / L silver nitrate standard solution to the sample solution, without adding sodium chloride solution, and repeat steps S4 and S5 to measure the chloride ion content of the cement sample. S35, when the measured V NH4SCN If the value is greater than 2.60 mL, it indicates that the chloride ion content in the cement sample is less than 0.3 mg. Weigh the cement sample again and repeat steps S1 and S2 to obtain the sample solution. Add 2 mL of 0.00141 mol / L sodium chloride standard solution and 1 mL of 0.141 mol / L silver nitrate standard solution to the sample solution, repeat steps S4 and S5, and measure the chloride ion content of the cement sample. S36, when the measured V NH4SCN If the chloride ion content in the cement sample is greater than or equal to 0.75 mL and less than or equal to 2.60 mL, it indicates that the chloride ion content in the cement sample is between 0.3 mg and 4 mg. The cement sample is not weighed again, and the chloride ion content of the cement sample is measured. S4. To prepare the filtrate, add a small amount of filter paper pulp to the beaker. Take an Erlenmeyer flask, a funnel, a glass rod, and a rapid filter paper that has been pre-washed with nitric acid (1+100). Attach the rapid filter paper to the funnel and the bottom of the glass rod to the filter paper. Tilt the beaker so that the mouth of the beaker is attached to the top of the glass rod. Pour the solution from the beaker into the Erlenmeyer flask. The solution flows along the glass rod, filter paper, and funnel neck into the Erlenmeyer flask. Wash the beaker and stirring equipment with nitric acid (1+100) until the solution in the Erlenmeyer flask reaches 200 mL. Place the beaker in a dim light or dark place and cool the solution to below 25°C. S5. After cooling, add 5 mL of ferric ammonium sulfate indicator solution to the conical flask. Titrate the excess silver ions in the conical flask with ammonium thiocyanate standard solution until the solution turns reddish-brown and the color does not disappear when the conical flask is shaken. Stop the titration and record the volume V of ammonium thiocyanate consumed at this time. NH4SCN ; S6, without adding cement sample, repeat S2-S5 to perform blank tests, and record the volume V of ammonium thiocyanate used in the blank test titration. NH4SCN空白 The chloride ion content of the cement sample was calculated.

2. The method for detecting chloride ions in cement according to claim 1, characterized in that, The specific method for measuring the chloride ion content of the cement sample is as follows: When the chloride ion content of a cement sample is between 0-4 mg, the chloride ion content of the cement sample is: ， When the chloride ion content of the cement sample is between 4 and 15.9525 mg, and |n AgNO3 -n NH4SCN空白 | / n AgNO3 ≤2%, |C AgNO3 V AgNO3 -C NH4SCN V NH4SCN空白 | / (C AgNO3 V AgNO3 When the chloride ion content is ≤2%, the chloride ion content of the cement sample is: ， When the chloride ion content of the cement sample is between 4 and 15.9525 mg, and |n AgNO3 -n NH4SCN空白 | / n AgNO3 >2%, |C AgNO3 V AgNO3 -C NH4SCN V NH4SCN空白 | / (C AgNO3 V AgNO3 When the chloride ion content is greater than 2%, the chloride ion content of the cement sample is: ； in, This indicates the percentage of chloride ions by mass relative to the mass of the cement sample, expressed as %. This indicates the concentration of the ammonium thiocyanate standard solution, expressed in mol / L. This indicates the volume of ammonium thiocyanate solution consumed in the blank experiment, in mL. This indicates the volume of ammonium thiocyanate solution consumed by the cement sample, in mL. This indicates the mass of the cement sample, expressed in grams. This indicates the concentration of the added silver nitrate standard solution, expressed in mol / L. This indicates the volume of added silver nitrate standard solution, in mL.

3. The method for detecting chloride ions in cement according to claim 1, characterized in that, The concentration of the ammonium thiocyanate standard solution used in S5 is 0.0564 mol / L. If the chloride ion content of the cement sample is between 4 and 15.9525 mg and |n AgNO3 -n NH4SCN空白 | / n AgNO3 For samples with a chloride ion content >2%, or cement samples with a chloride ion content between 0-4 mg, a calibration measurement should be performed before titration with ammonium thiocyanate standard solution. Specifically: Measure 25 mL of calibrated silver nitrate standard solution (concentration 0.0564 mol / L) into a 250 mL Erlenmeyer flask. Add 50 mL of distilled or deionized water to the flask and stir well. Then add 2 mL of nitric acid (1+2) and 1 mL of ferric ammonium sulfate solution to the flask. Titrate the silver nitrate solution with the ammonium thiocyanate standard solution to be calibrated. Titrate until the solution in the flask turns reddish-brown. After shaking the flask, if the color does not fade, the titration is complete. Record the volume of ammonium thiocyanate consumed during the titration. And calculate the concentration of the ammonium thiocyanate solution, specifically: , where C NH4SCN C represents the concentration of the ammonium thiocyanate solution. AgNO3 V represents the concentration of the silver nitrate solution. AgNO3 V is the volume of silver nitrate solution added to the conical flask. NH4SCN This represents the volume of ammonium thiocyanate solution consumed.

4. The method for detecting chloride ions in cement according to claim 1, characterized in that, The specific preparation of filter paper pulp in S4 is as follows: tear a quantitative amount of filter paper into pieces and put it into a beaker, add water to submerge it, and heat it to a boil while stirring for 10 minutes. Then cool it to room temperature for later use.

5. The method for detecting chloride ions in cement according to claim 1, characterized in that, The specific steps for preparing the ferric ammonium sulfate indicator solution are as follows: Add 100 mL of saturated aqueous solution of ferric ammonium sulfate dodecahydrate [NH4Fe(SO4)2·12H2O] to 10 mL of nitric acid (1+2) and stir until homogeneous to obtain the ferric ammonium sulfate indicator solution.

6. The method for detecting chloride ions in cement according to claim 1, characterized in that, The concentration of the ammonium thiocyanate standard solution is 0.0564 mol / L. The preparation method of the ammonium thiocyanate standard solution is as follows: weigh 4.2932 g of ammonium thiocyanate (NH4SCN), dissolve it in distilled water or deionized water, and dilute it to 1 L to obtain the ammonium thiocyanate standard solution.

7. The method for detecting chloride ions in cement according to claim 1, characterized in that, The specific steps for preparing the sodium chloride solution are as follows: Sodium chloride is placed in a high-temperature furnace and ignited for 40-50 minutes until constant weight is achieved. 0.8429g of the ignited sodium chloride is weighed and dissolved in water in a 1L volumetric flask. Diluent is added according to the required concentration of the prepared sodium chloride solution to complete the preparation of the sodium chloride solution. The temperature in the high-temperature furnace is 550±50℃.

8. The method for detecting chloride ions in cement according to claim 7, characterized in that, The sodium chloride is heated to constant weight in a high-temperature furnace as follows: The first step is to place sodium chloride in a high-temperature furnace and ignite it for 15 minutes, then cool and weigh the ignited sodium chloride. The second step is to repeat the first step, subjecting the sodium chloride to multiple high-temperature incinerations, cooling, and weighing. When the mass difference between two consecutive weighings is less than 0.0005g, the sodium chloride reaches a constant weight.

9. The method for detecting chloride ions in cement according to claim 1, characterized in that, The calculated chloride ion content should be rounded. If a burette with an accuracy of 0.05 mL is used, it should be rounded to 0.002%; if a burette with an accuracy of 0.01 mL is used, it should be rounded to 0.0004%.

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