Accurate determination method of ammonia nitrogen in complex water bodies

Abstract

The present invention provides a method for accurately determining ammonia nitrogen in complex water bodies. First, a suitable amount of ethylenediaminetetraacetic acid, silver nitrate, potassium sodium tartrate and an indicator are added to a complex water sample, and a heating distillation is performed, and then a suitable amount of sodium thiosulfate and an indicator are added for secondary heating distillation; by preparing ammonia nitrogen standard solutions with different ammonia nitrogen contents and combining Nessler's reagent to determine absorbance, a standard curve of mass and absorbance is drawn, and finally the sample to be detected after secondary distillation is tested by the same method, the absorbance is determined, and the corresponding ammonia nitrogen content is obtained from the standard curve. The present invention selects a suitable masking agent to cooperate with a secondary distillation process to mask the interference of urea, volatile amines, chloramines, sulfides, heavy metals, etc. contained in the complex water sample on the determination of ammonia nitrogen, and selects a suitable mixed absorption liquid to achieve accurate determination of ammonia nitrogen in complex water bodies. The method has good precision and accuracy and high application value.

Description

Technical Field

[0001] The present invention relates to the technical field of ammonia nitrogen detection in water bodies, and in particular to a method for accurately determining ammonia nitrogen in complex water bodies. Background Art

[0002] Ammonia nitrogen usually refers to the free ammonia (NH3) and ammonium salt (NH 4+ ) in the form of nitrogen. These nitrogen sources are mainly caused by industrial waste, industrial wastewater, urban domestic sewage, and water generated by agricultural activities. After these nitrogen sources enter the water body, they will cause eutrophication of the water environment and affect the water quality and the balance of the aquatic ecosystem. Therefore, ammonia nitrogen is regarded as an important indicator in water environment pollution and ecological protection. Ammonia nitrogen is a common item in water quality testing. The determination of various forms of nitrogen compounds in water is helpful to evaluate water pollution and the "self-purification status" of water bodies. Therefore, it is very important to accurately determine the content of ammonia nitrogen in water quality.

[0003] At present, the main detection methods of ammonia nitrogen in water include Nessler reagent spectrophotometry, salicylic acid spectrophotometry, gas phase molecular absorption spectroscopy, distillation-neutralization titration, flow injection method, ion chromatography and electrode method. Among them, Nessler reagent spectrophotometry is widely used due to its convenience, short process, high efficiency and high sensitivity. A certain existing invention patent discloses a method for determining the ammonia nitrogen content in water by Nessler reagent spectrophotometry, comprising the following steps: (1) taking the original water sample to be analyzed; (2) pre-treating the original water sample; (3) taking the pre-treated water sample in a colorimetric tube, shaking it well, adding Nessler reagent to the colorimetric tube and shaking it well again, and after standing for a certain period of time, using a spectrophotometer to test the absorbance of the water sample; (4) using distilled water to perform a blank test to obtain a blank absorbance; (5) subtracting the blank absorbance measured in step (4) from the absorbance measured in step (3) to obtain a corrected absorbance, drawing a standard curve, and calculating the ammonia nitrogen content in water. Although this method avoids the interference of suspended matter and residual chlorine on the test results by pre-treating the original water samples and improves the test results, it is unable to detect water bodies polluted by industrial waste, industrial wastewater, urban domestic sewage, etc. The polluted water bodies often contain one or more interfering elements such as urea, volatile amines, chloramines, sulfides, heavy metals, etc. These substances bring great difficulties to the analysis of ammonia nitrogen in water bodies.

[0004] In view of this, it is necessary to design an improved method for accurate determination of ammonia nitrogen in complex water bodies to solve the above problems. Summary of the invention

[0005] The purpose of the present invention is to provide a method for accurately determining ammonia nitrogen in complex water bodies, select a suitable masking agent and cooperate with a secondary distillation process to mask the interference of urea, volatile amines, chloramines, sulfides, heavy metals, etc. contained in complex water samples on the determination of ammonia nitrogen; and select boric acid and sulfuric acid as mixed absorption liquids to solve the problem of low liquid measurement values ​​after the sample is distilled, so as to achieve accurate determination of ammonia nitrogen in complex water bodies.

[0006] To achieve the above-mentioned object of the invention, the present invention provides a method for accurately determining ammonia nitrogen in complex water bodies, comprising the following steps:

[0007] S1. Take a certain amount of complex water sample and place it in a distillation bottle, add appropriate amount of ethylenediaminetetraacetic acid solution, silver nitrate solution and potassium sodium tartrate solution, shake well and let it stand;

[0008] S2, placing a certain amount of mixed absorption liquid in a receiving bottle, and ensuring that the outlet of the condenser connecting the distillation flask and the receiving bottle is below the liquid level of the mixed absorption liquid;

[0009] S3, drip bromothymol blue indicator into the distillation flask, connect a condenser to carry out a heating distillation, obtain a distillate, after the distillation is completed, transfer the distillate in the receiving bottle to a new distillation flask, add an appropriate amount of sodium thiosulfate solution, and drip bromothymol blue indicator to carry out secondary heating distillation to obtain a secondary distillate; the concentration of the sodium thiosulfate solution is 4-6 g / L;

[0010] S4, using sodium hydroxide solution to adjust the secondary distillate obtained in step S3 to a pH of 6.0-7.0, to obtain a sample to be tested;

[0011] S5. Prepare ammonia nitrogen standard solutions with different ammonia nitrogen contents, place them in colorimetric tubes respectively, add potassium sodium tartrate and shake well, then add Nessler's reagent and shake well and let stand, use a blank tube with zero concentration as a reference, measure the absorbance, and draw a standard curve of mass and absorbance;

[0012] S6. Place the sample to be tested obtained in step S4 in a colorimetric tube, and perform testing in the same manner as in step S5 to measure the absorbance, and obtain the corresponding ammonia nitrogen content from the standard curve of step S5.

[0013] As a further improvement of the present invention, in step S2, the mixed absorption liquid is a mixed solution of boric acid solution and sulfuric acid solution, wherein the concentration of the boric acid solution is 20~30 g / L, the concentration of the sulfuric acid solution is 0.0002~0.0006 mol / L, and the volume ratio of the two is (3.5~4.5):1.

[0014] As a further improvement of the present invention, in step S3, during the first heating distillation and the second heating distillation, after the bromothymol blue indicator is added dropwise, the liquid pH is adjusted to 6.0-7.4 using sodium hydroxide solution or hydrochloric acid solution, and 0.2-0.3 g of light magnesium oxide and a few glass beads are added; the concentration of the bromothymol blue indicator is 0.4-0.6 g / L.

[0015] As a further improvement of the present invention, in step S3, the amount of the sodium thiosulfate solution added is the same as the amount of the silver nitrate solution added in step S1, and the distillate rate of the first heating distillation and the second heating distillation is 8-12 mL / min.

[0016] As a further improvement of the present invention, in step S1, the amount of the complex water sample is 250~280 mL, in step S2, the amount of the mixed absorption liquid added is 50~60 mL, in step S3, the amount of the first distillate is 200~220 mL, and the first heating distillation is stopped; the amount of the second distillate is 180~200 mL, and the second heating distillation is stopped.

[0017] As a further improvement of the present invention, in step S1, the amount of the ethylenediaminetetraacetic acid solution added is 5-8 mL, and the volume ratio of the added amounts of the ethylenediaminetetraacetic acid solution, the silver nitrate solution and the potassium sodium tartrate solution is 1:1:(0.3-0.5); the concentration of the ethylenediaminetetraacetic acid solution is 80-120 g / L, the concentration of the silver nitrate solution is 0.1-0.3 mol / L, and the solubility of the potassium sodium tartrate is 180-200 g / L.

[0018] As a further improvement of the present invention, in step S6, the calculation formula of the ammonia nitrogen content is:

[0019]

[0020] Where: C ——Concentration of ammonia nitrogen, mg / L;

[0021] m ——The mass of ammonia nitrogen is found from the calibration curve, μg;

[0022] m 0 ——The mass of the blank sample is found from the calibration curve, μg;

[0023] V ——The volume of the sample taken during color development, mL;

[0024] d ——The dilution factor of the sample is equal to the sample fixed volume divided by the sample sampling volume.

[0025] As a further improvement of the present invention, in step S5, the amount of potassium sodium tartrate added is 0.03 to 0.05 times the volume of the ammonia nitrogen standard solution, and the volume ratio of the amount of Nessler's reagent added to the amount of potassium sodium tartrate added is 1:(1.5 to 2.5).

[0026] As a further improvement of the present invention, in step S5, the ammonia nitrogen standard solutions with different ammonia nitrogen contents include standard solutions with ammonia nitrogen contents of 0.0 μg, 5.0 μg, 10.0 μg, 20.0 μg, 40.0 μg, 60.0 μg, 80.0 μg, and 100.0 μg, respectively.

[0027] As a further improvement of the present invention, the preparation method of the Nessler's reagent is: weigh 16.0 g of sodium hydroxide, dissolve it in 50 mL of water, and cool it to room temperature to obtain a sodium hydroxide solution; weigh 7.0 g of potassium iodide and 10.0 g of mercuric iodide, dissolve them in water, slowly add the solution to the sodium hydroxide solution under stirring, and dilute it with water to 100 mL to obtain the Nessler's reagent.

[0028] The beneficial effects of the present invention are:

[0029] 1. The invention discloses an accurate determination method for ammonia nitrogen in complex water bodies. The method adopts ethylenediaminetetraacetic acid to complex most heavy metal ions, eliminates the interference of heavy metal ions in the water body, and adopts silver nitrate to mask chloramine and sulfide contained in the sample, eliminates the influence of chloramine and sulfide on the determination of ammonia nitrogen. At the same time, the method cleverly adopts a secondary distillation process combined with the addition of a masking agent sodium thiosulfate to mask the interference of urea and volatile amines contained in the complex water sample on the determination of ammonia nitrogen. Sodium thiosulfate cannot be introduced to mask the interference of urea and volatile amines during the first distillation because the presence of other reagents will destroy the masking effect of sodium thiosulfate. Therefore, the invention adopts a combination of secondary distillation and a masking agent to solve the interference of impurities in the complex water body on the determination of ammonia nitrogen and improve the accuracy of the determination of ammonia nitrogen.

[0030] 2. The present invention selects a suitable masking agent in combination with a secondary distillation process to mask the interference of urea, volatile amines, chloramines, sulfides, heavy metals, etc. contained in complex water samples on the determination of ammonia nitrogen, and selects boric acid and sulfuric acid in a certain volume ratio as a mixed absorption liquid, which solves the problem of low measurement values ​​when boric acid or sulfuric acid is used alone as an absorption liquid after the sample is distilled, and realizes the accurate determination of ammonia nitrogen in complex water bodies. This method has good precision and accuracy and high application value. DETAILED DESCRIPTION

[0031] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is described in detail below in conjunction with specific embodiments.

[0032] It should also be noted that, in order to avoid obscuring the present invention due to unnecessary details, only structures and / or processing steps closely related to the scheme of the present invention are shown in the embodiments, while other details that are not closely related to the present invention are omitted.

[0033] In addition, it should be noted that the terms "comprises", "includes" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article or apparatus that includes a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or apparatus.

[0034] A method for accurately determining ammonia nitrogen in complex water bodies comprises the following steps:

[0035] S1. Take a certain amount of complex water sample and place it in a distillation bottle, add appropriate amount of ethylenediaminetetraacetic acid solution, silver nitrate solution and potassium sodium tartrate solution, shake well and let it stand; use ethylenediaminetetraacetic acid to complex most of the heavy metal ions to eliminate the interference of heavy metal ions in the water, use silver nitrate to mask the chloramine and sulfide contained in the sample, and eliminate the influence of chloramine and sulfide on the determination of ammonia nitrogen;

[0036] S2. Place a certain amount of mixed absorption liquid in the receiving bottle, and ensure that the outlet of the condenser connecting the distillation flask and the receiving bottle is below the liquid level of the mixed absorption liquid;

[0037] S3, drop bromothymol blue indicator into the distillation flask, connect the condenser to perform a heating distillation to obtain a distillate, after the distillation is completed, transfer the distillate in the receiving bottle to a new distillation flask, add an appropriate amount of sodium thiosulfate solution, and drop bromothymol blue indicator to perform a second heating distillation to obtain a secondary distillate; the concentration of the sodium thiosulfate solution is 4-6 g / L; the secondary distillation process is combined with the addition of a masking agent sodium thiosulfate to mask the interference of urea and volatile amines contained in the complex water sample on the determination of ammonia nitrogen. Sodium thiosulfate cannot be introduced during the first distillation to mask the interference of urea and volatile amines because the presence of other reagents will destroy the masking effect of sodium thiosulfate. Therefore, the scheme adopts a combination of secondary distillation and masking agents to solve the interference of impurities in complex water bodies on the determination of ammonia nitrogen and improve the accuracy of ammonia nitrogen determination;

[0038] S4, using sodium hydroxide solution to adjust the secondary distillate obtained in step S3 to a pH of 6.0-7.0, to obtain a sample to be tested;

[0039] S5. Prepare ammonia nitrogen standard solutions with different ammonia nitrogen contents, place them in colorimetric tubes respectively, add potassium sodium tartrate and shake well, then add Nessler's reagent and shake well and let stand, use a blank tube with zero concentration as reference, measure the absorbance, and draw a standard curve of mass and absorbance;

[0040] S6. Place the sample to be tested obtained in step S4 in a colorimetric tube, and perform testing in the same manner as in step S5 to measure the absorbance, and obtain the corresponding ammonia nitrogen content from the standard curve of step S5.

[0041] Specifically, in step S2, the mixed absorption liquid is a mixed solution of boric acid solution and sulfuric acid solution, wherein the concentration of the boric acid solution is 20-30 g / L, the concentration of the sulfuric acid solution is 0.0002-0.0006 mol / L, and the volume ratio of the two is (3.5-4.5): 1. By selecting a certain volume ratio of boric acid and sulfuric acid as the mixed absorption liquid, the problem of low measured values ​​when using boric acid or sulfuric acid alone as the absorption liquid after the sample is distilled is solved, and the accurate determination of ammonia nitrogen in complex water bodies is achieved.

[0042] In step S3, during the first heating distillation and the second heating distillation, after the bromothymol blue indicator is added dropwise, the pH of the liquid is adjusted to 6.0-7.4 using a sodium hydroxide solution or a hydrochloric acid solution, and 0.2-0.3 g of light magnesium oxide and a few glass beads are added; the concentration of the bromothymol blue indicator is 0.4-0.6 g / L. The distillate rate of the first heating distillation and the second heating distillation is 8-12 mL / min, and the amount of sodium thiosulfate solution added is the same as the amount of silver nitrate solution added in step S1.

[0043] In step S1, the amount of the complex water sample is 250~280 mL, and in step S2, the amount of the mixed absorption liquid added is 50~60 mL. The amount of the first distillate is 200~220 mL, and the first heating distillation is stopped; the amount of the second distillate is 180~200 mL, and the second heating distillation is stopped. It should be noted that the amount of the mixed absorption liquid added, the amount of the first distillate, and the amount of the second distillate here all correspond to the situation when the sampling amount of the complex water sample is 250~280 mL. Those skilled in the art should know that if the sampling amount of the complex water sample changes, the amount of the mixed absorption liquid added, the amount of the first distillate, and the amount of the second distillate can be adjusted accordingly.

[0044] In step S1, the amount of ethylenediaminetetraacetic acid solution added is 5-8 mL, and the volume ratio of the added amounts of ethylenediaminetetraacetic acid solution, silver nitrate solution and potassium sodium tartrate solution is 1:1:(0.3-0.5); the concentration of the ethylenediaminetetraacetic acid solution is 80-120 g / L, the concentration of the silver nitrate solution is 0.1-0.3 mol / L, and the solubility of the potassium sodium tartrate is 180-200 g / L.

[0045] In step S6, the calculation formula of ammonia nitrogen content is:

[0046]

[0047] Where: C ——Concentration of ammonia nitrogen, mg / L;

[0048] m ——The mass of ammonia nitrogen is found from the calibration curve, μg;

[0049] m 0 ——The mass of the blank sample is found from the calibration curve, μg;

[0050] V ——The volume of the sample taken during color development, mL;

[0051] d ——The dilution factor of the sample is equal to the sample fixed volume divided by the sample sampling volume.

[0052] In step S5, the amount of potassium sodium tartrate added is 0.03 to 0.05 times the volume of the ammonia nitrogen standard solution, and the volume ratio of the amount of Nessler's reagent added to the amount of potassium sodium tartrate added is 1: (1.5 to 2.5). In step S5, the ammonia nitrogen standard solutions with different ammonia nitrogen contents include standard solutions with ammonia nitrogen contents of 0.0 μg, 5.0 μg, 10.0 μg, 20.0 μg, 40.0 μg, 60.0 μg, 80.0 μg, and 100.0 μg, respectively.

[0053] The preparation method of Nessler's reagent is as follows: weigh 16.0 g of sodium hydroxide, dissolve it in 50 mL of water, and cool it to room temperature to obtain a sodium hydroxide solution; weigh 7.0 g of potassium iodide and 10.0 g of mercuric iodide, dissolve them in water, slowly add this solution to the sodium hydroxide solution under stirring, and dilute it to 100 mL with water to obtain Nessler's reagent.

[0054] Example 1

[0055] This embodiment provides a method for accurately determining ammonia nitrogen in complex water bodies, comprising the following steps:

[0056] S1. Take 250 mL of complex water sample (ammonia nitrogen sample containing interfering elements such as urea, volatile amines, chloramines, sulfides, heavy metals, etc.) and place it in a distillation bottle. Add 5 mL of ethylenediaminetetraacetic acid solution (concentration of 100 g / L), 5 mL of silver nitrate solution (concentration of 0.2 mol / L) and 2 mL of potassium sodium tartrate solution (concentration of 2 00g / L), shake well and let it stand for 10 min.

[0057] Among them, the preparation method of 100 g / L EDTA-disodium solution is as follows: weigh 10.0g EDTA-disodium and dissolve it in water, dilute it to 100mL, and shake it well; 0.2 mol / L silver nitrate solution: weigh 3.3974g standard purity silver nitrate and dissolve it in water, dilute it to 100mL, shake it well, store it in a brown reagent bottle, and use it after shaking well; 200g / L potassium sodium tartrate solution: weigh 20.0g potassium sodium tartrate (KNaC4H6O6•4H2O) and dissolve it in 100mL water, heat and boil it to drive off ammonia, and dilute it to 100mL after sufficient cooling;

[0058] S2. Place 50 mL of mixed absorption liquid in the receiving bottle, and ensure that the outlet of the condenser connecting the distillation flask and the receiving bottle is below the liquid level of the mixed absorption liquid;

[0059] The preparation method of the mixed absorption liquid is as follows: weigh 25 g of boric acid and dissolve it in water, dilute it to 1000 mL, and shake it to obtain a 25 g / L boric acid solution; measure 14.0 mL of concentrated sulfuric acid and add it to water, dilute it to 250 mL, take 10 mL of it and dilute it to 500 mL to obtain a 0.0004 mol / L sulfuric acid solution; mix the boric acid solution and the sulfuric acid solution in a volume ratio of 4:1 to prepare a mixed absorption liquid;

[0060] S3. Add bromothymol blue indicator (concentration of 0.5 g / L) to the distillation flask, adjust the pH to between 6.0 (indicator is yellow) and 7.4 (indicator is blue) with sodium hydroxide solution or hydrochloric acid solution, add 0.25 g of light magnesium oxide and several glass beads, connect the nitrogen ball and the condenser to perform a heating distillation to obtain a distillate at a distillate rate of 10 mL / min. When the distillate reaches 200 mL, stop the distillation, rinse the distillate conduit with a small amount of water, and remove the receiving tube;

[0061] Transfer the primary distillate in the receiving bottle to a new distillation bottle, add 5 mL of sodium thiosulfate solution (concentration of 5 g / L), and drop bromothymol blue indicator, adjust the pH to between 6.0 (indicator is yellow) and 7.4 (indicator is blue) with sodium hydroxide solution or hydrochloric acid solution, add 0.25 g of light magnesium oxide and several glass beads for secondary heating and distillation to obtain secondary distillate; the distillate rate is 10 mL / min, and when the distillate reaches 180 mL, stop distillation, rinse the distillate conduit with a small amount of water, and remove the receiving tube;

[0062] Among them, the preparation method of 0.5 g / L bromothymol blue indicator is as follows: weigh 0.05g bromothymol blue and dissolve it in 50 mL water, add 10 mL anhydrous ethanol, dilute it to 100 mL with water, and shake well; the preparation method of sodium hydroxide solution (40 g / L) is as follows: weigh 40g sodium hydroxide and dissolve it in water, dilute it to 1000 mL, shake well, and store it in a polyethylene plastic container; hydrochloric acid solution 1 mol / L: take 8.5 mL hydrochloric acid in a 100 mL volumetric flask and dilute it to the mark with water; the preparation method of sodium thiosulfate solution with a concentration of 5 g / L is as follows: weigh 5.0g sodium thiosulfate and dissolve it in water, dilute it to 1000 mL, and shake well;

[0063] S4, using 10 g / L sodium hydroxide solution to adjust the secondary distillate obtained in step S3 to a pH of 6.0-7.0, and dilute to 250 mL to obtain a sample to be tested; wherein, 10 g / L sodium hydroxide solution: weigh 10 g of sodium hydroxide and dissolve it in water, dilute to 1000 mL, shake well, and store in a polyethylene plastic container;

[0064] S5, preparing ammonia nitrogen standard solutions with different ammonia nitrogen contents;

[0065] Take 8 50 mL stoppered colorimetric tubes, add 0 mL, 0.50 mL, 1.00 mL, 2.00 mL, 4.00 mL, 6.00 mL, 8.00 mL, and 10.00 mL of ammonia nitrogen standard solution, respectively, add water to the mark, the corresponding ammonia nitrogen content is 0.0 μg, 5.0 μg, 10.0 μg, 20.0 μg, 40.0 μg, 60.0 μg, 80.0 μg, and 100.0 μg, respectively, add 2 mL of potassium sodium tartrate, shake well, then add 1.0 mL of Nessler's reagent, shake well, let it stand for 15 minutes at a wavelength of 420 nm, use a 20 mm colorimetric dish and a zero concentration blank tube as a reference, measure the absorbance, and draw a curve of mass and absorbance;

[0066] Among them, Nessler's reagent: weigh 16.0 g sodium hydroxide (NaOH), dissolve in 50 mL water, cool to room temperature, weigh 7.0 g potassium iodide (KI) and 10.0 g mercuric iodide (HgI2), dissolve in water, then slowly add this solution to the above 50 mL sodium hydroxide solution under stirring, dilute with water to 100 mL, store in a polyethylene bottle, cover with a rubber stopper or polyethylene cap, and store in a dark place. The validity period is one year;

[0067] S6. Take 50 mL of the sample to be tested obtained in step S4 and place it in a colorimetric tube. Use the same method as step S5 to test and measure the absorbance. From the standard curve of step S5, the corresponding ammonia nitrogen content m is 34.5 μg. At the same time, the mass m0 of the blank sample is determined to be 2.20 μg. The calculation formula of the ammonia nitrogen content is:

[0068]

[0069] Where: C ——Concentration of ammonia nitrogen, mg / L;

[0070] m ——The mass of ammonia nitrogen is found from the calibration curve, μg;

[0071] m 0 ——The mass of the blank sample is found from the calibration curve, μg;

[0072] V ——The volume of the sample taken during color development, mL;

[0073] d ——The dilution factor of the sample is equal to the sample fixed volume divided by the sample sampling volume.

[0074] Specifically The calculated ammonia nitrogen concentration is 0.65 mg / L.

[0075] Example 2

[0076] This embodiment provides a method for accurately determining ammonia nitrogen in complex water bodies. Compared with Embodiment 1, the difference is that in step S1, the source of the complex water body sample used is different.

[0077] In step S6, 25 mL of the sample to be tested obtained in step S4 is placed in a colorimetric tube, diluted to the mark with water and shaken, and tested in the same way as in step S5. After measuring the absorbance, the corresponding ammonia nitrogen content m is obtained from the standard curve in step S5 to be 92.0 μg. At the same time, the mass m0 of the blank sample is measured to be 2.00 μg. The calculation formula for the ammonia nitrogen content is:

[0078] The calculated ammonia nitrogen concentration is 3.60 mg / L.

[0079] Example 3

[0080] This embodiment provides an accurate determination method for ammonia nitrogen in complex water bodies. Compared with Example 1, the difference is that in step S1, the standard sample GSB07-3164-2014200585 replaces the complex water sample, and the rest is roughly the same as Example 1 and will not be repeated here.

[0081] In step S6, after measuring the absorbance, the corresponding ammonia nitrogen content m is obtained from the standard curve of step S5 as 63.20 μg. At the same time, the mass m0 of the blank sample is measured as 2.00 μg. The calculation formula of the ammonia nitrogen content is:

[0082] The calculated ammonia nitrogen concentration is 1.22 mg / L.

[0083] The standard value of standard sample GSB07-3164-2014200585 is 1.22, the expanded uncertainty is 0.06, and the measured value is consistent with the standard value.

[0084] Comparative Example 1

[0085] Comparative Example 1 provides an accurate determination method for ammonia nitrogen in complex water bodies. Compared with Example 3, the difference is that in step S2, the mixed absorption liquid is changed to 50 mL of 25 g / L boric acid solution, and the rest is roughly the same as Example 3, which will not be repeated here.

[0086] Comparative Example 2

[0087] Comparative Example 2 provides an accurate determination method for ammonia nitrogen in complex water bodies. Compared with Example 3, the difference is that in step S2, the mixed absorption liquid is changed to 50 mL of 0.0004 mol / L sulfuric acid solution, and the rest is roughly the same as Example 3, which will not be repeated here.

[0088] Comparative Example 3

[0089] Comparative Example 3 provides an accurate determination method for ammonia nitrogen in complex water bodies. Compared with Example 3, the difference is that in step S2, the ratio of boric acid solution and sulfuric acid solution in the mixed absorption liquid is changed to 2:1, and the rest is roughly the same as Example 3 and will not be repeated here.

[0090] Comparative Example 4

[0091] Comparative Example 4 provides an accurate determination method for ammonia nitrogen in complex water bodies. Compared with Example 3, the difference is that in step S2, the ratio of boric acid solution to sulfuric acid solution in the mixed absorption liquid is changed to 1:4, and the rest is roughly the same as Example 3 and will not be repeated here.

[0092] The measurement result in Comparative Example 1 is 0.74 mg / L, the measurement result in Comparative Example 2 is 0.58 mg / L, the measurement result in Comparative Example 3 is 1.02 mg / L, and the measurement result in Comparative Example 4 is 1.68 mg / L. It can be seen that the results of Comparative Examples 1 to 3 are significantly lower than the standard values, and the result of Comparative Example 4 is significantly higher, and the accuracy is poor. It can be seen from the results of Example 3 and Comparative Examples 1 to 4 that the use of a certain volume ratio of boric acid and sulfuric acid as a mixed absorption liquid solves the problem of inaccurate measurement values ​​when using boric acid or sulfuric acid alone as an absorption liquid after the sample is distilled, and realizes the accurate determination of ammonia nitrogen in complex water bodies, and at the same time proves that the added masking agent and secondary distillation do not affect the accurate determination of ammonia nitrogen.

[0093] Example 4

[0094] This embodiment provides an accurate determination method for ammonia nitrogen in complex water. Compared with Embodiment 1, the difference is that in step S1, the standard sample GSB07-3164-2014200588 replaces the complex water sample.

[0095] In step S6, 20 mL of the sample to be tested obtained in step S4 is placed in a colorimetric tube, diluted to the mark with water and shaken, and tested in the same way as in step S5. After measuring the absorbance, the corresponding ammonia nitrogen content m is obtained from the standard curve of step S5 to be 94.60 μg. At the same time, the mass m0 of the blank sample is measured to be 2.03 μg. The calculation formula of the ammonia nitrogen content is:

[0096] The calculated ammonia nitrogen concentration is 4.63 mg / L.

[0097] The standard value of standard sample GSB07-3164-2014200588 is 4.60, the expanded uncertainty is 0.16, and the measured value is consistent with the standard value.

[0098] Comparative Example 5

[0099] Comparative Example 5 provides a method for accurately determining ammonia nitrogen in complex water bodies. Compared with Example 4, the difference is that in step S1, no silver nitrate solution is added, and the rest is roughly the same as Example 4, which will not be repeated here.

[0100] Comparative Example 6

[0101] Comparative Example 6 provides a method for accurately determining ammonia nitrogen in complex water bodies. Compared with Example 4, the difference is that in step S1, EDTA solution is not added, and the rest is roughly the same as Example 4, which will not be repeated here.

[0102] Comparative Example 7

[0103] Comparative Example 7 provides a method for accurately determining ammonia nitrogen in complex water bodies. Compared with Example 4, the difference is that in step S3, sodium thiosulfate solution is not added, and the rest is roughly the same as Example 4, which will not be repeated here.

[0104] Comparative Example 8

[0105] Comparative Example 8 provides a method for accurately determining ammonia nitrogen in complex water bodies. Compared with Example 4, the difference is that in step S1, sodium thiosulfate solution is added, and in step S3, the secondary heating distillation step is not performed. The rest is roughly the same as Example 4 and will not be repeated here.

[0106] The results of Comparative Example 5 were 3.58 mg / L, the results of Comparative Example 6 were 4.10 mg / L, the results of Comparative Example 7 were 6.16 mg / L, and the results of Comparative Example 8 were 6.22 mg / L. It can be seen that the results of Comparative Examples 5 to 8 are less accurate than the standard values. Urea, volatile amines, chloramines, sulfides, heavy metals, etc. will interfere with the determination of ammonia nitrogen. The present invention cleverly adopts a combination of secondary distillation and the addition of a masking agent, sodium thiosulfate, to mask the interference of urea and volatile amines contained in the sample on the determination of ammonia nitrogen; it can be seen from Comparative Example 8 that sodium thiosulfate cannot be introduced to mask urea and volatile amines during a single distillation, because other reagents will destroy the masking effect of sodium thiosulfate. Silver nitrate is used to mask the chloramines and sulfides contained in the sample to eliminate the influence of chloramines and sulfides on the determination of ammonia nitrogen. Most heavy metal ions are complexed by EDTA to eliminate the interference of heavy metal ions.

[0107] In summary, the present invention provides an accurate determination method for ammonia nitrogen in complex water bodies, which uses ethylenediaminetetraacetic acid to complex most heavy metal ions to eliminate the interference of heavy metal ions in the water body, and uses silver nitrate to mask chloramines and sulfides contained in the sample to eliminate the influence of chloramines and sulfides on the determination of ammonia nitrogen; at the same time, the secondary distillation process is cleverly combined with the addition of a masking agent sodium thiosulfate to mask the interference of urea and volatile amines contained in the complex water sample on the determination of ammonia nitrogen. Sodium thiosulfate cannot be introduced to mask the interference of urea and volatile amines during the first distillation because the presence of other reagents will destroy the masking effect of sodium thiosulfate. The present invention selects a suitable masking agent in combination with a secondary distillation process to mask the interference of urea, volatile amines, chloramines, sulfides, heavy metals and the like contained in complex water samples on the determination of ammonia nitrogen, and selects boric acid and sulfuric acid in a certain volume ratio as a mixed absorption liquid, thereby solving the problem of low measurement values ​​when boric acid or sulfuric acid is used alone as an absorption liquid after the sample is treated with distillation, and realizes the accurate determination of ammonia nitrogen in complex water bodies. The method has good precision and accuracy and high application value.

[0108] The above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solution of the present invention.

Claims

1. A method for accurately determining ammonia nitrogen in complex water bodies, characterized in that: The following steps are involved: S1. Take a certain amount of complex water sample and place it in a distillation bottle, add appropriate amount of ethylenediaminetetraacetic acid solution, silver nitrate solution and potassium sodium tartrate solution, shake well and let it stand; S2, placing a certain amount of mixed absorption liquid in a receiving bottle, and ensuring that the outlet of the condenser connecting the distillation flask and the receiving bottle is below the liquid level of the mixed absorption liquid; S3, drip bromothymol blue indicator into the distillation flask, connect a condenser to carry out a heating distillation, obtain a distillate, after the distillation is completed, transfer the distillate in the receiving bottle to a new distillation flask, add an appropriate amount of sodium thiosulfate solution, and drip bromothymol blue indicator to carry out secondary heating distillation to obtain a secondary distillate; the concentration of the sodium thiosulfate solution is 4-6 g / L; S4, using sodium hydroxide solution to adjust the secondary distillate obtained in step S3 to a pH of 6.0-7.0, to obtain a sample to be tested; S5. Prepare ammonia nitrogen standard solutions with different ammonia nitrogen contents, place them in colorimetric tubes respectively, add potassium sodium tartrate and shake well, then add Nessler's reagent and shake well and let stand, use a blank tube with zero concentration as a reference, measure the absorbance, and draw a standard curve of mass and absorbance; S6. Place the sample to be tested obtained in step S4 in a colorimetric tube, and perform testing in the same manner as in step S5 to measure the absorbance, and obtain the corresponding ammonia nitrogen content from the standard curve of step S5.

2. The method for accurately determining ammonia nitrogen in complex water bodies according to claim 1, characterized in that: In step S2, the mixed absorption liquid is a mixed solution of boric acid solution and sulfuric acid solution, wherein the concentration of the boric acid solution is 20-30 g / L, the concentration of the sulfuric acid solution is 0.0002-0.0006 mol / L, and the volume ratio of the two is (3.5-4.5):

1.

3. The method for accurately determining ammonia nitrogen in complex water bodies according to claim 1, characterized in that: In step S3, during the primary heating distillation and the secondary heating distillation, after adding bromothymol blue indicator, sodium hydroxide solution or hydrochloric acid solution is used to adjust the liquid pH to 6.0-7.4, and 0.2-0.3 g of light magnesium oxide and several glass beads are added; the concentration of the bromothymol blue indicator is 0.4-0.6 g / L.

4. The method for accurately determining ammonia nitrogen in complex water bodies according to claim 1, characterized in that: In step S3, the amount of the sodium thiosulfate solution added is the same as the amount of the silver nitrate solution added in step S1, and the distillate rate of the first heating distillation and the second heating distillation is 8-12 mL / min.

5. The method for accurately determining ammonia nitrogen in complex water bodies according to claim 1, characterized in that: In step S1, the amount of the complex water sample is 250~280 mL, in step S2, the amount of the mixed absorption liquid added is 50~60 mL, in step S3, the amount of the first distillate is 200~220 mL, and the first heating distillation is stopped; the amount of the second distillate is 180~200 mL, and the second heating distillation is stopped.

6. The method for accurately determining ammonia nitrogen in complex water bodies according to claim 5, characterized in that: In step S1, the amount of the ethylenediaminetetraacetic acid solution added is 5-8 mL, and the volume ratio of the ethylenediaminetetraacetic acid solution, the silver nitrate solution and the potassium sodium tartrate solution added is 1:1:(0.3-0.5); the concentration of the ethylenediaminetetraacetic acid solution is 80-120 g / L, the concentration of the silver nitrate solution is 0.1-0.3 mol / L, and the solubility of the potassium sodium tartrate is 180-200 g / L.

7. The method for accurately determining ammonia nitrogen in complex water bodies according to claim 1, characterized in that: In step S6, the calculation formula of ammonia nitrogen content is: Where: C ——Concentration of ammonia nitrogen, mg / L; m ——The mass of ammonia nitrogen is found from the calibration curve, μg; m 0 ——The mass of the blank sample is found from the calibration curve, μg; V ——The volume of the sample taken during color development, mL; d ——The dilution factor of the sample is equal to the sample fixed volume divided by the sample sampling volume.

8. The method for accurately determining ammonia nitrogen in complex water bodies according to claim 1, characterized in that: In step S5, the amount of potassium sodium tartrate added is 0.03-0.05 times the volume of the ammonia nitrogen standard solution, and the volume ratio of the amount of Nessler's reagent added to the amount of potassium sodium tartrate added is 1:(1.5-2.5).

9. The method for accurately determining ammonia nitrogen in complex water bodies according to claim 1, characterized in that: In step S5, the ammonia nitrogen standard solutions with different ammonia nitrogen contents include standard solutions with ammonia nitrogen contents of 0.0 μg, 5.0 μg, 10.0 μg, 20.0 μg, 40.0 μg, 60.0 μg, 80.0 μg, and 100.0 μg, respectively.

10. The method for accurately determining ammonia nitrogen in complex water bodies according to claim 8, characterized in that: The preparation method of the Nessler's reagent is as follows: weigh 16.0 g of sodium hydroxide, dissolve it in 50 mL of water, and cool it to room temperature to obtain a sodium hydroxide solution; weigh 7.0 g of potassium iodide and 10.0 g of mercuric iodide, dissolve them in water, slowly add the solution into the sodium hydroxide solution under stirring, and dilute it with water to 100 mL to obtain the Nessler's reagent.

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