Method for measuring total alkalinity of pore water in salt lake water or marine sediments

Abstract

The invention provides a method for measuring the total alkalinity of pore water in salt lake water or marine sediments, aiming at the characteristics of huge alkalinity variation range of a salt lake sample and small sample amount of the pore water in the sediments, the traditional step-by-step dilution alkalinity measuring method is complicated to operate, and simple dilution of the sample can change the balance of a water body carbonate system. The accuracy of total alkalinity determination is influenced, and the uncertainty is large. The method for measuring the total alkalinity of the salt lake water with the high total alkalinity range and the marine sediment pore water with the low sample amount can be used for batch measurement of the total alkalinity of the samples, repeated dilution is not needed, one-time rapid measurement is achieved, the sample measurement accuracy is improved, and manpower and material resources are saved. In the method, the alkalinity of the prepared NaOH standard solution is calibrated more accurately based on the standard curve adding method, the effective measurement range can be expanded by adjusting the concentration of the prepared NaOH standard solution, and the method is wide in applicability.

Description

Technical Field

[0001] The invention belongs to the technical field of water quality analysis and detection, and specifically relates to a method for measuring the total alkalinity of pore water in salt lake water or marine sediments. Background Art

[0002] Salt lakes are one of the important types of lakes in the world. They are a natural resource that integrates inorganic salts, organic matter and organisms. They are the product of the joint action of the hydrosphere, atmosphere, biosphere and lithosphere. Salt lakes are an important component of inland water bodies, accounting for about 44% of the total volume and 23% of the total area of ​​lakes on the earth. At the same time, salt lakes are important carbon reservoirs on the earth, collecting a large amount of carbon components from terrestrial surface water. Salt lakes play an important role in regulating regional climate, maintaining regional carbon budget and ecosystem balance through lake-air CO2 exchange. The Qinghai-Tibet Plateau is the world's most important salt lake distribution area, and the salt lakes with the highest alkalinity characteristics in the world are mainly distributed in my country's Qinghai-Tibet Plateau.

[0003] As for salt lake water, its total alkalinity can regulate the CO2 partial pressure (pCO2) of the water, and then regulate the CO2 absorption and release process at the interface between the water and the atmosphere. Accurate determination of the total alkalinity of salt lake water is a key prerequisite for the analysis of its carbonate system, which is used to evaluate the acidification buffering capacity of salt lake water and its ability to absorb atmospheric CO2 (carbon sink).

[0004] The determination of total alkalinity of salt lake water is mainly based on the indicator titration method. The conventional operation method is to accurately transfer 20 mL of salt lake water, dilute it with 100 mL of CO2-free cooled high-purity water, add 10 drops of methyl red indicator, and use a hydrochloric acid standard solution (0.05 mol L -1 ) until the solution just turns red, then boil on a hot plate for 3 min to drive out the CO2, remove and cool, and if the red color fades, add hydrochloric acid standard solution (0.05 mol L -1 ) titrate until the color just turns red as the endpoint, and do the blank test at the same time. Calculate the total alkalinity of the salt lake water based on the total amount of hydrochloric acid standard solution consumed.

[0005] However, the above method may be affected by ionic strength when testing high-salinity water samples, resulting in inaccurate results. Moreover, under strong alkaline or strong acidic conditions, the color change of the indicator may not be obvious, making it difficult to determine the end point. In addition, the total alkalinity between different salt lake water bodies may vary by 0 to 300 times, with a large fluctuation range. If the dilution method is used to measure the salt lake water diluted with pure water, the dilution method will change the balance of the carbonate system in the water body, affecting the accuracy of the total alkalinity measurement, and the operation is cumbersome.

[0006] Pore ​​water exists between tiny particles of soil, sediments, etc., and is also called interstitial water. Marine sediment pore water is a water solution existing between the pores of sediment particles. It mainly comes from the seawater entrained by the sediment particles during deposition. The chemical composition of pore water that has not been transformed by diagenesis mainly reflects the properties of the original seawater buried with the sediments and the biogeochemical reactions inside the sediments.

[0007] After obtaining the total alkalinity of marine sediment pore water samples, the total alkalinity of the upper sediment pore water can be compared with the total alkalinity of the overlying seawater. If the total alkalinity of the upper sediment pore water is higher, it indicates that the total alkalinity of the sediment can be released to the overlying seawater. In addition, in marine sediments, the total alkalinity of pore water is closely related to the redox process of substances in the sediments, and can be used to trace the biochemical reaction process in the sediments and evaluate the ecological and environmental health of marine sediments.

[0008] The existing methods for detecting the total alkalinity of marine sediment pore water mainly include pH titration, indicator titration, dilution determination and single-point spike method, but all of them have certain defects.

[0009] 1) The pH titration method refers to adding an excess of hydrochloric acid solution of known concentration to the water sample to neutralize the alkali in the water sample, and then using a pH meter to measure the pH value of the mixed solution. The remaining acid in the mixed solution is calculated from the measured pH value, and then the remaining acid is subtracted from the total amount of acid added to obtain the amount of acid added to the water sample. The total alkalinity of the water sample is further calculated based on the amount of acid used. This method requires a large amount of sample (>25ml) and cannot be used for small-volume samples. The pH measurement range is between 3.40 and 3.90. If it is not within this range, hydrochloric acid solution must be added again or the water sample must be re-measured for pH, which is cumbersome.

[0010] 2) Indicator titration method refers to taking a certain volume of sediment pore water sample, adding methyl red-methylene blue indicator, and directly titrating with hydrochloric acid standard solution. When the solution changes from orange-yellow to stable light purple-red, it is the titration end point, indicating that the alkaline substances in the water sample have been neutralized to the CO2 titration end point. In this method, the indicator is alkaline, which will essentially consume the hydrochloric acid standard solution and affect the measurement results; the titration end point needs to be judged by observing the color change of the solution during the titration process, which has uncontrollable human influence; and the sample volume requirement is large, usually >5mL.

[0011] 3) The dilution method for total alkalinity determination refers to taking a certain volume of sediment pore water sample, adding distilled water or deionized water to dilute it, and then using pH titration, indicator titration or Gran titration to determine it. In this method, since distilled water or deionized water contains HCO3 - and CO3 2-Therefore, measuring the pore water in marine sediments after dilution will lead to systematic errors.

[0012] 4) The single-point addition method refers to accurately taking 10mL of seawater sample with known total alkalinity and 2mL of sediment pore water sample into a 60mL beaker, placing a magnetic rotor, inserting a clean pH electrode (calibrated with standard buffer solutions of pH 4.00, 7.00 and 10.00 before use), and titrating with hydrochloric acid standard solution. When the pH value is between 3.40 and 3.90, record the titration volume and pH value of the hydrochloric acid standard solution, calculate the total alkalinity of the mixed solution using the formula in GB / T12763.4-2007, and then calculate the total alkalinity of the added pore water sample based on the added sediment pore water sample volume. This method requires a small amount of sample, but the single-point addition method has high requirements for operation and is cumbersome to operate, so it is not suitable for the determination of large quantities of samples.

[0013] Considering the wide variation range of total alkalinity in salt lake water and the high total alkalinity in marine sediment pore water, but the sample volume is extremely small (<3ml), it is difficult to measure the above samples by conventional titration, while the measurement after dilution has large uncertainty and cumbersome operation. Summary of the invention

[0014] The purpose of the present invention is to provide a method for determining the total alkalinity of pore water in salt lake water or marine sediments. The method of the present invention does not require multiple dilutions and can quickly determine the total alkalinity of salt lake water with a high total alkalinity variation range or pore water in marine sediments with a low sample amount at one time, thereby making up for the shortcomings of the prior art.

[0015] The method provided by the present invention for determining the total alkalinity of pore water in salt lake water or marine sediments comprises the following steps:

[0016] 1) Draw a six-point calibration standard curve TA-V diagram

[0017] Accurately pipette different volumes (0, 2, 4, 6, 8, 10 ml) of prepared NaOH standard solution into (50 ml) aged seawater to form mixed samples with spiked volume gradients. Then, the alkalinity of different mixed samples was determined by Gran titration method, and a six-point calibration standard curve (TA-V diagram) of total alkalinity and spiked volume of NaOH standard solution was drawn.

[0018] The standard curve formula is as follows: y = 33.28x + 2008.4, R 2 =0.9972, where y is the total alkalinity and x is the spiked volume of NaOH standard solution;

[0019] The aged seawater is obtained from the surface seawater of an open sea area more than 200 km offshore, sealed and stored in a dark and cool place for at least two years;

[0020] 2) Calculate the exact total alkalinity value (TA) of the added NaOH solution according to the TA-V diagram NaOH )

[0021] First, obtain the function of the TA-V linear relationship, obtain its slope and intercept, and use the formula TA NaOH =((slope × 10 + intercept) × 60 - intercept × 50) / 10 to calculate the accurate total alkalinity value of the prepared NaOH solution;

[0022] 3) Determination of pore water samples in salt lake water or marine sediments

[0023] Accurately measure (X ml) of salt lake water or pore water sample in marine sediments and mix it with (50 ml) of aged seawater to make a mixed solution, and determine its total alkalinity value (TA) based on the Gran titration method. X );

[0024] According to the total alkalinity value of the spiked sample, the volume of the equivalent added NaOH standard solution (V X ), the calculation process is based on the formula V X =(TA X -intercept) / slope, and then calculate the true total alkalinity value (TA) of the sample based on the sampling volume and the total alkalinity value of the prepared NaOH solution. 样品 ), the calculation process is based on the formula TA 样品 =TA NaOH ×V x / X, where X is the volume of the water sample to be tested (ml), and 2ml is recommended as the best.

[0025] In view of the huge alkalinity variation range of salt lake samples and the small sample size of sediment pore water, the traditional step-by-step dilution alkalinity measurement method is cumbersome to operate, and simple dilution of the sample will change the balance of the carbonate system of the water body, affecting the accuracy of the total alkalinity determination, and has large uncertainty. The method provided by the present invention for determining the total alkalinity of salt lake water with a high total alkalinity range and marine sediment pore water with a low sample size can be used for batch determination of the total alkalinity of the above samples. It does not require multiple dilutions, and a one-time rapid determination improves the accuracy of sample determination, saving manpower and material resources. In this method, it is more accurate to calibrate the alkalinity of the prepared NaOH standard solution based on the spiked standard curve method, and the effective measurement range can be expanded by adjusting the concentration of the prepared NaOH standard solution, and it has wide applicability. This method can be intuitively measured by the volume of the equivalently added NaOH standard solution (V X ) and the alkalinity of NaOH standard solution to characterize the alkalinity value of the measured sample, which is easier to understand and convenient for experimental operators with different basic skills to perform sample measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 : Sampling information diagram of aged seawater used in the present invention;

[0027] Figure 2 :Schematic diagram of the preparation method of NaOH standard solution;

[0028] Figure 3 :Schematic diagram of the NaOH standard solution spike method;

[0029] Figure 4 : Data processing diagram;

[0030] Figure 5 : Schematic diagram of preparation of spiked mixed samples;

[0031] Figure 6 :Schematic diagram of the operation of determining the total alkalinity of salt lake water and marine sediment pore water based on the dilution method;

[0032] Figure 7 :Schematic diagram of the operation of determining the total alkalinity of pore water in salt lake water or marine sediments based on the single-point spike method. DETAILED DESCRIPTION

[0033] Total alkalinity (TA) refers to the total amount of hydrogen ions consumed when a water sample is titrated with a low concentration of strong acid to reach the CO2 titration endpoint (pH value is about 4.4). The specific formula is as follows: TA = [HCO3-] + 2 [CO3 2- ]+[B(OH)4 2- ]+[OH - ]+[HPO4 2- ]+2[PO4 3- ]+[H3SiO4 - ]+[N3H]+[HS - ]-[H + ]-[HSO4 - ]-[HF]-[H3PO4].

[0034] The first object of the present invention is to provide a method for determining the total alkalinity of salt lake water with a high total alkalinity variation range, with an effective determination range of 1800-20000 μmol L -1 For samples with particularly high total alkalinity, the measuring range can be extended by increasing the concentration of the spiked NaOH solution.

[0035] The second object of the present invention is to provide a method for determining the total alkalinity of pore water in marine sediments with a low sample volume, with an effective determination range of 1800-20000 μmol L -1 For samples with particularly high total alkalinity, the measuring range can be extended by increasing the concentration of the spiked NaOH solution.

[0036] The method of the present invention uses aged seawater with stable properties as the background condition for the spike operation, providing a background condition with salinity and carbonate system buffering capacity. Using aged seawater as the background condition can effectively reduce the disturbance of the carbonate system by the addition of NaOH solution. Specifically, the aged seawater needs to be taken from the surface seawater of an open sea area more than 200 km offshore and less disturbed by humans, sealed and stored in a dark and cool place for at least two years, so that the organic matter in the water body is fully decomposed, the properties are stable, and the total alkalinity changes little.

[0037] The total alkalinity of the preparation of the present invention is about 4000 μmol L -1 The NaOH standard solution is used to simulate the total alkalinity values ​​of salt lake water and marine sediment pore water with different total alkalinity values. Specifically, weigh 16g of NaOH solid sample and place it in a 100ml beaker, dissolve it with deionized water, transfer it to a 1L volumetric flask, rinse the beaker twice and transfer all of it to the volumetric flask. Add deionized water to dilute it to less than the constant volume scale line, and after it is completely cooled, make up the volume, and mix it upside down. At this time, the concentration of the NaOH solution is about 0.4molL -1 Take 10 ml of the prepared NaOH solution and place it in a 1L volumetric flask, dilute to 1L, and mix up and down. At this time, the concentration of the NaOH solution is about 0.004 mol L -1 (That is, the total alkalinity is about 4000 μmol L -1 For samples with particularly high total alkalinity, the measuring range can be extended by increasing the concentration of the spiked NaOH solution.

[0038] The preferred steps for drawing the spiked standard curve are as follows:

[0039] Accurately pipette 0, 2, 4, 6, 8, 10 ml of the prepared NaOH solution into 50 ml of aged seawater to prepare mixed samples and form a spiked volume gradient. Gran titration was used to determine the alkalinity of different mixed samples, and a six-point calibration standard curve (TA-V graph) of total alkalinity and NaOH solution spiked volume was drawn. Then, the accurate total alkalinity value (TA) of the added NaOH solution was calculated based on the TA-V graph. NaOH Specifically, first obtain the function of the TA-V linear relationship, obtain its slope and intercept, and then use the formula TA NaOH =((slope×10+intercept)×60-intercept×50) / 10 to calculate the accurate total alkalinity value of the prepared NaOH solution.

[0040] The present invention is described in detail below in conjunction with embodiments and drawings.

[0041] Example 1: Determination of total alkalinity of salt lake water and marine sediment pore water based on the spiked standard curve method

[0042] Step 1: Take advantage of the voyage opportunity to collect surface seawater from the open sea, seal it in a cool place for at least two years, and make aged seawater with stable properties ( Figure 1 ).

[0043] Step 2: If Figure 2 Prepare NaOH standard solution as shown

[0044] Weigh 16g of solid NaOH into a 100ml beaker, dissolve it with deionized water, transfer it to a 1L volumetric flask, rinse the beaker twice and transfer it to the volumetric flask. Add deionized water to dilute it to less than the scale line, and after it is completely cooled, make up the volume and mix it upside down. At this time, the concentration of the NaOH solution is about 0.4mol L -1 ; Take 10 ml of the prepared NaOH solution and place it in a 1L volumetric flask, dilute to 1L, and mix up and down. At this time, the concentration of the NaOH solution is about 0.004 mol L -1 (Total alkalinity is about 4000 μmol L -1 For samples with particularly high total alkalinity, the measuring range can be extended by increasing the concentration of the spiked NaOH solution.

[0045] Step 3: If Figure 3 As shown, NaOH solution was spiked.

[0046] Accurately measure 0, 2, 4, 6, 8, and 10 ml of the prepared NaOH solution and add them to 50 ml of aged seawater to form a concentration gradient.

[0047] Step 4: As shown in Table 1 and Figure 4 As shown, the total alkalinity of different mixed samples was measured based on the Gran titration method, and a standard curve of total alkalinity and NaOH spike volume (TA-V diagram) was drawn.

[0048] Table 1: NaOH standard solution spike volume and its corresponding total alkalinity value determined by Gran titration method

[0049]

[0050]

[0051] Step 5: If Figure 4 As shown, the exact total alkalinity value of the added NaOH is calculated based on the TA-V diagram.

[0052] First, obtain the function of the linear relationship of the TA-V graph and obtain its slope and intercept. Here, the intercept of the TA-V linear relationship graph represents the alkalinity of 50ml aged seawater without adding NaOH; the slope represents the effect of adding 1ml NaOH on the total alkalinity of the mixed sample based on 50ml aged seawater. Figure 4Taking the experimental results of as an example, the intercept represents the accurate alkalinity value of aged seawater, which is 2008.4 μmol L -1 The slope is that the total alkalinity of the mixed sample increases by 33.3 μmol L for every 1 ml of NaOH added. -1 .

[0053] Secondly, according to the slope and intercept obtained, use the formula TA NaOH =((slope × 10 + intercept) × 60 - intercept × 50) / 10 Calculate the exact total alkalinity value (TA) of the prepared NaOH NaOH ). Where (slope × 10 + intercept) is the total alkalinity value of the mixed solution of 50 ml aged seawater and 10 ml NaOH, which is 2341.4 μmol L -1 ; (slope × 10 + intercept) × 60 is the total alkalinity of 60 ml of mixed solution, which is 140.5 μmol; (intercept × 50) is the total alkalinity of 50 ml of aged seawater, which is 100.4 μmol; ((slope × 10 + intercept) × 60 - intercept × 50) is the change in total alkalinity after adding 10 ml of NaOH solution, that is, the total alkalinity value corresponding to 10 ml of NaOH, which is 40.1 μmol; ((slope × 10 + intercept) × 60 - intercept × 50) / 10 is the exact total alkalinity value per ml of NaOH, which is 4005.2 μmol L -1 .

[0054] Step 6: If Figure 5 As shown, X ml of sample was added to 50 ml of aged seawater to prepare a mixed solution, and its total alkalinity value (TA) was determined based on the Gran titration method. X In the experiment of this method, 2 ml of salt lake water sample was added to 50 ml of aged seawater, and the alkalinity value TAX was measured to be 2048.2 μmol L -1 .

[0055] Step 7: According to Figure 4 The volume of NaOH added equivalently (V X ), the calculation process is based on the formula V X =(TA X In the experiments of this method, the addition of spiked seawater is equivalent to the addition of 1.196 ml of NaOH solution.

[0056] Step 8: According to Figure 4 According to the method described in the sample volume and the total alkalinity of the prepared NaOH solution, the true total alkalinity value (TA) of the sample is calculated. 样品 ), the calculation process is based on the formula TA 样品 =TA NaOH ×Vx / X, where X is the volume of the water sample added to be tested (ml). In the experiment of this method, the alkalinity of the salt lake water sample tested was 2395μmol L -1 The effective determination range of this method is 1800-20000 μmol L -1 For samples with particularly high total alkalinity, the range can be extended by increasing the concentration of the spiked NaOH solution.

[0057] Example 2: Determination of total alkalinity of salt lake water and marine sediment pore water based on dilution method

[0058] Step 1: If Figure 6 , take X ml of sample, add Y ml of distilled water or deionized water for dilution (the specific dilution factor depends on the total alkalinity value of the actual sample and the range of the Gran titration method). In the experiment of this method, take 50 ml of salt lake water sample and add 10 ml of deionized water for dilution.

[0059] Step 2: Determine the total alkalinity (TA) of the diluted sample using Gran titration 稀释 ), 2041.7 μmol L -1 .

[0060] Step 3: The total alkalinity of the sample is calculated based on the alkalinity measured after dilution and the dilution multiple, which is (Y+X) / X×TA 稀释 In the experiment of this method, the alkalinity of the salt lake water sample was calculated to be 2450 μmol L -1 .

[0061] Example 3: Determination of total alkalinity of salt lake water and marine sediment pore water based on single-point spike method

[0062] Step 1: If Figure 7 , X ml of sample was added to Y ml of aged seawater to prepare a mixed solution, and its total alkalinity value (TA) was determined based on the Gran titration method. X In the experiment of this method, 2 ml of salt lake water sample was added to 50 ml of aged seawater, and the total alkalinity value TAX was determined to be 2024.0 μmol L -1 .

[0063] Step 2: Determine the total alkalinity value (TA) of aged seawater 陈化海水 ). In the experiment of this method, the alkalinity of aged seawater was 2008.4 μmol L -1 .

[0064] Step 3: Calculate the total alkalinity of the sample, which is ((Y+X)×TA X -Y×TA 陈化海水) / X. In the experiment of this method, the final alkalinity of the salt lake water sample was calculated to be 2415 μmol L -1 .

[0065] The measurement results of the above three methods showed that the results based on the spiked standard curve method were closest to the true value and had a very obvious advantage (Table 2).

[0066] Table 2: Comparison of the results of three alkalinity measurement methods

[0067]

[0068] The above is only a specific implementation of the present application, but the protection scope of the present application is not limited thereto. Any person skilled in the art who is familiar with the present technical field can easily think of changes or substitutions within the technical scope disclosed in the present application, which should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. A method for determining the total alkalinity of pore water in salt lake water or marine sediments, characterized in that: The method comprises the following steps: 1) Draw a six-point calibration standard curve TA-V diagram Accurately pipette different volumes of prepared NaOH standard solution into aged seawater to form mixed samples with spiked volume gradients, then use Gran titration to determine the alkalinity of different mixed samples, and draw a six-point calibration standard curve TA-V diagram of total alkalinity and NaOH standard solution spiked volume; 2) Calculate the exact total alkalinity value of the added NaOH solution based on the TA-V diagram First, obtain the function of the TA-V linear relationship, obtain its slope and intercept, and use the formula TA NaOH =((slope × 10 + intercept) × 60 - intercept × 50) / 10 to calculate the accurate total alkalinity value of the prepared NaOH solution; 3) Determination of pore water samples in salt lake water or marine sediments Accurately measure the pore water sample from salt lake water or marine sediments and mix it with aged seawater to make a mixed solution, and determine its total alkalinity value TA based on the Gran titration method X ; According to the total alkalinity value of the spiked sample, the volume V of the equivalent added NaOH standard solution is calculated on the TA-V curve. X , the calculation process is based on the formula V X =(TA X -intercept) / slope, and then calculate the true total alkalinity value TA of the sample based on the sampling volume and the total alkalinity value of the prepared NaOH solution 样品 , the calculation process is based on the formula TA 样品 =TA NaOH ×V x / X, where X is the volume of the water sample added to be tested.

2. The method according to claim 1, characterized in that In the above 1), 0, 2, 4, 6, 8 or 10 ml of the prepared NaOH standard solution is added to 50 ml of aged seawater to form a mixed sample with a spiked volume gradient.

3. The method according to claim 1, characterized in that The formula of the standard curve is y=33.28x+2008.4, R 2 =0.9972, where y is the total alkalinity and x is the spiked volume of NaOH standard solution.

4. The method according to claim 1, characterized in that The total alkalinity of the NaOH standard solution is 4000 μmol L -1 .

5. The method according to claim 1, characterized in that The aged seawater is prepared by taking the surface seawater from the open sea area more than 200 km offshore, sealing and storing it in a dark and cool place for at least two years.

6. The method according to claim 1, characterized in that The volume of the water sample to be tested added in the above 3) is 2 ml.

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