A method for eliminating phosphate interference in determining the content of sodium carbonate and sodium bicarbonate in a compound water-retaining agent

Abstract

The invention provides a method for determining the content of sodium carbonate and sodium bicarbonate in a compound water-retaining agent by eliminating the interference of phosphate. The method adopts an ion chromatography method and a standard solution titration method to determine the content of sodium bicarbonate and sodium carbonate in the compound water-retaining agent. The method adopts an ion chromatography method to qualitatively and quantitatively detect phosphate in a sample, and then uses a 0.1 mol / L hydrochloric acid standard solution and a double indicator method to perform acid-base titration. The two methods are combined to respectively determine the content of sodium carbonate and sodium bicarbonate in a mixed sample. The error of the detection result is within ±1%, the repeatability is good, and the accuracy can meet the quantitative analysis requirement. The method of the invention is simple to operate, fast, accurate in result, low in detection cost, can quickly determine a large number of samples, and provides a convenient detection means for production, supervision, research, etc.

Description

Technical Field

[0001] The invention relates to the field of carbonate (bicarbonate) determination, in particular to a method for eliminating phosphate interference in determining the content of sodium carbonate and sodium bicarbonate in a composite water retaining agent. Background Art

[0002] Compound water-retaining agents refer to a class of substances that can improve the stability of products, maintain the water retention of foods, and improve the shape, flavor, and color of foods after being added during food processing. They are commonly used in meat products, dairy products, rice and flour products, etc. The commonly used components of compound water-retaining agents used in food are phosphates, sodium citrate, sodium carbonate, sodium bicarbonate, and salt, among which more than 90% of compound water-retaining agents contain three main ingredients: phosphates, sodium carbonate, and sodium bicarbonate, and most samples contain more than 70% of the three main ingredients. Due to the differences in particle size of these three main ingredients, it is not easy to mix them evenly. Therefore, in compound water-retaining agent production units, it is necessary to analyze the content of the main active ingredients of phosphate and carbonate (bicarbonate) in the samples, monitor the production process, and control the quality of the products. Therefore, it is necessary to establish a set of convenient, accurate, and low-cost detection methods to guide production and provide a basis for production process and finished product inspection.

[0003] At present, the main method for detecting the content of sodium carbonate and sodium bicarbonate is the national standard method - hydrochloric acid standard solution titration method. Most of the samples of compound water-retaining agents contain phosphates, and the phosphate content is usually 30%-70%. When phosphates (orthophosphates, pyrophosphates, tripolyphosphates, acid pyrophosphates, metaphosphates) are dissolved in water, HPO4 will be hydrolyzed in the solution. 2- 、H2PO4 - , can be combined with H + Combined with, consuming the hydrochloric acid standard solution, resulting in the determination of HCO3 during the titration process - 、CO3 2- The volume of the consumed standard solution is measured with error, interfering with HCO3 - 、CO3 2- Quantitative analysis. When methyl orange is used as the indicator at the second titration endpoint, the color change pH value is about 4.5. The color change is not sharp and it is not easy to judge the endpoint. It is easy to fail to accurately judge the titration endpoint and cause a large error. Other methods such as Raman spectroscopy have expensive equipment and consumables, high detection costs, and difficult to master operating methods. It is not easy to promote this method within the enterprise. Chinese patent CN114216812A discloses a method for determining the content of sodium carbonate and sodium bicarbonate in a compound moisture-retaining agent, which combines the gas generation method with the drying loss method, which can control the measurement error to about ±3%, but the measurement error is still large. Summary of the invention

[0004] In order to solve the above technical problems, the present invention provides a method for eliminating the interference of phosphate in determining the content of sodium carbonate and sodium bicarbonate in a compound water-retaining agent, which adopts a combination of ion chromatography and standard solution titration to eliminate the influence of phosphate in the compound water-retaining agent on the determination of the content of sodium carbonate and sodium bicarbonate, thereby reducing the error.

[0005] The technical solution adopted by the present invention is as follows:

[0006] The invention discloses a method for determining the content of sodium carbonate and sodium bicarbonate in a compound water-retaining agent by eliminating the interference of phosphate. The method adopts ion chromatography and standard solution titration-double indicator method to determine the content of sodium bicarbonate and sodium carbonate in the compound water-retaining agent.

[0007] Preferably, the method specifically comprises the following steps:

[0008] (1) Sample preparation: Accurately weigh 0.5-1 g of sample and dissolve it in distilled water in a beaker and dilute to 500 mL to obtain a sample solution;

[0009] (2) Phosphate analysis: The sample solution is loaded onto an ion chromatograph for qualitative and quantitative analysis of the phosphate therein to determine the type and content of the phosphate.

[0010] (3) Phosphate primary titration: Accurately weigh an equal amount of phosphate from step (2) into a beaker, add 50 mL of decarbonated water, add 3 drops of phenolphthalein indicator and titrate with 0.1 mol / L hydrochloric acid standard solution until the red color disappears, and record the volume V1;

[0011] (4) Phosphate secondary titration: add methyl red-bromocresol green indicator to the solution after titration in step (3), titrate with 0.1 mol / L hydrochloric acid standard solution until the solution turns dark red, and record the volume V2 when the second titration endpoint is reached;

[0012] (5) Sample primary titration: Accurately weigh 0.5-1 g of sample into a beaker, add 50 mL of decarbonated water, add 3 drops of phenolphthalein indicator and titrate with 0.1 mol / L hydrochloric acid standard solution until the red color disappears, and record the volume V3;

[0013] (6) Secondary titration of the sample: add methyl red-bromocresol green indicator to the solution after titration in step (5), titrate with 0.1 mol / L hydrochloric acid standard solution until the solution turns dark red, and record the volume V4 when the second titration endpoint is reached;

[0014] (7) Result calculation: The content of sodium bicarbonate and sodium carbonate in the sample was calculated using the following formula:

[0015]

[0016]

[0017] Where: C(HCl) is the concentration of the standard solution, in mol / L;

[0018] M(Na2CO3) is the molar mass of sodium carbonate, in g / mol;

[0019] M(NaHCO3) is the molar mass of sodium bicarbonate, in g / mol;

[0020] m is the mass of the sample in g.

[0021] Preferably, the ion chromatograph described in step (2) uses an anion analysis column, the column flow rate is 1.5 mL / min, the injection volume is 0.25 mL, the detection temperature is 30°C, and the elution method of the chromatographic column is 30mM, 50mM, 90mM KOH gradient elution.

[0022] The concentrations of the methyl red-bromocresol green indicator in steps (4) and (6) are 1 g / L methyl red and 1 g / L bromocresol green, and the volume ratio of methyl red to bromocresol green is 1:3; the volume ratio of the indicator to the solution is 0.1-0.2:50.

[0023] The beneficial effects obtained by the present invention are as follows: firstly, the type and content of phosphate in the composite water-retaining agent are determined by ion chromatography, then the consumption of the standard solution by phosphate that will affect the determination of the content of sodium carbonate and sodium bicarbonate is determined by hydrochloric acid standard solution titration, and finally the accurate consumption of the standard hydrochloric acid solution by sodium carbonate and sodium bicarbonate in the composite water-retaining agent is determined by hydrochloric acid standard solution titration, thereby obtaining the content of sodium carbonate and sodium bicarbonate within an error of ±1%, with high accuracy and good repeatability. At the same time, by adopting the double indicator method for acid-base titration, a methyl red-bromocresol green indicator with more obvious color change is selected, so that the determination of the error end point is easier, and the accuracy of measurement is further improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a procedure diagram for determining phosphate in the composite water-retaining agent sample FP-1 in Example 4, wherein A is a peak time diagram of each substance, and B is a standard curve used in the procedure determination.

[0025] Figure 2 This is a chromatogram showing the determination of the phosphate content in the composite water-retaining agent sample FP-1 in Example 4.

[0026] Figure 3 This is a diagram of the solution color change using methyl orange as the indicator in Example 6, wherein A is the solution color before the titration endpoint, and B is the solution color at the titration endpoint.

[0027] Figure 4This is a diagram of the solution color change using methyl red-bromocresol green indicator in Example 6, wherein A is the solution color before the titration endpoint, and B is the solution color at the titration endpoint. DETAILED DESCRIPTION

[0028] The embodiments of the present invention are described in detail below in conjunction with the accompanying drawings and examples, but those skilled in the art will understand that the following examples are only used to illustrate the present invention and should not be regarded as limiting the scope of the present invention.

[0029] The compound moisture retaining agent sample FP-1 used in the following examples is a commonly used compound moisture retaining agent in laboratories, which contains about 20-50% phosphate, 10-30% sodium bicarbonate, 10-30% sodium carbonate and 10-20% sodium chloride.

[0030] Example 1 Determination of interfering factors in compound water-retaining agents

[0031] The influence factors of phosphates commonly used in compound water-retaining agents on the content of sodium bicarbonate and sodium carbonate detected by hydrochloric acid titration were detected. The phosphates commonly used in compound water-retaining agents are sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, sodium pyrophosphate, sodium tripolyphosphate, sodium hexametaphosphate and disodium dihydrogen pyrophosphate.

[0032] The specific method is as follows:

[0033] (1) Accurately weigh several commonly used phosphates into a beaker, add 50 mL of carbon dioxide water, add 3 drops of phenolphthalein indicator, and titrate with 0.1 mol / L hydrochloric acid standard solution until the red color disappears, and record the volume V1;

[0034] (2) Add 3 drops of methyl red-bromocresol green indicator to the solution and titrate with 0.1 mol / L hydrochloric acid standard solution until the solution turns dark red. Record the volume V2.

[0035] Each experiment was repeated three times to obtain the average value. The volume of hydrochloric acid consumed by various phosphates is shown in Table 1:

[0036] Table 1 Volume of hydrochloric acid solution consumed by phosphate

[0037] category Sample mass (g) V1 V2 Sodium dihydrogen phosphate 0.7012 × × Disodium hydrogen phosphate 0.4047 × × Trisodium phosphate 0.4257 1.72 2.28 Sodium pyrophosphate 0.7026 1.22 3.6 Sodium tripolyphosphate 0.7314 0.52 3.1 Sodium Hexametaphosphate 0.7167 × × Disodium dihydrogen pyrophosphate 0.7096 × ×

[0038] The data in Table 1 show that among the 7 phosphates commonly used in compound water-retaining agents, 3 phosphates will consume hydrochloric acid solution, namely trisodium phosphate, sodium pyrophosphate and sodium tripolyphosphate, which will affect the determination of sodium carbonate and sodium bicarbonate content; while the other four phosphates will not consume phosphates and will not affect the determination of sodium carbonate and sodium bicarbonate content. Therefore, the above three phosphates will be mainly used in subsequent experiments.

[0039] Example 2 Effect of Phosphate on Sodium Carbonate Content Determination

[0040] The trisodium phosphate, sodium pyrophosphate and sodium tripolyphosphate determined in Example 1 were mixed with sodium carbonate respectively, and a hydrochloric acid titration experiment was performed to detect the influence level of different phosphates on the determination of sodium carbonate content; the specific method is as follows:

[0041] (1) Take 50 mL of decarbonated water in a beaker, add 3 drops of phenolphthalein indicator, and titrate with 0.1 mol / L hydrochloric acid standard solution until the red color disappears. Record the volume V0;

[0042] (2) Weigh a certain amount of trisodium phosphate, sodium pyrophosphate and sodium tripolyphosphate respectively, and put them in a beaker with an appropriate amount of sodium carbonate, add 50 mL of decarbonated water, add 3 drops of phenolphthalein indicator, and titrate with 0.1033 mol / L hydrochloric acid standard solution until the red color disappears, and record the volume V1;

[0043] Each experiment was repeated three times to obtain the average value, and the content of sodium carbonate in the sample was calculated. The results are shown in Table 2, and the calculation formula is:

[0044]

[0045] Table 2 The influence level of phosphate on the determination of sodium carbonate content

[0046] Sample composition Trisodium phosphate + sodium carbonate Sodium pyrophosphate + sodium carbonate Sodium tripolyphosphate + sodium carbonate Phosphate Quality 0.1117g 0.3024g 0.1252g Sodium carbonate quality 0.1552g 0.2315g 0.1192g total 0.2669g 0.5339g 0.2444g V0(mL) 0.02mL 0.02mL 0.02mL V1(mL) 17.72mL 24.50mL 11.72mL Hydrochloric acid concentration (mol / L) 0.1033mol / L 0.1033mol / L 0.1033mol / L Theoretical content of sodium carbonate 58.15％ 43.36％ 48.77％ Sodium carbonate content determination 72.62％ 50.21％ 52.42％ error 14.47％ 6.85％ 3.65％

[0047] The results are shown in Table 2. Trisodium phosphate, sodium pyrophosphate, and sodium tripolyphosphate all consume H+ during the titration process, resulting in the sodium carbonate content determination result being higher than the actual result; among them, trisodium phosphate has the highest error impact on the determination of sodium carbonate content, which is 14.47%; sodium tripolyphosphate has the lowest error impact on the determination of sodium carbonate content, which is 3.65%.

[0048] Example 3 Effect of Phosphate on Sodium Bicarbonate Content Determination

[0049] The trisodium phosphate, sodium pyrophosphate and sodium tripolyphosphate determined in Example 1 were mixed with sodium bicarbonate respectively, and a hydrochloric acid titration experiment was performed to detect the influence level of different phosphates on the determination of sodium bicarbonate content; the specific method is as follows:

[0050] (1) Take 50 mL of decarbonated water in a beaker, add 3 drops of phenolphthalein indicator, and titrate with 0.1033 mol / L hydrochloric acid standard solution until the red color disappears. Record the volume V0;

[0051] (2) Weigh a certain amount of trisodium phosphate, sodium pyrophosphate and sodium tripolyphosphate respectively, and put them in a beaker with an appropriate amount of sodium bicarbonate, add 50 mL of decarbonated water, add 3 drops of phenolphthalein indicator, and use 0.1 mol / L hydrochloric acid standard solution to titrate until the red color disappears, and record the volume V1;

[0052] Each experiment was repeated three times to take the average value and calculate the content of sodium bicarbonate in the sample. The results are shown in Table 3. The calculation formula is:

[0053]

[0054] Table 3 The influence level of phosphate on the determination of bicarbonate content

[0055] Sample composition Trisodium phosphate + sodium bicarbonate Sodium pyrophosphate + sodium bicarbonate Sodium tripolyphosphate + sodium bicarbonate Phosphate Quality 0.1477g 0.1521g 0.1635g Sodium bicarbonate quality 0.1259g 0.1352g 0.1234g total 0.2736g 0.2873g 0.2869g V0 0.02mL 0.02mL 0.02mL V2 18.12mL 20.57mL 17.24mL Hydrochloric acid concentration 0.1033mol / L 0.1033mol / L 0.1033mol / L Sodium bicarbonate actual content 46.02％ 47.06％ 43.01％ Sodium bicarbonate content 57.40％ 62.07％ 52.08％ error 11.38％ 15.01％ 9.07％

[0056] The results are shown in Table 3. Trisodium phosphate, sodium pyrophosphate, and sodium tripolyphosphate all consume H+ during the titration process, resulting in the sodium bicarbonate content determination result being higher than the actual result; among them, sodium tripolyphosphate has the lowest error effect on the determination of sodium bicarbonate content, which is 9.07%; sodium pyrophosphate has the highest error effect on the determination of sodium bicarbonate content, which is 15.01%.

[0057] Example 4 Phosphate content in composite water-retaining agent by ion chromatography

[0058] Ion chromatography was used to determine the type and content of phosphate in the compound water-retaining agent sample FP-1. The specific method is as follows:

[0059] (1) Accurately weigh 1.0034 g of sample into a beaker, add a small amount of distilled water, stir until completely dissolved, transfer to a 500 mL volumetric flask, take 5 mL of the solution and filter through a 0.45 μM filter, then inject into the ion chromatography sample bottle;

[0060] (2) A Thero AS11-HC anion analysis column was used with an injection volume of 0.25 mL, a flow rate of 1.5 mL / min, a temperature of 30°C, a suppressor current of 120 mA, and a gradient elution of 30 mM, 50 mM, and 90 mM KOH.

[0061] The external standard method was used to determine the phosphate in the compound water-retaining agent sample FP-1 by ion chromatography. Figure 1 The four blue Xs in B are on the same line, indicating that the four standards are all on the curve and have not shifted outside the line, indicating that the linearity of the curve meets the test requirements.

[0062] The results show that Figure 1-2and Table 4), the sodium pyrophosphate content in 1.0034g sample is 14.3478ppm, and the sodium tripolyphosphate content is 559.8429ppm. It can be calculated that the sodium pyrophosphate content in the sample is 0.71%, and the sodium tripolyphosphate content is 27.90%.

[0063] Table 4 Chromatographic results of phosphate content measurement in composite water-retaining agent sample FP-1

[0064]

[0065] In the table, sodium pyrophosphate means sodium pyrophosphate, poly means sodium tripolyphosphate, meta 1 means the first determination of sodium metaphosphate, and meta 2 means the second determination of sodium metaphosphate.

[0066] Example 5 National Standard Method for Determining the Content of Sodium Carbonate and Sodium Bicarbonate in the Composite Water Retention Agent

[0067] (1) Accurately weigh 0.6087 g of the compound water-retaining agent sample FP-1 into a small beaker, add 50 mL of decarbonated water, add 3 drops of phenolphthalein indicator, and titrate with 0.1033 mol / L hydrochloric acid standard solution until the red color disappears, and record the volume V3;

[0068] (2) Add 3 drops of methyl red-bromocresol green indicator to the solution after titration in step (1), and titrate with 0.1 mol / L hydrochloric acid standard solution. The solution turns dark red. When the second titration endpoint is reached, record the volume V4.

[0069] (3) Add 3 drops of phenolphthalein indicator to 50 mL of decarbonated water and titrate with 0.1033 mol / L hydrochloric acid standard solution until the red color disappears and record the volume V0;

[0070]

[0071]

[0072] The results are shown in Table 5:

[0073] Table 5 National standard method for determining the content of sodium carbonate and sodium bicarbonate

[0074]

[0075]

[0076] The results showed that the error in measuring the content of sodium carbonate and sodium bicarbonate in compound water-retaining agents using the national standard method was large, with the error of sodium carbonate reaching 3.93% and the error of sodium bicarbonate as high as 11.63%. This was extremely different from the actual content, seriously affecting the judgment of the content of sodium carbonate and sodium bicarbonate in the compound water-retaining agent, and seriously affecting the use of the compound water-retaining agent in subsequent experiments. Therefore, the national standard method is not suitable for measuring the content of sodium carbonate and sodium bicarbonate in compound water-retaining agents containing phosphates.

[0077] Example 6

[0078] The content of each substance in the mixture of sodium carbonate and sodium bicarbonate is determined by the national standard method, wherein the second indicator is replaced with a methyl orange indicator or a methyl red-bromocresol green indicator; the specific method is as follows:

[0079] (1) Accurately weigh 4 groups of sodium carbonate and sodium bicarbonate, mix them and place them in a beaker, add 50 mL of decarbonated water, add 3 drops of phenolphthalein indicator, and use 0.4893 mol / L hydrochloric acid standard solution to titrate until the red color disappears, and record the volume V1;

[0080] (2) adding 3 drops of methyl orange indicator or methyl red-bromocresol green indicator respectively, and then titrating with 0.4893 mol / L hydrochloric acid standard solution, and recording the volume V2 of the hydrochloric acid standard solution consumed when the second endpoint is reached; wherein groups 1 and 2 use methyl orange as the second indicator, and groups 3 and 4 use methyl red-bromocresol green as the indicator;

[0081] (3) Add 3 drops of phenolphthalein indicator to 50 mL of decarbonated water and titrate with 0.4893 mol / L hydrochloric acid standard solution until the red color disappears and record the volume V0;

[0082]

[0083]

[0084] The results are shown in Table 6:

[0085] Table 6 Determination of sodium carbonate and sodium bicarbonate content using different second indicators

[0086]

[0087]

[0088] As shown in Table 6, when methyl orange is used as the second indicator to measure the content of sodium carbonate and sodium bicarbonate, the error is above ±1%, while when methyl red-bromocresol green is used as the second indicator, the measurement error is around ±0.5%. This may be because when methyl orange is used as the indicator, the solution color changes from yellow to orange at the titration endpoint ( Figure 3 ), the color change is not obvious, and the determination of the titration endpoint is inaccurate; while when methyl red-bromocresol green is used as the indicator, the solution color changes from green to dark red at the titration endpoint ( Figure 4 ), the color change is obvious, and it is easy to determine the titration endpoint, so the measurement error is smaller. Therefore, methyl red-bromocresol green indicator is used as the second indicator in subsequent experiments.

[0089] Example 7 Titration method for measuring the volume of hydrochloric acid standard solution consumed by phosphate in a sample

[0090] (1) According to the contents of sodium pyrophosphate and sodium tripolyphosphate measured in Example 4, weigh 0.0071 g of pyrophosphate and 0.2790 g of sodium tripolyphosphate respectively into a beaker, add 50 mL of decarbonated water, add 3 drops of phenolphthalein indicator, and titrate with 0.1 mol / L hydrochloric acid standard solution until the red color disappears, and record the volume V1;

[0091] (2) Add another 3 drops of methyl red-bromocresol green indicator and titrate with 0.1 mol / L hydrochloric acid standard solution until the solution turns dark red. When the second titration endpoint is reached, record the volume V2.

[0092] Repeat three times and take the average value. The titration results are shown in Table 7:

[0093] Table 7 Volume of hydrochloric acid standard solution consumed by phosphate

[0094]

[0095] Example 8 Titration method for measuring the content of sodium bicarbonate and sodium carbonate in the composite water-retaining agent

[0096] The contents of sodium bicarbonate and sodium carbonate in the compound water-retaining agent sample FP-1 were measured by titration using a phenolphthalein indicator and a methyl red-bromocresol green indicator using a hydrochloric acid standard solution. The specific steps are as follows:

[0097] (1) Accurately weigh 0.6087 g of the compound water-retaining agent sample FP-1 into a small beaker, add 50 mL of decarbonated water, add 3 drops of phenolphthalein indicator, and titrate with 0.1033 mol / L hydrochloric acid standard solution until the red color disappears, and record the volume V3;

[0098] (2) Add methyl red-bromocresol green indicator and titrate with 0.1033 mol / L hydrochloric acid standard solution. The solution turns dark red. When the second titration endpoint is reached, record the volume V4.

[0099] Repeat three times and take the average value. The titration results are shown in Table 8:

[0100] Table 8 Volume of hydrochloric acid standard solution consumed by composite water-retaining agent sample FP-1

[0101]

[0102] The following calculation formula is used to calculate the content of sodium carbonate and sodium bicarbonate. The calculation results are shown in Table 8:

[0103]

[0104]

[0105] Table 9 Content and error of sodium carbonate and sodium bicarbonate

[0106]

[0107] The results showed that the qualitative and quantitative analysis of phosphate in the compound water-retaining agent by ion chromatography and then the measurement of the sodium carbonate and sodium bicarbonate contents by standard solution titration-double indicator method could significantly reduce the measurement error and obtain more accurate measurement results. The error of sodium carbonate could be controlled within about ±0.1%, and the measurement error of sodium bicarbonate could be controlled within about ±1%.

Claims

1. A method for eliminating phosphate interference in determining the content of sodium carbonate and sodium bicarbonate in a composite water-retaining agent, characterized in that: The method combines ion chromatography and standard solution titration-double indicator method to determine the content of sodium bicarbonate and sodium carbonate in the compound water-retaining agent; the method comprises the following steps: (1) Sample preparation: Accurately weigh 0.5-1.0 g (accurate to 0.001 g) of sample and dissolve it in distilled water in a beaker, and dilute to 500 mL to obtain a sample solution; (2) Phosphate analysis: Load the sample solution into an ion chromatograph to conduct qualitative and quantitative analysis of the phosphate therein to determine the type and content of the phosphate; (3) Phosphate primary titration: Accurately weigh an equal amount of phosphate from step (2) into a beaker, add decarbonated water, add phenolphthalein indicator and titrate with standard solution until the red color disappears, and record the volume V1; (4) Phosphate secondary titration: Add methyl red-bromocresol green indicator to the solution after titration in step (3), and titrate with standard solution until the solution turns dark red. When the second titration endpoint is reached, record the volume V2; (5) Sample primary titration: Accurately weigh 0.5-1g of sample into a beaker, add decarbonated water, add phenolphthalein indicator and titrate with standard solution until the red color disappears, record the volume V3; (6) Secondary titration of the sample: Add methyl red-bromocresol green indicator to the solution after titration in step (5), and titrate with the standard solution until the solution turns dark red. When the second titration endpoint is reached, record the volume V4; (7) Calculation of results: The following formula is used to calculate the content of sodium bicarbonate and sodium carbonate in the sample: ； ； Where: C (HCl) is the concentration of the standard solution, in mol / L; M (Na2CO3) is the molar mass of sodium carbonate, in g / mol; M (NaHCO3) is the molar mass of sodium bicarbonate, in g / mol; m is the mass of the sample in g.

2. A method for eliminating phosphate interference in determining the content of sodium carbonate and sodium bicarbonate in a composite water-retaining agent according to claim 1, characterized in that: The ion chromatograph described in step (2) uses an anion analysis column with a column flow rate of 1.5 mL / min.

3. A method for eliminating phosphate interference in determining the content of sodium carbonate and sodium bicarbonate in a composite water-retaining agent according to claim 1, characterized in that: The standard solution in steps (3) to (6) is a 0.1 mol / L hydrochloric acid standard solution.

4. A method for eliminating phosphate interference in determining the content of sodium carbonate and sodium bicarbonate in a composite water-retaining agent according to claim 1, characterized in that: The phenolphthalein indicator in steps (3) and (5) has a concentration of 1 g / L, and the volume ratio of the indicator to the solution is 0.1-0.2:

50.

5. A method for eliminating phosphate interference in determining the content of sodium carbonate and sodium bicarbonate in a composite water-retaining agent according to claim 1, characterized in that: The concentrations of the methyl red-bromocresol green indicator in steps (4) and (6) are 1 g / L methyl red and 1 g / L bromocresol green, with a volume ratio of methyl red to bromocresol green of 1:3; and a volume ratio of the indicator to the solution of 0.1-0.2:50.

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