Method for detecting content of citric acid in citric gypsum

Abstract

The invention belongs to the field of citric acid production and detection, and particularly provides a method for detecting the content of citric acid in citric acid gypsum, which comprises the following steps: reacting sodium carbonate with citric acid gypsum to form a sodium sulfate solution and a calcium carbonate precipitate, and completely releasing citric acid embedded in the citric acid gypsum in the reaction process. And then the citric acid is detected by using a high performance liquid chromatograph, so that the detection result is more accurate and effective, and compared with the traditional acid-base titration detection method, the citric acid content detection accuracy is improved by about 30%.

Description

Technical Field

[0001] The invention belongs to the field of citric acid production and detection, and particularly relates to a method for detecting the citric acid content in citric acid gypsum. Background Art

[0002] Citric acid gypsum is a byproduct of the reaction between precipitated calcium citrate and sulfuric acid during the production of citric acid. Its main component is calcium sulfate dihydrate and contains citric acid. As an industrial waste residue, citric acid gypsum can be used in building materials, cement industry and environmental protection fields, which can realize the resource utilization of waste and has important economic and environmental benefits.

[0003] The purpose of testing the citric acid content in citric acid gypsum is, on the one hand, to evaluate whether chemical changes have occurred in the citric acid during the production or processing process and to understand the conversion of citric acid as a data reference for optimizing the production process; on the other hand, citric acid, as a retarder, will affect the performance of subsequent products of citric acid gypsum, so testing the citric acid content is crucial to ensure its effective and safe application in construction, industry and other fields.

[0004] The detection method of citric acid content in citric acid gypsum in the prior art is acid-base titration, but this method can only detect the content of citric acid soluble in water in citric acid gypsum, and cannot detect the citric acid embedded in the citric acid gypsum, resulting in inaccurate final detection results. Therefore, how to further accurately detect the citric acid content in citric acid gypsum is a technical problem to be solved in the art. Summary of the invention

[0005] In order to solve the problems existing in the above-mentioned prior art, the present invention provides a method for detecting the citric acid content in citric acid gypsum, wherein sodium carbonate is reacted with the citric acid gypsum to form a sodium sulfate solution and a calcium carbonate precipitate, and during the reaction process, all the citric acid embedded in the citric acid gypsum is released, and the citric acid is detected by a high performance liquid chromatography, so that the detection result is more accurate and effective.

[0006] The citric acid gypsum targeted by the present invention is a by-product in the production process of citric acid. The calcium citrate in the production process has been completely reacted after continuous acid hydrolysis and water washing, so the citric acid gypsum finally obtained does not contain calcium citrate components.

[0007] The technical solution adopted by the present invention is specifically as follows:

[0008] A method for detecting the content of citric acid in citric acid gypsum, comprising the following specific steps:

[0009] (1) Sample preparation: Accurately weigh a certain amount of citric acid gypsum, and by calculation, weigh a certain amount of sodium carbonate and place it in a beaker. Dissolve it fully with deionized water to form a nearly saturated solution. After heating to a certain temperature, add citric acid gypsum and stir. The citric acid gypsum decomposes during the reaction and the embedded citric acid is released. The reaction mechanism is: CaSO4+Na2CO3→CaCO3↓+Na2SO4;

[0010] Among them, the amount of sodium carbonate used must be greater than the theoretical calculated value for complete reaction with citric acid gypsum. Excessive sodium carbonate can make the reaction complete, and the heating temperature is 70-80°C.

[0011] (2) Preparation of sample solution: After the reaction is completed, filter with filter paper, rinse the precipitate and beaker with deionized water several times in small amounts to obtain a filtrate; add a certain amount of hydrochloric acid to the filtrate to neutralize the excess sodium carbonate, adjust the pH of the solution to 2-5, transfer to a volumetric flask and make up to volume to obtain a solution to be tested.

[0012] The end point after the completion of the reaction is determined by observing the crystal form under a microscope: the citric acid gypsum is transformed from a flake crystal form to a granular crystal form; medium-speed filter paper is used as the filter paper, and the hydrochloric acid concentration is 240g / L.

[0013] (3) Preparation of standard solutions: Accurately weigh 0.10 g of citric acid standard, dissolve it in mobile phase to make up to 100 mL, shake well, and prepare a 1 g / L citric acid standard stock solution. Then prepare a series of citric acid standard solutions of 10 mg / mL, 20 mg / mL, 40 mg / mL, 80 mg / mL, and 160 mg / mL in sequence;

[0014] The mobile phase used was a mixture of 30 mmol sodium dihydrogen phosphate solution with a pH of 2 and methanol (95:5, V / V).

[0015] (4) Preparation of standard curve: Inject the standard solutions into the HPLC, draw the standard curve with the citric acid concentration as the abscissa and the corresponding peak area as the ordinate;

[0016] (5) Sample determination: The sample solution is injected into a high performance liquid chromatograph to obtain the corresponding citric acid peak area, and the concentration of citric acid in the test solution is obtained according to the standard curve;

[0017] Among them, the conditions of the high performance liquid chromatography are as follows: the chromatographic column is a C18 column (Waters Atlantis dC18, size is 4.6mm×150mm, 5.0μm), the detector is an ultraviolet detector, the wavelength is 210nm, isocratic elution is adopted, the flow rate is 0.6mL / min, the column temperature is 30°C, the injection volume is 20μL, and the mobile phase is a mixture of 30mmol sodium dihydrogen phosphate solution and methanol (95:5, V / V) at pH=2.

[0018] Compared with the prior art, the inventors have optimized the chromatographic conditions as follows to adapt to the conditions of the system to be tested in the present invention:

[0019] ① The present invention compares the separation effects of water-methanol, water-acetonitrile, and 30mmol sodium dihydrogen phosphate solution-methanol as mobile phases when selecting the mobile phase. When the water-methanol and water-acetonitrile systems are used, it is found that the citric acid peak type is asymmetric and easy to tail; when the 30mmol sodium dihydrogen phosphate solution-methanol system is used, the symmetry of the citric acid peak type meets the requirements, and the separation degree, sensitivity, and peak type of the target compound are better than other mobile phase systems. At the same time, the present invention experiments with different flow rates and column temperatures, and finds that the flow rate and column temperature have little effect on the citric acid peak type, so the present invention uses 30mmol sodium dihydrogen phosphate solution-methanol as the mobile phase, as well as a relatively moderate flow rate and column temperature.

[0020] ② The chromatographic conditions of the present invention are adjusted on the basis of the HPLC detection of citric acid content, making it more suitable for low citric acid content detection. Under the premise of keeping the volume ratio of sodium dihydrogen phosphate solution and methanol basically unchanged, the pH value of the mobile phase is adjusted with phosphoric acid. When the mobile phase pH = 2, the peak shape and separation degree of citric acid are more stable.

[0021] Compared with the prior art, the method for detecting the citric acid content in citric acid gypsum provided by the present invention uses sodium carbonate to react with citric acid gypsum to release all the citric acid embedded in the citric acid gypsum, and uses a high performance liquid chromatograph to detect the citric acid, so that the detection result of the citric acid content is more accurate. Thanks to the high precision and high sensitivity of high performance liquid chromatography, the method has higher detection accuracy and smaller error. Compared with the traditional acid-base titration detection method, the detection accuracy of the citric acid content is improved by about 30%. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The standard curve of citric acid in Example 1;

[0023] Figure 2 is the HPLC chromatogram of the citric acid standard product of Example 1;

[0024] Figure 3This is the high performance liquid chromatogram of citric acid gypsum 1# in Example 1;

[0025] Figure 4 This is the high performance liquid chromatogram of citric acid gypsum 2# in Example 1. DETAILED DESCRIPTION

[0026] The present invention will be further described below in conjunction with specific embodiments, so that those skilled in the art can understand the present invention more comprehensively, but the present invention is not limited in any way. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in the field without creative work are within the scope of protection of the present invention.

[0027] Example 1

[0028] A method for detecting the content of citric acid in citric acid gypsum, comprising the following specific steps:

[0029] (1) Sample preparation: Take two batches of citric acid gypsum samples from the workshop, named citric acid gypsum 1# and citric acid gypsum 2#, and make parallel samples for each batch; accurately weigh 10g (water-containing) citric acid gypsum from the workshop. By calculation, weigh 5.6g of sodium carbonate and place it in a beaker, dissolve it completely with deionized water to form a nearly saturated solution, heat it to 80°C, add the citric acid gypsum sample to the sodium carbonate solution, and stir.

[0030] (2) Preparation of sample solution: observe under a microscope that the citric acid gypsum changes from a flaky crystal form to a granular crystal form, and the reaction is complete. Filter with medium-speed filter paper. After filtration, rinse the reacted calcium carbonate and beaker with deionized water in small amounts and multiple times to obtain a filtrate; add an appropriate amount of 240g / L hydrochloric acid to the filtrate, adjust the solution pH to 2-5, transfer to a 100mL volumetric flask to make up the volume, obtain a test solution, filter through a 0.22μm organic filter membrane, and put it on the machine for use.

[0031] (3) Preparation of standard solutions: Accurately weigh 0.10 g of citric acid standard, dissolve it in mobile phase to make up to 100 mL, shake well, and prepare a 1 g / L citric acid standard stock solution. Then prepare a series of citric acid standard solutions of 10 mg / L, 20 mg / L, 40 mg / L, 80 mg / L, and 160 mg / L in sequence.

[0032] (4) Standard curve determination: According to the above chromatographic conditions, the standard curve is drawn with the citric acid concentration as the horizontal axis and the corresponding peak area as the vertical axis. Figure 1 The results showed that the citric acid content had a good linear relationship within a certain range.

[0033] (5) Sample determination: The citric acid content in the test solution was detected by high performance liquid chromatography. The conditions of the high performance liquid chromatography were as follows: the chromatographic column was a C18 column, the column temperature was 30°C, the flow rate was 0.6 mL / min, the injection volume was 20 μL, and the mobile phase was a mixture of 30 mmol sodium dihydrogen phosphate solution and methanol (95:5, V / V) at pH = 2. The chromatogram of the standard is shown in Figure 2 The chromatogram of the actual sample is shown in Figure 3 , Figure 4 In the figure, oxalic acid is a trace amount of organic acid produced during the fermentation of citric acid. Its peak time is significantly different from that of citric acid and does not affect the detection results of citric acid.

[0034] (6) Precision experiment: The methodological verification of the high performance liquid chromatography method of the present invention was carried out, and a precision experiment was carried out. A citric acid standard sample solution (40 mg / L) was taken, passed through a 0.22 μm organic filter membrane, and tested on the machine according to the sample chromatography conditions. The test was repeated 7 times to determine the citric acid content. The RSD was 1.42%, indicating that the precision of the instrument and the detection method were good. The results are shown in Table 1.

[0035] Table 1 Precision test results

[0036]

[0037] (7) Spiked recovery experiment: The methodological verification of the high performance liquid chromatography of the present invention was carried out, and the spiked recovery experiment was carried out. According to the sample preparation operation method, an appropriate amount of citric acid was added, and the citric acid content was detected according to the sample determination method. Each concentration level was measured twice. The results are shown in Table 2. The citric acid recovery rate is 96.6% to 101.5%, indicating that the sample recovery rate determined by this method is high, stable and reliable.

[0038] Table 2 Spike recovery experimental data

[0039]

[0040] Comparative Example 1

[0041] The citric acid content of the two batches of citric acid gypsum samples was determined by direct method. The specific steps are as follows:

[0042] 10 g of citric acid gypsum was weighed, diluted with mobile phase to 100 mL without treatment, filtered through a 0.22 μm organic filter membrane, and tested on the machine. The results are shown in Table 3.

[0043] Table 3 Citric acid content test results

[0044]

[0045] It can be seen from Comparative Example 1 that the detection amount of citric acid in the citric acid gypsum treated by the pretreatment method of the present invention is increased by nearly 30%. This significant improvement shows that the pretreatment method of the present invention can more effectively release and extract citric acid in the citric acid gypsum, thereby improving the accuracy and reliability of the detection.

[0046] Comparative Example 2

[0047] The citric acid content of the two batches of citric acid gypsum samples was directly tested by acid-base titration. The specific steps are as follows:

[0048] Accurately weigh 10g of citric acid gypsum, add 150mL of deionized water and mix thoroughly, use phenolphthalein solution as an indicator, and titrate with 0.1mol / L sodium hydroxide solution. The specific calculation formula is:

[0049]

[0050] V1: Volume of sodium hydroxide standard titrant consumed in sample titration, mL;

[0051] V0: Volume of sodium hydroxide standard titrant consumed in blank titration, mL;

[0052] c: concentration of sodium hydroxide standard titration solution, mol / L;

[0053] 0.06404: the number of grams of anhydrous citric acid equivalent to 1.00 mL of sodium hydroxide [c(NaOH) = 1.000 mol / L];

[0054] m: sample mass, g.

[0055] Another batch of citric acid gypsum samples were taken and treated according to the pretreatment method of the present invention, and then the citric acid content was detected by the above-mentioned acid-base titration method. The test results are shown in Table 4:

[0056] Table 4 citric acid content test results

[0057]

[0058] The method for detecting the content of citric acid in citric acid gypsum provided by the present invention is used, and the detection result shows that: by reacting sodium carbonate with citric acid gypsum, all the citric acid embedded in the citric acid gypsum is released, and then the citric acid is detected by high performance liquid chromatography. Compared with direct detection of citric acid gypsum, the detection result of the citric acid content is more accurate and effective. At the same time, it can be seen from the result that the high performance liquid chromatography method has higher accuracy and smaller error.

[0059] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any equivalent substitutions, modifications, etc. made by technicians in this field without any creative work within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for detecting the citric acid content in citric acid gypsum, characterized in that: The specific steps are as follows: (1) Sample preparation: Accurately weigh citric acid gypsum, weigh a certain amount of sodium carbonate and place it in a beaker, fully dissolve it with deionized water to form a nearly saturated solution, heat it to a certain temperature, add citric acid gypsum, stir, and the citric acid gypsum will decompose during the reaction, and the embedded citric acid will be released; (2) Sample solution preparation: After the reaction is completed, filter with filter paper, rinse the precipitate and the beaker with deionized water several times in small amounts to obtain a filtrate; add a certain amount of hydrochloric acid to the filtrate to neutralize the excess sodium carbonate, adjust the pH of the solution to 2-5, transfer to a volumetric flask and make up to volume to obtain a solution to be tested; (3) Preparation of standard solutions: Accurately weigh 0.10 g of citric acid standard, dissolve it in mobile phase to make up to 100 mL, shake well, and prepare a 1 g / L citric acid standard stock solution. Then prepare a series of citric acid standard solutions of 10, 20, 40, 80, and 160 mg / mL in sequence. (4) Preparation of standard curve: Inject the standard solutions into the HPLC, draw the standard curve with the citric acid concentration as the abscissa and the corresponding peak area as the ordinate; (5) Sample determination: The sample solution is injected into a high performance liquid chromatograph to obtain the corresponding peak area of ​​citric acid, and the concentration of citric acid in the test solution is obtained according to the standard curve; The conditions of the high performance liquid chromatography are as follows: the chromatographic column is a C18 column, the detector is an ultraviolet detector, the wavelength is 210 nm, isocratic elution is adopted, the flow rate is 0.6 mL / min, the column temperature is 30°C, and the injection volume is 20 μL.

2. The method for detecting the citric acid content in citric acid gypsum according to claim 1, characterized in that: The heating temperature in step (1) is 70-80°C.

3. The method for detecting the citric acid content in citric acid gypsum according to claim 1, characterized in that: The end point after the reaction in step (2) is completed is determined by observing the crystal form under a microscope: the citric acid gypsum is transformed from a sheet crystal form to a granular crystal form; the filter paper is a medium-speed filter paper, and the hydrochloric acid concentration is 240 g / L.

4. The method for detecting the citric acid content in citric acid gypsum according to claim 1, characterized in that: The mobile phase in steps (3) and (4) is a mixture of 30 mmol sodium dihydrogen phosphate solution at pH = 2 and methanol (95:5, V / V).

5. The method for detecting the citric acid content in citric acid gypsum according to claim 1, characterized in that: The C18 column in step (4) has a size of 4.6 mm×150 mm and a column diameter of 5.0 μm.

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