Method for detecting content of citric acid in citric acid gypsum

Abstract

The present application belongs to the field of citric acid production and detection, and specifically provides a method for detecting the content of citric acid in citric acid gypsum, which adopts sodium carbonate to react with citric acid gypsum to form a sodium sulfate solution and a calcium carbonate precipitate, and in the reaction process, all the citric acid embedded in the citric acid gypsum is released, and then the citric acid is detected by 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 accuracy of the citric acid content detection is improved by about 30%.

Description

Technical Field

[0001] This invention belongs to the field of citric acid production and testing, and specifically relates to a method for detecting the citric acid content in citric acid gypsum. Background Technology

[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, which contains citric acid. As an industrial waste residue, citric acid gypsum can be used in building materials, the cement industry, and environmental protection, realizing the resource utilization of waste and possessing significant economic and environmental benefits.

[0003] Testing the citric acid content in citric acid gypsum serves two purposes: firstly, to assess whether citric acid has undergone chemical changes during production or processing, and to understand its transformation, providing data for optimizing the production process; secondly, because citric acid acts as a retarder and can affect the performance of subsequent citric acid gypsum products, testing its content is crucial for ensuring its effective and safe application in construction, industry, and other fields.

[0004] The existing method for detecting citric acid content in citric acid gypsum is acid-base titration. However, this method can only detect water-soluble citric acid in citric acid gypsum, and cannot detect citric acid embedded within the gypsum, leading to inaccurate results. Therefore, how to more accurately detect the citric acid content in citric acid gypsum is a pressing technical problem to be solved in this field. Summary of the Invention

[0005] To address the problems existing in the prior art, this invention provides a method for detecting citric acid content in citric acid gypsum. The method involves reacting sodium carbonate with citric acid gypsum to form a sodium sulfate solution and a calcium carbonate precipitate. During the reaction, all the citric acid embedded in the citric acid gypsum is released. The citric acid is then detected using high-performance liquid chromatography (HPLC), resulting in more accurate and effective detection results.

[0006] The citric acid gypsum targeted in this invention is a byproduct of the citric acid production process. The calcium citrate in the production process has been completely reacted after continuous acid hydrolysis and water washing, so the final citric acid gypsum does not contain calcium citrate.

[0007] The specific technical solution adopted in this invention is as follows:

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

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

[0010] The amount of sodium carbonate used must be greater than the theoretically calculated value for complete reaction with citric acid gypsum. Excess sodium carbonate can make the reaction complete. The heating temperature is 70-80℃.

[0011] (2) Sample solution preparation: After the reaction is completed, filter with filter paper. After filtration, rinse the precipitate and beaker with deionized water in small amounts several times to obtain 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 the test solution.

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

[0013] (3) Preparation of standard solutions: Accurately weigh 0.10 g of citric acid standard, dissolve and dilute to 100 mL with mobile phase, 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 at pH 2 and methanol (95:5, V / V).

[0015] (4) Preparation of standard curve: Inject the standard solutions into the high performance liquid chromatograph and plot the standard curve with citric acid concentration as the abscissa and the corresponding peak area as the ordinate.

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

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

[0018] Compared with the prior art, the inventors optimized the chromatographic conditions as follows to suit the test system of the present invention:

[0019] ① In this invention, the separation effects of water-methanol, water-acetonitrile, and 30 mmol sodium dihydrogen phosphate solution-methanol as mobile phases were compared. Using water-methanol and water-acetonitrile systems, it was found that the citric acid peak shape was asymmetrical and prone to tailing. However, using the 30 mmol sodium dihydrogen phosphate solution-methanol system, the peak symmetry of citric acid met the requirements, and its resolution, sensitivity, and the peak shape of the target compound were superior to other mobile phase systems. Furthermore, experiments were conducted at different flow rates and column temperatures, revealing that flow rate and column temperature had minimal impact on the citric acid peak shape. Therefore, this invention uses 30 mmol sodium dihydrogen phosphate solution-methanol as the mobile phase, along with a more suitable flow rate and column temperature.

[0020] ② The chromatographic conditions of this invention are adjusted based on the high-performance liquid chromatography (HPLC) method for detecting citric acid content, making them more suitable for detecting low citric acid content. While maintaining a relatively constant volume ratio of sodium dihydrogen phosphate solution to methanol, the pH of the mobile phase is adjusted with phosphoric acid. When the mobile phase pH = 2, the peak shape and resolution of citric acid are more stable.

[0021] Compared with existing technologies, the method for detecting citric acid content in citric acid gypsum provided by this invention releases all the citric acid embedded in the citric acid gypsum by reacting sodium carbonate with it. The citric acid is then detected using high-performance liquid chromatography (HPLC), resulting in more accurate citric acid content detection. Thanks to the high precision and sensitivity of HPLC, this method has higher detection accuracy and smaller errors. Compared with traditional acid-base titration methods, the accuracy of citric acid content detection is improved by approximately 30%. Attached Figure Description

[0022] Figure 1 This is the standard curve for citric acid in Example 1;

[0023] Figure 2 This is the high-performance liquid chromatogram of the citric acid standard from Example 1;

[0024] Figure 3The high-performance liquid chromatogram of citric acid gypsum 1# in Example 1 is shown below.

[0025] Figure 4 This is the high-performance liquid chromatogram of citric acid gypsum 2# from Example 1. Detailed Implementation

[0026] The present invention will be further described below with reference to specific embodiments, which will enable those skilled in the art to have a more comprehensive understanding of the invention, but will not limit the invention in any way. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0027] Example 1

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

[0029] (1) Sample preparation: Take two batches of citric acid gypsum samples from the workshop and name them citric acid gypsum 1# and citric acid gypsum 2# respectively. Make parallel samples for each batch. Accurately weigh 10g of citric acid gypsum (containing water) from the workshop. Through calculation, weigh 5.6g of sodium carbonate and place it in a beaker. Dissolve it completely in deionized water to form a nearly saturated solution. Heat it to 80℃, add the citric acid gypsum sample to the sodium carbonate solution, and stir.

[0030] (2) Sample solution preparation: The reaction was observed to be complete by microscopic observation of the transformation of citric acid gypsum from flake crystal form to granular crystal form. The solution was filtered with medium-speed filter paper. After filtration, the calcium carbonate and beaker were rinsed with deionized water in small amounts several times to obtain the filtrate. An appropriate amount of 240 g / L hydrochloric acid was added to the filtrate to adjust the pH of the solution to 2-5. The solution was then transferred to a 100 mL volumetric flask and diluted to volume to obtain the test solution. After filtration through a 0.22 μm organic filter membrane, the solution was ready for use.

[0031] (3) Preparation of standard solutions: Accurately weigh 0.10 g of citric acid standard, dissolve and dilute to 100 mL with the mobile phase, 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: The determination was performed under the chromatographic conditions described above. A standard curve was plotted with citric acid concentration on the x-axis and the corresponding peak area on the y-axis. (See figure...) Figure 1 The results showed that the citric acid content exhibited a good linear relationship within a certain range.

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

[0034] (6) Precision Experiment: The high-performance liquid chromatography (HPLC) method of this invention was validated, and a precision experiment was conducted. A citric acid standard sample solution (40 mg / L) was filtered through a 0.22 μm organic filter membrane and analyzed using the HPLC method under the specified chromatographic conditions. The analysis was repeated seven times to determine the citric acid content. The RSD was 1.42%, indicating that the instrument's precision and the detection method were effective. The results are shown in Table 1.

[0035] Table 1. Precision Experiment Results

[0036]

[0037] (7) Spike recovery experiment: The high performance liquid chromatography method of the present invention was validated by a spike recovery experiment. Following the sample preparation procedure, an appropriate amount of citric acid was added, and the citric acid content was determined according to the sample determination method. Each concentration level was measured twice individually. The results are shown in Table 2. The citric acid recovery rate was 96.6%–101.5%, indicating that the sample recovery rate determined by this method is high, stable, and reliable.

[0038] Table 2 Spike Recovery Experiment Data

[0039]

[0040] Comparative Example 1

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

[0042] 10g of citric acid gypsum was weighed, diluted with the mobile phase and brought to a final volume of 100mL without any pretreatment. The solution was then filtered through a 0.22μm organic filter membrane and analyzed. The results are shown in Table 3.

[0043] Table 3. Results of Citric Acid Content Detection

[0044]

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

[0046] Comparative Example 2

[0047] The citric acid content of the two batches of citric acid gypsum samples was directly determined 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 as follows:

[0049]

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

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

[0052] c: Concentration of sodium hydroxide standard titrant, 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 was processed according to the pretreatment method of this invention, and the citric acid content was detected by the acid-base titration method described above. The test results are shown in Table 4.

[0056] Table 4. Results of Citric Acid Content Detection

[0057]

[0058] The method for detecting citric acid content in citric acid gypsum provided by this invention 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 (HPLC). Compared with direct detection of citric acid gypsum, the detection result of citric acid content is more accurate and effective. At the same time, the results show that the accuracy of HPLC is higher and the error is smaller.

[0059] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any equivalent substitutions or modifications made by those skilled in the art within the spirit and principle of the present invention without any creative effort should be included within 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, dissolve it fully with deionized water to form a near-saturated solution, heat it to a certain temperature, add citric acid gypsum, stir, citric acid gypsum decomposes during the reaction, and the embedded citric acid is released. (2) Sample solution preparation: After the reaction is completed, filter with filter paper, and rinse the precipitate and beaker with deionized water in small amounts several times to obtain 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 the test solution; The endpoint of the reaction in step (2) is determined by observing the crystal form under a microscope: the citric acid gypsum changes from a flaky crystal form to a granular crystal form; medium-speed filter paper is used, and the hydrochloric acid concentration is 240 g / L; (3) Preparation of standard solutions: Accurately weigh 0.10 g of citric acid standard, dissolve and dilute to 100 mL with mobile phase, 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 high performance liquid chromatograph and plot the standard curve with the citric acid concentration as the abscissa and the corresponding peak area as the ordinate. The mobile phase in steps (3) and (4) is a mixture of 30 mmol sodium dihydrogen phosphate solution at pH=2 and methanol in a volume ratio of 95:5; (5) Sample determination: The test solution is injected into the high performance liquid chromatograph to obtain the peak area of ​​citric acid, and the concentration of citric acid in the test solution is obtained according to the standard curve; The conditions for the high performance liquid chromatograph were as follows: C18 column, UV detector, wavelength 210 nm, isocratic elution, flow rate 0.6 mL / min, column temperature 30 ℃, and injection volume 20 μL.

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

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

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