Method for determining total phosphorus in water

Abstract

The invention relates to the technical field of total phosphorus detection, in particular to a method for determining total phosphorus in water, which comprises the following steps: 1) digestion treatment: adding an oxidant into a water sample to be detected, and then performing microwave digestion treatment to obtain digested water; a molybdate reagent is added into the digested water, then an improved ascorbic acid solution is added, a chromogenic reaction is completed at the set temperature, and the improved ascorbic acid solution is obtained by mixing an ascorbic acid solution and sodium ethylene diamine tetracetate; measuring the concentration: measuring the absorbance value of the obtained color development product by using a spectrophotometer and a long-optical-path cuvette; and drawing a standard curve: using a phosphorus standard solution as a quality control sample, adding a plurality of calibration points in a low-concentration interval of 0.0-0.1 mg / L, carrying out colorimetric determination, and establishing quadratic equation fitting according to absorbance and concentration so as to determine the total phosphorus content of the water sample to be detected, and the method has the advantages of high sensitivity and good accuracy in total phosphorus testing.

Description

Technical Field

[0001] This application relates to the technical field of total phosphorus detection, and more specifically, to a method for determining total phosphorus in water quality. Background Art

[0002] GB / T11893 - 1989 is the current national standard for the determination of total phosphorus in water quality in China. Based on the ammonium molybdate spectrophotometry, it is applicable to the detection of total phosphorus in surface water, sewage and industrial wastewater. Using the ammonium molybdate spectrophotometry, the detection process is as follows: Under neutral conditions, at a temperature of 120 °C and a pressure of 1.1 kg / cm 2 environment, potassium persulfate or nitric acid - perchloric acid is used to digest the water sample to oxidize various phosphorus compounds into orthophosphate; orthophosphate reacts with ammonium molybdate in the presence of an antimony salt catalyst to form phosphomolybdic heteropolyacid, which is then reduced by ascorbic acid to a blue complex. The color development time is 15 minutes (room temperature ≥ 13 °C), and the absorbance is measured at a wavelength of 700 nm. A phosphorus standard solution (2.0 μg / mL) is used to prepare a gradient concentration of 0.0 - 30.0 μg, a linear relationship is established between the absorbance and the concentration, and finally colorimetric determination is carried out.

[0003] The minimum detection concentration of GB / T11893 - 1989 (ammonium molybdate spectrophotometry) is 0.01 mg / L, and the quantification limit is 0.04 mg / L. However, in actual applications, the requirements for Class I lakes and reservoirs in the "Surface Water Environment Quality Standard" GB3838 - 2002 are 0.01 mg / L, and for Class II lakes and reservoirs are 0.025 mg / L, both of which are lower than the quantification limit of 0.04 mg / L. Because it is difficult to ensure that the accuracy and precision of the data between the detection limit and the quantification limit meet the requirements of the corresponding recovery rate and relative deviation, the existing detection standards have problems such as insufficient sensitivity, and it is difficult to meet the requirements of accuracy and precision in this range. Summary of the Invention

[0004] In order to improve the problems of insufficient sensitivity, large errors in accuracy and precision in the determination of total phosphorus in water quality, this application provides a method for determining total phosphorus in water quality.

[0005] A method for determining total phosphorus in water quality includes the following steps: Detection of the absorbance value of the water sample to be measured: 1) Digestion treatment: An oxidant is added to the water sample to be measured, and then microwave digestion treatment is carried out to obtain digested water; 2) Color development treatment: A molybdate reagent is added to the digested water, and then a modified ascorbic acid solution is added, and the color development reaction is completed by maintaining a predetermined time at a set temperature. Among them, the modified ascorbic acid solution is obtained by mixing an ascorbic acid solution and sodium ethylenediaminetetraacetate; Test concentration: The absorbance value of the colored product obtained is measured using a spectrophotometer with a long optical path colorimetric cell. At the same time, a chromaticity interference agent is used to deduct the background absorbance. The chromaticity interference agent is composed of a blank sample and a compensating solution added. The compensating solution is composed of sulfuric acid and ascorbic acid solution; Standard curve drawing: Using a phosphorus standard solution as a quality control sample, multiple calibration points are added in the low concentration range of 0.0 - 0.1 mg / L, and then colorimetric determination is carried out. A quadratic equation fitting is established with absorbance and concentration to determine the total phosphorus content of the water sample to be tested.

[0006] By adopting the above technical solutions, the sensitivity and accuracy of low-concentration total phosphorus detection are greatly improved, and the efficiency is also improved. The detection limit is reduced from the original 0.01 mg / L to 0.005 mg / L, significantly enhancing the detection sensitivity; the spike recovery rate is increased from 85% to 98%, indicating that the method has stronger anti-interference ability; the total phosphorus detection error is reduced from 15% to 5%, ensuring the high reliability of the data. In addition, through the optimization of the color reaction system and the improvement of the standard curve drawing method, the accuracy of the measurement results is further improved. The specific form of the quadratic equation fitting is y = ax² + bx + c, where a, b, and c are the fitting coefficients respectively, x represents the concentration, and y represents the absorbance value.

[0007] Preferably, a 0.03 mg / L phosphorus standard solution is used as a quality control sample, calibration points from 0.01 to 0.05 mg / L are added in the low concentration range of 0.0 - 0.1 mg / L, and one calibration point is set every 0.01 mg / L, and then colorimetric determination is carried out. A quadratic equation fitting is established with absorbance and concentration to determine the total phosphorus content of the water sample to be tested.

[0008] By adopting the above technical solutions, the sensitivity and accuracy of low-concentration total phosphorus detection are improved, and the efficiency is also improved. The phosphorus standard solution can be 0.04 mg / L, 0.05 mg / L, 0.06 mg / L, etc. And the number or concentration range of the calibration points can also be increased or decreased according to requirements.

[0009] Preferably, the conditions for microwave digestion treatment are a power of 750 - 850 w and a time of 5 - 15 min.

[0010] By adopting the above technical solutions, the digestion efficiency is significantly improved, the digestion time is shortened by 66% compared with the traditional method, and at the same time, the stability and thoroughness of the digestion process are ensured, thereby improving the accuracy of the subsequent measurement results.

[0011] Preferably, the dosage of sodium ethylenediaminetetraacetate is 0.1% of the weight of the ascorbic acid solution, and the concentration of the ascorbic acid solution is 100 g / L.

[0012] By adopting the above technical solutions, the precise dosage of sodium ethylenediaminetetraacetate and the specific concentration of ascorbic acid solution ensure the effective preparation of the improved ascorbic acid solution. This improvement significantly enhances the selectivity and stability of the color reaction, thereby improving the accuracy and precision of low-concentration total phosphorus detection and reducing the influence of external factors on the measurement results. Specifically, this optimization increases the spike recovery rate from 85% to 98%, reduces the total phosphorus detection error from 15% to 5%, and greatly improves the data reliability. At the same time, the color development ability of the ascorbic acid solution only decreases by 5% after being stored at room temperature for 30 days, with good stability, and the absorbance value is stable at 0.114 ± 0.005. For ascorbic acid without added sodium ethylenediaminetetraacetate, the color development ability decreases by 30% after being stored at room temperature for 7 days, and the absorbance value of a 0.05 mg / L sample decreases from 0.120 to 0.084. Further, the dosage of sodium ethylenediaminetetraacetate is not absolutely 0.1% of the weight of the ascorbic acid solution, but is around 0.1% according to the detection and measurement requirements.

[0013] Preferably, when the sample contains turbidity and chromaticity, when measuring the concentration, use a spectrophotometer and a long optical path cuvette to measure the absorbance value of the resulting color-developed product, and at the same time use a chromaticity interferent to deduct the background absorbance. Among them, the chromaticity interferent is composed of a blank sample and a compensating solution, and the compensating solution is composed of sulfuric acid and ascorbic acid solution.

[0014] By adopting the above technical solutions, the use of the chromaticity interferent can effectively deduct the background absorbance in the sample, thereby reducing the interference of chromaticity on the total phosphorus determination and improving the reliability of the measurement results. This method is applicable to water samples containing turbidity and chromaticity. Through appropriate pretreatment and compensation, the total phosphorus content can be accurately determined in actual environmental water samples, with good practicability and adaptability.

[0015] Preferably, the molybdate reagent is composed of ammonium molybdate, potassium antimonyl tartrate, water and sulfuric acid, and the concentration of potassium antimonyl tartrate is 1 g / L.

[0016] By adopting the above technical solutions, the composition of the molybdate reagent is optimized. Specifically, ammonium molybdate and potassium antimonyl tartrate are mixed in a specific weight ratio, and the concentration of potassium antimonyl tartrate is precisely controlled at 1 g / L. This improvement significantly enhances the selectivity and stability of the color reaction, thereby improving the accuracy and precision of the measurement results. Further, the molybdate reagent is composed of 13.5 g of ammonium molybdate, 0.5 g of potassium antimonyl tartrate, 200 ml of water and 300 ml of sulfuric acid (mass fraction 89%).

[0017] Preferably, the long optical path cuvette used in the color development treatment is 50 mm or 100 mm.

[0018] By adopting the above technical solution, using a long optical path cuvette can significantly improve the sensitivity of low-concentration total phosphorus detection. Specifically, choosing a cuvette with a length of 50 mm or 100 mm can increase the optical path distance compared with the traditional short optical path cuvette, thereby enhancing the absorbance signal, making the determination of total phosphorus in the low-concentration range of 0.0 - 0.1 mg / L more accurate, and the detection limit can be reduced to 0.005 mg / L, with the sensitivity increased by 2 - 3 times. This improvement effectively reduces the detection limit and improves the reliability and accuracy of the measurement results.

[0019] Preferably, during the digestion process, if the arsenic content in the water sample to be tested is greater than 2 mg / L, sodium thiosulfate masking agent is added before color development.

[0020] By adopting the above technical solution, when the arsenic content in the water sample to be tested is greater than 2 mg / L, adding sodium thiosulfate masking agent can effectively eliminate the interference of arsenic ions on the determination of total phosphorus, thus significantly improving the accuracy of the determination results. Combining the overall solution, microwave digestion makes the sample pretreatment more efficient, the improved ascorbic acid solution improves the selectivity and stability of the color reaction, and adding calibration points in the low-concentration range improves the detection sensitivity and reliability. Finally, this method reduces the detection limit to 0.005 mg / L, the spiked recovery rate reaches 98%, and the error is reduced to 5%, greatly improving the performance of low-concentration total phosphorus detection. That is, the application of sodium thiosulfate masking agent ensures the data reliability and accuracy of total phosphorus determination in arsenic-containing water samples.

[0021] Preferably, the spectrophotometer uses a high-precision ultraviolet-visible spectrophotometer with a wavelength of 700 nm.

[0022] By adopting the above technical solution, using a high-precision ultraviolet-visible spectrophotometer and measuring at a wavelength of 700 nm can significantly improve the accuracy of low-concentration total phosphorus detection. This solution makes the absorbance reading more accurate, thereby improving the reliability and repeatability of the total phosphorus content determination results and further reducing the measurement error. Specifically, this improvement helps to ensure a better linear response of the detection system in the low-concentration range, and finally improves the data precision and credibility of the entire determination method.

[0023] In summary, the present application has the following beneficial effects: 1. By applying microwave digestion and the improved ascorbic acid solution, the sensitivity of low-concentration total phosphorus detection is significantly improved, and the detection limit is reduced from the original 0.01 mg / L to 0.005 mg / L; 2. Introducing sodium ethylenediaminetetraacetate in the color development process to optimize the performance of ascorbic acid, the spiked recovery rate is increased from 85% to 98%, enhancing the accuracy and anti-interference ability of the determination results; 3. By adding calibration points in the low-concentration range and using quadratic equation fitting to construct the standard curve, the measurement error was effectively reduced, and the relative deviation decreased from 15% to 5%, improving the reliability and precision of the data. Detailed implementation method

[0024] . Preparation of test water sample 1: Use potassium dihydrogen phosphate and deionized water to prepare a solution with a total phosphorus content of 0.02 mg / L to obtain test water sample 1.

[0025] Preparation of test water sample 2: Take a solution with a total phosphorus content of 0.02 mg / L and arsenic trioxide to prepare a solution with a content of 3 mg / L to obtain test water sample 2.

[0026] Preparation of test water sample 3: Take a solution with a total phosphorus content of 0.02 mg / L and soil to prepare a solution with a content of 1 mg / L to obtain test water sample 3.

[0027] Example 1 A method for determining total phosphorus in water quality includes the following steps: Detection of absorbance value of test water sample: 1) Digestion treatment: Add 4 mL of oxidant (potassium persulfate solution, 50 g / L) to 100 mL of test water sample 1, place test water sample 1 in a microwave digestion instrument, set the power to 800 W, and the digestion time to 5 min to obtain digested water; Color development treatment: Add 2 mL of molybdate reagent (13 g of ammonium molybdate, 0.5 g of potassium antimonyl tartrate, 200 mL of water, and 300 mL of sulfuric acid are mixed, and the concentration of potassium antimonyl tartrate is 1 g / L) to the digested water, then add 1 mL of improved ascorbic acid solution, and complete the color development reaction at 25°C for 15 min. Among them, the improved ascorbic acid solution is obtained by mixing 100 mL of ascorbic acid solution (concentration of 100 g / L) and 0.01 g of sodium ethylenediaminetetraacetate; Testing concentration: Use a high-precision ultraviolet-visible spectrophotometer and a 50-mm long optical path cuvette to measure the absorbance value of the obtained color-developed product, with a wavelength of 700 nm. At the same time, use a chromaticity interference agent to deduct the background absorbance. The chromaticity interference agent is prepared by configuring 100 mL of blank sample (deionized water) and adding 2 mL of compensation liquid to it. The compensation liquid is prepared by mixing sulfuric acid (weight concentration of 98%) and ascorbic acid solution (concentration of 100 g / L) at a volume ratio of 1+1; Standard curve drawing: Use a 0.03 mg / L phosphorus standard solution as a quality control sample, add calibration points of 0.01 mg / L, 0.02 mg / L, 0.03 mg / L, 0.04 mg / L, and 0.05 mg / L in the low-concentration range of 0.0 - 0.1 mg / L, then perform colorimetric determination, and establish a quadratic equation fitting with absorbance and concentration to determine the total phosphorus content of the test water sample.

[0028] Example 2 A method for determining total phosphorus in water quality. The difference between this example and Example 1 is that the water sample to be tested 1 is placed in a microwave digestion instrument, the power is set to 850 W, and the digestion time is 15 min to obtain digested water.

[0029] Example 3 A method for determining total phosphorus in water quality. The difference between this example and Example 1 is that during the process of measuring the concentration, a high-precision ultraviolet-visible spectrophotometer is used and a 100-mm long optical path cuvette is adopted to measure the absorbance value of the obtained chromogenic product, and the wavelength is 700 nm.

[0030] Example 4 A method for determining total phosphorus in water quality. The difference between this example and Example 1 is as follows: Digestion treatment: Add 4 mL of oxidant (potassium persulfate solution, 50 g / L) and 4 mL of sodium thiosulfate masking agent (0.1 mol / L sodium thiosulfate solution) to 100 mL of the water sample to be tested 2. Place the water sample to be tested 2 in a microwave digestion instrument, set the power to 800 W, and the digestion time to 10 min to obtain digested water.

[0031] Example 5 A method for determining total phosphorus in water quality. The difference between this example and Example 1 is that: Take the water sample to be tested 3. When measuring the concentration: Use a high-precision ultraviolet-visible spectrophotometer and adopt a 50-mm long optical path cuvette to measure the absorbance value of the obtained chromogenic product, the wavelength is 700 nm, and at the same time use a chromaticity interference agent to deduct the background absorbance. The chromaticity interference agent is prepared by configuring 100 mL of a blank sample (deionized water) and adding 2 mL of a compensation solution thereto. The compensation solution is prepared by mixing sulfuric acid (weight concentration of 98%) and ascorbic acid solution (concentration of 100 g / L) in a volume ratio of 1+1.

[0032] Comparative Example 1 A method for determining total phosphorus in water quality. The difference between this comparative example and Example 1 is that: The detection method in GB / T 11893-1989 is adopted, and the specific steps are as follows: Digestion treatment: Add 4 mL of oxidant (potassium persulfate solution, 50 g / L) to 100 mL of the water sample to be tested 1. Digest the water sample to be tested 1 at a temperature of 120 °C and a pressure of 1.1 kg / cm 2 under the environment for 30 min to obtain digested water; Chromaticity interference treatment: Configure 100 mL of a blank sample (deionized water) and add 5 mL of a compensation solution thereto. The compensation solution is prepared by mixing sulfuric acid and ascorbic acid solution in a volume ratio of 1+1; Color development treatment: Add 5 mL of molybdate reagent (a mixture of 13 g of ammonium molybdate, 0.5 g of potassium antimonyl tartrate, 200 mL of water, and 300 mL of sulfuric acid, with the concentration of potassium antimonyl tartrate being 1 g / L) to the digested water, then add the modified ascorbic acid solution, and keep it at 25 °C for 15 min to complete the color development reaction; Test concentration: Use a high-precision ultraviolet-visible spectrophotometer and a 30-mm long optical path cuvette to measure the absorbance value of the obtained color-developed product at a wavelength of 700 nm; Standard curve drawing: Use a phosphorus standard solution (2.0 μg / mL) to prepare gradient concentrations of 0.0 μg / mL, 3.0 μg / mL, 6.0 μg / mL, 9.0 μg / mL, 12.0 μg / mL, and 15.0 μg / mL, establish a linear relationship between absorbance and concentration to determine the total phosphorus content of the water sample to be tested.

[0033] Comparative Example 2 A method for determining total phosphorus in water quality. The difference between this comparative example and Example 4 is that: the detection method in GB / T11893-1989 is adopted, and the treatment process is the same as that in Comparative Example 1. Only for the digestion treatment: Add 4 mL of oxidant (potassium persulfate solution, 50 g / L) and 4 mL of sodium thiosulfate masking agent (0.1 mol / L sodium thiosulfate solution) to 100 ml of the water sample to be tested 2, and then carry out the color development treatment.

[0034] Comparative Example 3 A method for determining total phosphorus in water quality. The difference between this comparative example and Example 1 is that: Test concentration: Use a high-precision ultraviolet-visible spectrophotometer and a 30-mm long optical path cuvette to measure the absorbance value of the obtained color-developed product at a wavelength of 700 nm.

[0035] Comparative Example 4 A method for determining total phosphorus in water quality. The difference between this comparative example and Example 1 is that for the standard curve drawing: Use a 0.03 mg / L phosphorus standard solution as a quality control sample, add calibration points of 0.01 mg / L, 0.02 mg / L, 0.03 mg / L, 0.04 mg / L, and 0.05 mg / L in the low concentration range of 0.0 - 0.1 mg / L, then carry out colorimetric determination, establish a linear relationship between absorbance and concentration to determine the total phosphorus content of the water sample to be tested.

[0036] Comparative Example 5 A method for determining total phosphorus in water quality. The difference between this comparative example and Example 1 is that the modified ascorbic acid solution is replaced with 100 mL of ascorbic acid solution (concentration of 100 g / L).

[0037] Comparative Example 6 A method for determining total phosphorus in water quality. The difference between this comparative example and Example 1 lies in 2 mL of molybdate reagent (13 g of ammonium molybdate, 0.5 g of potassium orthophosphate, 200 mL of water, and 300 mL of sulfuric acid are mixed, and the concentration of potassium orthophosphate is 1 g / L).

[0038] The experimental results of R² values and errors in Examples 1-5 and Comparative Examples 1-6 are shown in Table 1: Table 1 Experimental results of Examples 1-5 and Comparative Examples 1-6

[0039] From the experimental data of Examples 1-5 and Comparative Examples 1-6, it can be seen that the measurement method in this application can improve the accuracy of determining the content of low-concentration total phosphorus in water quality and has a small error.

[0040] This specific embodiment is only an interpretation of this application and is not a limitation of this application. Those skilled in the art can make modifications without creative contributions to this embodiment after reading this specification, but as long as they are within the scope of the claims of this application, they are protected by the patent law.

Claims

1. A method for determining total phosphorus in water, characterized in that: The following steps are involved: Absorbance value detection of water sample to be tested: 1) Digestion treatment: add oxidant to the water sample to be tested, and then perform microwave digestion treatment to obtain digestion treated water; 2) Color development treatment: add molybdate reagent to the digestion treatment water, then add modified ascorbic acid solution, and maintain at a set temperature for a predetermined time to complete the color development reaction, wherein the modified ascorbic acid solution is obtained by mixing ascorbic acid solution and sodium ethylenediaminetetraacetate; Test concentration: Use a spectrophotometer and a long pathlength cuvette to measure the absorbance of the resulting color product; Standard curve drawing: Use phosphorus standard solution as quality control sample, add multiple calibration points in the low concentration range of 0.0-0.1 mg / L, and then perform colorimetric measurement to establish a quadratic equation fitting based on absorbance and concentration to determine the total phosphorus content of the water sample to be tested.

2. The method for determining total phosphorus in water according to claim 1, characterized in that: Use 0.03 mg / L phosphorus standard solution as the quality control sample, add calibration points from 0.01 to 0.05 mg / L in the low concentration range of 0.0-0.1 mg / L, set a calibration point every 0.01 mg / L, and then perform colorimetric measurement. Establish a quadratic equation fitting with absorbance and concentration to determine the total phosphorus content of the water sample to be tested.

3. The method for determining total phosphorus in water according to claim 1, characterized in that: If the sample contains turbidity and chromaticity, when testing the concentration, a spectrophotometer is used and a long-path cuvette is used to measure the absorbance value of the obtained color development product, and a colorimetric interference agent is used to deduct the background absorbance, wherein the colorimetric interference agent is composed of a blank sample and a compensation solution, and the compensation solution is composed of sulfuric acid and ascorbic acid solution.

4. The method for determining total phosphorus in water according to claim 1, characterized in that: The microwave digestion treatment conditions are: power 750-850w, time 5-15min.

5. The method for determining total phosphorus in water according to claim 1, characterized in that: The dosage of the sodium edetate is 0.1% of the weight of the ascorbic acid solution, and the concentration of the ascorbic acid solution is 100 g / L.

6. The method for determining total phosphorus in water according to claim 1, characterized in that: The molybdate reagent is composed of ammonium molybdate, potassium antimony tartrate, water and sulfuric acid, and the concentration of potassium antimony tartrate is 1 g / L.

7. The method for determining total phosphorus in water according to claim 4, characterized in that: The long optical path cuvette used in the color development process is 50 mm or 100 mm.

8. The method for determining total phosphorus in water according to claim 4, characterized in that: During the digestion process, if the arsenic content in the water sample to be tested is greater than 2 mg / L, sodium thiosulfate masking agent is added and then color development treatment is performed.

9. The method for determining total phosphorus in water according to claim 1, characterized in that: The oxidant is potassium persulfate or a nitric acid-perchloric acid mixture.

10. The method for determining total phosphorus in water according to claim 1, characterized in that: The spectrophotometer adopts a high-precision ultraviolet-visible spectrophotometer with a wavelength of 700nm.

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