Method for detecting total phosphorus in water

Abstract

The invention relates to the technical field of water quality analysis and testing, in particular to a method for detecting total phosphorus in water. The detection method comprises the following steps: (1) adding a potassium persulfate solution into a phosphorus standard solution, uniformly mixing, then adding an ammonium molybdate sulfuric acid solution, an ascorbic acid solution, a colloid stabilizer solution and a metal ion solution, uniformly mixing, standing at room temperature for color development, and measuring the absorbance to obtain a phosphorus standard curve; (2) taking a water sample to be detected, adding a potassium persulfate solution, uniformly mixing, digesting, and standing to room temperature to obtain a digested water sample; and (3) adding an ammonium molybdate sulfuric acid solution into the digested water sample, uniformly mixing, adding an ascorbic acid solution, a colloid stabilizer solution and a metal ion solution, uniformly mixing, standing at room temperature for color development, measuring the absorbance, and calculating the total phosphorus content in the water sample to be measured according to the phosphorus standard curve obtained in the step (1). The detection method is high in sample detection upper limit, rapid and stable in color development and accurate in test.

Description

Technical Field

[0001] The present invention relates to the technical field of water quality analysis and testing, and particularly to a method for detecting total phosphorus in water quality. Background Art

[0002] The detection of total phosphorus in water quality is a key indicator for evaluating water eutrophication and pollution control. Generally, the "GB / T 11893-1989 Determination of Total Phosphorus in Water - Ammonium Molybdate Spectrophotometry" is used to achieve quantitative analysis through oxidation digestion combined with the phosphomolybdenum blue color reaction. However, this method has the following limitations: (1) Limited detection range: The upper detection limit of the standard method is only 0.6 mg / L, which is difficult to meet the detection requirements of high-concentration industrial wastewater (such as chemical and metallurgical wastewater). It is necessary to dilute the sample multiple times, resulting in cumbersome operations and error accumulation; (2) Poor color development stability: The color development process requires standing at room temperature for 15 minutes at a wavelength of 700 nm, and the test needs to be completed within 30 minutes. The color development time window is narrow, sensitive to experimental conditions (such as temperature and light), and is prone to absorbance fluctuations due to operation delays, affecting data accuracy.

[0003] In response to the above problems, researchers have proposed various improvement schemes, but there are still obvious defects. Patent CN20211115618.2 improves the color development stability by extending the water bath time (20 minutes), but the test still needs to be completed within 30 minutes after color development, and the time window has not been significantly improved, and the detection upper limit has not broken through the limitations of traditional methods. Inductively coupled plasma optical emission spectrometry (ICP-OES): Although it can avoid the color development step and the detection range is extended to 0.1-100 μg / mL, it relies on high-cost instruments (such as Thermo Fisher ICAP-6300), requires a complex digestion process (heating a mixture of sulfuric acid and perchloric acid to 250 °C), and is not suitable for on-site rapid detection. Microfluidics and photocatalysis technology: Patent CN201910355789.5 proposes a microfluidic detection system, which simplifies the process through photocatalytic digestion, but the miniaturization of the device results in low throughput, and it depends on specific electrolytes and ultraviolet light sources, with limited applicability. Fluorescent probe technology: For example, patent CN201910299972.8 is based on a composite ratio fluorescent probe. Although it has high selectivity, it is sensitive to Fe 3+ and requires customized reagents (N,S-CQDs / EuCPs), and the cost and operation threshold are significantly higher than traditional methods.

[0004] The above improvement schemes still have problems such as being limited by the detection range, relying on expensive equipment, having complex operation processes, or being difficult to balance high sensitivity, wide detection range, operation convenience, and cost-effectiveness. Therefore, there is an urgent need to develop a method for detecting total phosphorus in water quality that is suitable for high-concentration samples, has rapid and stable color development, and is easy to popularize. Summary of the Invention

[0005] In view of the above-mentioned disadvantages of the prior art, the present invention provides a method for detecting total phosphorus in water quality, which has a relatively high upper limit for sample detection, rapid and stable color development, and accurate testing.

[0006] The first aspect of the present invention provides a method for detecting total phosphorus in water quality, comprising the following steps:

[0007] (1) Add potassium persulfate solution to a series of phosphorus standard solutions with different concentrations. After mixing evenly, then add ammonium molybdate sulfuric acid solution, ascorbic acid solution, colloid stabilizer solution, and metal ion solution, mix evenly, stand for color development at room temperature, measure its absorbance, and obtain a phosphorus standard curve;

[0008] (2) Take the water sample to be tested, add potassium persulfate solution, mix evenly, then digest, and stand until room temperature to obtain the digested water sample;

[0009] (3) Add ammonium molybdate sulfuric acid solution to the digested water sample, mix evenly, then add ascorbic acid solution, colloid stabilizer solution, and metal ion solution, mix evenly, stand for color development at room temperature, measure its absorbance, and calculate the total phosphorus content in the water sample to be tested according to the phosphorus standard curve obtained in step (1).

[0010] In some embodiments of the present invention, in step (1), the series of phosphorus standard solutions with different concentrations include phosphorus standard solutions with concentrations of 0 ppm, 0.5 ppm, 1 ppm, 2 ppm, and 3 ppm.

[0011] In some embodiments of the present invention, in step (1) or step (3), the colloid stabilizer in the colloid stabilizer solution includes a mixture of two or more of NaCl, KCl, polyvinylpyrrolidone, polyvinyl alcohol, hydroxyethyl cellulose, fatty alcohol polyoxyethylene ether, sodium dodecylbenzenesulfonate, and sodium dodecyl sulfate.

[0012] In some embodiments of the present invention, in step (1) or step (3), the colloid stabilizer in the colloid stabilizer solution is a mixture composed of NaCl, polyvinylpyrrolidone, and sodium dodecylbenzenesulfonate; preferably, the mass ratio of NaCl, polyvinylpyrrolidone, and sodium dodecylbenzenesulfonate is 3:5:2.

[0013] In some embodiments of the present invention, in step (1) or step (3), the concentration of the potassium persulfate solution is 40-60 g / L, the concentration of the ammonium molybdate sulfate solution is 20-40 g / L, the concentration of the ascorbic acid solution is 60-100 g / L, the concentration of the colloid stabilizer solution is 0.5-20 g / L, and the concentration of the metal ion solution is 1-100 ppm. In some preferred embodiments of the present invention, in step (1), the volume ratio of the phosphorus standard solution to the 40-60 g / L potassium persulfate solution, the 20-40 g / L ammonium molybdate sulfate solution, the 60-100 g / L ascorbic acid solution, the 0.5-20 g / L colloid stabilizer solution, and the 1-100 ppm metal ion solution is 5:1:2:1:0.5:0.5; and / or, in step (3), the volume ratio of the water sample to be measured to the 40-60 g / L potassium persulfate solution, the 20-40 g / L ammonium molybdate sulfate solution, the 60-100 g / L ascorbic acid solution, the 0.5-20 g / L colloid stabilizer solution, and the 1-100 ppm metal ion solution is 5:1:2:1:0.5:0.5.

[0014] In some embodiments of the present invention, in step (1) or step (3), the metal ions in the metal ion solution are one or more of nickel ions, copper ions, iron ions, cobalt ions, and vanadium ions.

[0015] In some embodiments of the present invention, in step (1) or step (3), the metal ions in the metal ion solution are copper ions.

[0016] In some embodiments of the present invention, in step (2), the digestion is carried out in a digester; and / or, the temperature of the digestion is 120-180 °C, and the time of the digestion is 20-40 min. Preferably, the temperature of the digestion is 150 °C, and the time of the digestion is 30 min.

[0017] In some embodiments of the present invention, in step (1) or step (3), the time for standing and developing color at room temperature is 2 min.

[0018] In some embodiments of the present invention, in step (1) or step (3), when measuring the absorbance, the reagent blank solution treated synchronously is used for zero adjustment.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] 1. The total phosphorus detection method provided by the present invention adds metal ions as a catalyst to catalytically accelerate the reduction and color development of phosphomolybdic heteropolyacid. Compared with the ordinary method that requires waiting for 15 minutes and completing the detection within 30 minutes, the color development time is greatly shortened, and the reduction and color development of phosphomolybdic heteropolyacid can be completed within 2 minutes. By adding a colloid stabilizer, the reduced phosphomolybdenum blue is stably distributed in the solution to be measured, and the color is stable within 4 hours, with the absorbance change within 0.005, which can avoid the absorbance fluctuation caused by operation delay and improve the data accuracy.

[0021] 2. The total phosphorus detection method provided by the present invention has a relatively wide detection range of 0.006 - 3 ppm. Compared with the ordinary method whose detection upper limit is only 0.6 mg / L, it can reduce the number of sample dilutions, the detection is simpler, more accurate, more efficient, and convenient for on-site operation. Description of the Drawings

[0022] Figure 1 It is the phosphorus standard curve of Example 1. Detailed Embodiments

[0023] The following further illustrates a total phosphorus detection method for water quality according to the present invention with specific embodiments.

[0024] The first aspect of the present invention provides a total phosphorus detection method for water quality, including the following steps:

[0025] (1) Add potassium persulfate solution to a series of phosphorus standard solutions with different concentrations. After mixing evenly, then add ammonium molybdate sulfuric acid solution, ascorbic acid solution, colloid stabilizer solution, and metal ion solution, mix evenly, stand for color development at room temperature, measure its absorbance, and obtain the phosphorus standard curve;

[0026] (2) Take the water sample to be measured, add potassium persulfate solution, mix evenly, digest, and stand until room temperature to obtain the digested water sample;

[0027] (3) Add ammonium molybdate sulfuric acid solution to the digested water sample, mix evenly, then add ascorbic acid solution, colloid stabilizer solution, and metal ion solution, mix evenly, stand for color development at room temperature, measure its absorbance, and calculate the total phosphorus content in the water sample to be measured according to the phosphorus standard curve obtained in step (1).

[0028] The total phosphorus detection method provided by the present invention adds metal ions as a catalyst and simultaneously adds a colloid stabilizer, so that the reduction and color development of phosphomolybdic heteropolyacid are completed within 2 minutes, and the reduced phosphomolybdenum blue is stably distributed in the solution to be measured, and the color is stable within 4 hours, overcoming the limitation that the ordinary method requires standing at room temperature for 15 minutes at a wavelength of 700 nm during the color development process and needs to complete the test within 30 minutes. It does not require the use of complex instruments, is simple to operate, and has a relatively wide detection range.

[0029] In some embodiments of the present invention, in step (1), the series of phosphorus standard solutions with different concentrations include phosphorus standard solutions with concentrations of 0 ppm, 0.5 ppm, 1 ppm, 2 ppm, and 3 ppm. In some other embodiments of the present invention, more concentrations may also be included, such as 0.05 ppm and 0.06 ppm.

[0030] In some embodiments of the present invention, in step (1) or step (3), the colloid stabilizer in the colloid stabilizer solution includes a mixture of two or more of NaCl, KCl, polyvinylpyrrolidone, polyvinyl alcohol, hydroxyethyl cellulose, fatty alcohol polyoxyethylene ether, sodium dodecylbenzenesulfonate, and sodium dodecyl sulfate. Specifically, it may be a mixture of NaCl and polyvinylpyrrolidone, a mixture of KCl and polyvinyl alcohol, a mixture of hydroxyethyl cellulose and fatty alcohol polyoxyethylene ether, a mixture of NaCl and sodium dodecyl sulfate, and a mixture of NaCl, KCl, and polyvinylpyrrolidone, a mixture of KCl, sodium dodecylbenzenesulfonate, and sodium dodecyl sulfate, etc. In some preferred embodiments of the present invention, in step (1) or step (3), the colloid stabilizer in the colloid stabilizer solution is a mixture composed of NaCl, polyvinylpyrrolidone, and sodium dodecylbenzenesulfonate; more preferably, the mass ratio of NaCl, polyvinylpyrrolidone, and sodium dodecylbenzenesulfonate is 3:5:2.

[0031] In some embodiments of the present invention, in step (1) or step (3), the concentration of the potassium persulfate solution is 40 - 60 g / L, the concentration of the ammonium molybdate sulfuric acid solution is 20 - 40 g / L, the concentration of the ascorbic acid solution is 60 - 100 g / L, the concentration of the colloid stabilizer solution is 0.5 - 20 g / L, and the concentration of the metal ion solution is 1 - 100 ppm. The concentration of the metal ion solution added in the present invention is 1 - 100 ppm. It is found in the detection practice that when the concentration of the metal ion solution is below 100 ppm, it can play a role in catalyzing and accelerating the reduction and color development of phosphomolybdic heteropolyacid. However, when the concentration of the metal ion solution is too high, it will destroy the stability of the solution, resulting in deviation of the detection results.

[0032] In some preferred embodiments of the present invention, in step (1), the volume ratio of the phosphorus standard solution to the potassium persulfate solution at 40 - 60 g / L, the ammonium molybdate sulfuric acid solution at 20 - 40 g / L, the ascorbic acid solution at 60 - 100 g / L, the colloid stabilizer solution at 0.5 - 20 g / L, and the metal ion solution at 1 - 100 ppm is 5:1:2:1:0.5:0.5; in step (3), the volume ratio of the water sample to be measured to the potassium persulfate solution at 40 - 60 g / L, the ammonium molybdate sulfuric acid solution at 20 - 40 g / L, the ascorbic acid solution at 60 - 100 g / L, the colloid stabilizer solution at 0.5 - 20 g / L, and the metal ion solution at 1 - 100 ppm is 5:1:2:1:0.5:0.5.

[0033] In some embodiments of the present invention, in step (1) or step (3), the metal ions in the metal ion solution are one or more of nickel ions, copper ions, iron ions, cobalt ions, and vanadium ions. Specifically, it can be any one of nickel ions, copper ions, iron ions, cobalt ions, and vanadium ions, or a combination of two metal ions such as nickel ions and copper ions, iron ions and cobalt ions, cobalt ions and vanadium ions, nickel ions and iron ions, copper ions and cobalt ions, etc., or a combination of three metal ions such as nickel ions, copper ions and iron ions, iron ions, cobalt ions and vanadium ions, etc., or a combination of four metal ions such as nickel ions, copper ions, iron ions, cobalt ions, or copper ions, iron ions, cobalt ions, vanadium ions, etc., or a combination of five metal ions such as nickel ions, copper ions, iron ions, cobalt ions, and vanadium ions.

[0034] In some embodiments of the present invention, in step (1) or step (3), the metal ions in the metal ion solution are copper ions.

[0035] In some embodiments of the present invention, in step (2), the digestion is carried out in a digester. In some specific embodiments of the present invention, the water sample to be measured is placed in a high-temperature resistant spiral digestion tube, and then the potassium persulfate solution is added. After mixing evenly, it is placed in a digester matching the high-temperature resistant spiral digestion tube for digestion.

[0036] In some embodiments of the present invention, in step (2), the temperature of the digestion is 120 - 180 °C, which can be 120 - 130 °C, 130 - 140 °C, 140 - 150 °C, 150 - 160 °C, 160 - 170 °C, or can also be 170 - 180 °C; the digestion time is 20 - 40 min, which can be 20 - 25 min, 25 - 30 min, 30 - 35 min, or can also be 35 - 40 min. Preferably, the temperature of the digestion is 150 °C and the digestion time is 30 min.

[0037] In some embodiments of the present invention, in step (1) or step (3), the time for standing and developing color at room temperature is 2 min. Since the reduction and color development of phosphomolybdic heteropolyacid can be completed within 2 min after adding metal ion catalysis, the color development time can be controlled to 2 min.

[0038] In some embodiments of the present invention, in step (1) or step (3), when measuring the absorbance, the reagent blank solution after synchronous treatment is used for zero adjustment, that is, the absorbance value of the reagent blank solution is used as the zero point to eliminate the reagent background interference and instrument error. The reagent blank solution contains all the reagents used in the total phosphorus detection process but does not contain phosphorus element. Specifically, it includes the potassium persulfate solution, ammonium molybdate sulfuric acid solution, ascorbic acid solution, colloid stabilizer solution, metal ion solution and distilled water used in the detection process, and the reagent blank solution is digested and developed synchronously with the sample.

[0039] Before further describing the specific embodiments of the present invention, it should be understood that the protection scope of the present invention is not limited to the following specific embodiments; it should also be understood that the terms used in the embodiments of the present invention are for the purpose of describing specific embodiments and not for limiting the protection scope of the present invention.

[0040] Unless otherwise defined, all technical and scientific terms used in the present invention have the same meaning as commonly understood by those skilled in the art of this technology. Except for the specific methods, equipment, and materials used in the embodiments, according to the knowledge of those skilled in the art of this technology and the description of the present invention, any methods, equipment, and materials similar to or equivalent to those described in the embodiments of the present invention can also be used to implement the present invention.

[0041] Unless otherwise specified, the experimental methods, detection methods, and preparation methods disclosed in the present invention all adopt the conventional analytical chemistry and related conventional technologies in this technical field. Unless otherwise specified, the materials and equipment used in the present invention are all commercially available sources.

[0042] Example 1

[0043] Using the total phosphorus standard sample as the water sample to be detected, the detection method includes the following steps:

[0044] (1) Prepare 30 g / L ammonium molybdate sulfuric acid solution, 80 g / L potassium persulfate solution, 50 ppm copper ion solution, 1 g / L colloid stabilizer solution: 0.3 g of NaCl, 0.5 g of polyvinylpyrrolidone (K30), and 0.2 g of sodium dodecylbenzenesulfonate are fixed in a 1 L volumetric flask with pure water, and 0 - 3 ppm phosphorus standard solution.

[0045] (2) Pipette 5 ml of the prepared phosphorus standard solution with a concentration range of 0 - 3.0 ppm respectively, add 1 ml of potassium persulfate solution, mix well, then add 2 ml of ammonium molybdate sulfuric acid solution, mix well again, and then add 1 ml of ascorbic acid solution, 0.5 ml of colloid stabilizer solution, and 0.5 ml of copper ion solution. Mix well and let it stand at room temperature for 2 min. Using a spectrophotometer, with the reagent blank solution treated synchronously with the phosphorus standard solution as the reference, measure its absorbance at 710 nm. Taking the phosphorus content as the abscissa and the absorbance as the ordinate, establish a detection standard curve with a correlation coefficient r ≥ 0.999, as Figure 1 shown.

[0046] (3) Take 5 ml of the total phosphorus standard sample and place it in a high-temperature resistant screw digestion tube. Then add 1 ml of potassium persulfate solution, mix well, and place it in a digester matching the high-temperature resistant screw digestion tube. After digestion at 150 °C for 30 min, take it out and let it stand to room temperature to obtain the digested total phosphorus standard sample.

[0047] (4) Add 2 ml of ammonium molybdate sulfuric acid solution to the digested total phosphorus standard sample, shake well, then add 1 ml of ascorbic acid solution, 0.5 ml of colloid stabilizer solution, and 0.5 ml of copper ion solution. Mix well and let it stand at room temperature for 2 min. Use the detection standard curve in step (2), zero with the reagent blank solution treated synchronously (digested and colored) with the total phosphorus standard sample, and directly measure the absorbance to read the total phosphorus content.

[0048] The test results are shown in Table 1 below:

[0049] Table 1 Test Results of Total Phosphorus Standard Sample

[0050]

[0051] Example 2

[0052] The difference between this example and Example 1 is that: taking on-site water treatment samples (Sample 1#, Sample 2#, and Sample 3#) as the water samples to be measured, using the standard addition method (the total phosphorus addition amount for Sample 1# is 0.05 ppm, for Sample 2# is 0.5 ppm, and for Sample 3# is 2 ppm) to verify the data accuracy of the method. The detection method is the same as that in Example 1, and the detection results are shown in Table 2 below:

[0053] Table 2 Verification Results of Detection Accuracy

[0054]

[0055] In Sample 2#, the target analyte total phosphorus is determined by standard addition, and its standard addition recovery rate is calculated. The determination results are shown in Table 3 below:

[0056] Table 32# Test Results of Sample Spike Recovery

[0057]

[0058] Comparative Example 1

[0059] The difference between this comparative example and Example 1 is that: the concentration of the copper ion solution prepared in step (1) is 150 ppm, and the detection method is the same as that in Example 1. The measurement data are shown in Table 4 below:

[0060] Table 4 Test Results of Total Phosphorus Standard Samples in Comparative Example

[0061]

[0062] It can be seen from the data in Table 4 that when the metal ion concentration is too high, it will damage the solution stability and cause a large deviation in the detection results.

[0063] In summary, the detection method provided by the present invention is very accurate and reliable for the determination of the total phosphorus content in sewage, meeting the requirements for the determination of sample solutions.

[0064] The above embodiments are only illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Any person familiar with this technology can modify or change the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or changes completed by those with ordinary knowledge in the technical field without departing from the spirit and technical ideas disclosed by the present invention should still be covered by the claims of the present invention.

Claims

1. A method for detecting total phosphorus in water quality, characterized in that, It includes the following steps: (1) Add potassium persulfate solution to a series of phosphorus standard solutions with different concentrations. After mixing evenly, add ammonium molybdate sulfuric acid solution, ascorbic acid solution, colloid stabilizer solution, and metal ion solution, mix evenly, let it stand at room temperature for color development, measure its absorbance, and obtain a phosphorus standard curve; (2) Take the water sample to be measured, add potassium persulfate solution, mix evenly, then digest it, and let it stand to room temperature to obtain the digested water sample; (3) Add ammonium molybdate sulfuric acid solution to the digested water sample, mix evenly, then add ascorbic acid solution, colloid stabilizer solution, and metal ion solution, mix evenly, let it stand at room temperature for color development, measure its absorbance, and calculate the total phosphorus content in the water sample to be measured according to the phosphorus standard curve obtained in step (1).

2. The detection method of total phosphorus in water quality according to claim 1, characterized in that, In step (1), the series of phosphorus standard solutions with different concentrations include phosphorus standard solutions with concentrations of 0 ppm, 0.5 ppm, 1 ppm, 2 ppm, and 3 ppm.

3. The detection method of total phosphorus in water quality according to claim 1, characterized in that, In step (1) or step (3), the colloid stabilizer in the colloid stabilizer solution includes a mixture of two or more of NaCl, KCl, polyvinylpyrrolidone, polyvinyl alcohol, hydroxyethyl cellulose, fatty alcohol polyoxyethylene ether, sodium dodecylbenzenesulfonate, and sodium dodecyl sulfate.

4. The detection method of total phosphorus in water quality according to claim 3, characterized in that, In step (1) or step (3), the colloid stabilizer in the colloid stabilizer solution is a mixture composed of NaCl, polyvinylpyrrolidone, and sodium dodecylbenzenesulfonate; preferably, the mass ratio of NaCl, polyvinylpyrrolidone, and sodium dodecylbenzenesulfonate is 3:5:

2.

5. The detection method of total phosphorus in water quality according to claim 1, characterized in that In step (1) or step (3), the concentration of the potassium persulfate solution is 40 - 60 g / L, the concentration of the ammonium molybdate sulfuric acid solution is 20 - 40 g / L, the concentration of the ascorbic acid solution is 60 - 100 g / L, the concentration of the colloid stabilizer solution is 0.5 - 20 g / L, and the concentration of the metal ion solution is 1 - 100 ppm.

6. The detection method of total phosphorus in water quality according to claim 5, wherein, In step (1), the volume ratio of the phosphorus standard solution to the 40 - 60 g / L potassium persulfate solution, 20 - 40 g / L ammonium molybdate sulfuric acid solution, 60 - 100 g / L ascorbic acid solution, 0.5 - 20 g / L colloid stabilizer solution, and 1 - 100 ppm metal ion solution is 5:1:2:1:0.5:0.5; and / or, in step (3), the volume ratio of the water sample to be measured to the 40 - 60 g / L potassium persulfate solution, 20 - 40 g / L ammonium molybdate sulfuric acid solution, 60 - 100 g / L ascorbic acid solution, 0.5 - 20 g / L colloid stabilizer solution, and 1 - 100 ppm metal ion solution is 5:1:2:1:0.5:0.

5.

7. The detection method of total phosphorus in water quality according to claim 1, characterized in that, In step (1) or step (3), the metal ions in the metal ion solution are one or more of nickel ions, copper ions, iron ions, cobalt ions, and vanadium ions.

8. The detection method of total phosphorus in water quality according to claim 7, characterized in that, In step (1) or step (3), the metal ion in the metal ion solution is copper ion.

9. The detection method of total phosphorus in water quality according to claim 1, characterized in that, In step (2), the digestion is carried out in a digester; and / or, the temperature of the digestion is 120 - 180 °C, and the time of the digestion is 20 - 40 min.

10. The detection method of total phosphorus in water quality according to claim 1, characterized in that, In step (1) or step (3), the time for standing and developing color at room temperature is 2 min; and / or, when measuring absorbance, zero the instrument with the reagent blank solution that has been synchronously processed.

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