CN116067952A - Seawater COD detection method - Google Patents

Abstract

A method for detecting seawater COD belongs to the technical field of seawater detection. In order to solve the defects that the existing seawater oxygen demand titration method is complex in operation, complex in steps, long in measurement time and unsuitable for on-site rapid measurement, the invention adopts the following steps: weighing a quantitative seawater sample, adding a sodium hydroxide solution, uniformly mixing, adding a potassium permanganate solution for oxidation, carrying out constant-temperature timed digestion, and cooling to room temperature; sequentially adding sulfuric acid solution and potassium iodide solution, mixing, standing, and pouring into a colorimetric device; and placing the colorimetric device in a tester for colorimetric measurement to obtain the COD value of the seawater sample. The invention is mainly used for detecting seawater COD.

Description

A kind of detection method of seawater COD

technical field

The invention belongs to the technical field of seawater detection, in particular to a method for detecting seawater COD.

Background technique

When the seawater contains a large amount of organic matter, it will lead to the massive growth and reproduction of microorganisms in the seawater, thereby reducing the dissolved oxygen content in the seawater, causing the deterioration of the marine ecological environment, and having a great impact on mariculture. Therefore, it is necessary to continuously increase the Supervision of large marine environmental pollution. In order to standardize the quality inspection of the ecological environment in coastal waters, protect the ecological environment, and ensure the scientificity, accuracy, systematicness, comparability and representativeness of the national coastal waters environmental monitoring, the measurement method of the current national standard HJ/T132-2003 is to adopt Determination of chemical oxygen demand in seawater by basic potassium permanganate method.

COD (Chemical Oxygen Demand) is a chemical method to measure the amount of reducing substances that need to be oxidized in a water sample. The oxygen equivalent of substances (generally organic substances) that can be oxidized by strong oxidants in the water sample.

The basic potassium permanganate COD in the national standard adopts the titration method. First, potassium iodate is used to calibrate sodium thiosulfate; Potassium acid, add starch indicator, titrate with the calibrated sodium thiosulfate solution, and finally calculate the oxygen consumption according to the amount of sodium thiosulfate consumed. The titration method is to add a large amount of excess oxidant to ensure that the organic matter is oxidized, then titrate with sodium sulfate to consume the remaining oxidant, and then convert the oxidant actually consumed when oxidizing the organic matter. The operation is complicated, the steps are cumbersome, and the measurement time is long. Suitable for rapid determination in the field.

Therefore, there is a need for a seawater COD detection method that can be detected in batches, has high efficiency, short measurement time, and is not easily affected by the detection environment.

Contents of the invention

The purpose of the present invention is to solve the defects of the existing seawater oxygen demand titration method, which are complex in operation, cumbersome in steps, long in measurement time, and unsuitable for rapid on-site measurement, and provide a method capable of batch detection, high in efficiency, short in measurement time, A method for detecting seawater COD that is not easily affected by the detection environment.

A kind of assay method of seawater COD of the present invention, it comprises the steps:

S1, measure A ml seawater sample and be placed in the digestion tube, add (A/10) ml, the sodium hydroxide solution that concentration is 3mol/L in the digestion tube, mix to obtain primary mixed solution;

S2, add (A/10)ml, the potassium permanganate solution that concentration is 0.002mol/L in the mixed solution to oxidize, mix again and obtain secondary mixed solution;

S3, placing the secondary mixed liquid in a constant temperature digester for closed digestion, so that the organic matter in the secondary mixed liquid is oxidized to obtain a digested liquid;

S4. Take out the test tube immediately after the digestion, and cool the water to room temperature;

S5. Add (1+3) sulfuric acid solution (A/10)ml to the cooled digestion solution, mix well, then add potassium iodide solution with a concentration of 2.5g/100ml (A/10)ml, cover and mix Uniform;

S6. After standing still for 2-20 minutes, pour into the colorimetric device;

S7, placing the colorimetric device in the water quality measuring instrument for colorimetric measurement, or placing it in a spectrophotometer to read the absorbance for measurement;

S8. Obtain the COD value of the seawater sample based on the corresponding relationship between COD and the standard color card, or the standard curve between COD and absorbance.

Further: in S1, the sampling volume A of the seawater sample satisfies: 25ml≥A≥5ml.

Further: in S3, the temperature of the digester is 100-120°C.

Further: in S5, the (1+3) sulfuric acid solution is a solution obtained by mixing 1 part of 98% sulfuric acid (ρ=1.84g/ml) with 3 parts of distilled water.

Further: in S6, the rest time is 5 minutes.

Further: in S7, the colorimetric measurement is completed within half an hour after being poured into the colorimetric device.

Further: in S7, the colorimetric device is a cuvette or a cuvette.

The beneficial effects of the present invention are: the full name detection of the present invention can basically be controlled and completed within half an hour, which can save a lot of time compared with the traditional titration method, and can realize fast, convenient and accurate seawater COD suitable for laboratory and On-site detection methods. The detection wavelength of the present invention in the spectrophotometer is 420nm visible light. The reagents in the method exist simultaneously with chloride ions, and there is no obvious absorption in the adopted 420nm wavelength visible light, so the chloride ions have no influence on the method during the colorimetric process. In this scheme, the content of potassium permanganate satisfies the set optimal range, and this method makes the concentration of potassium iodide present an obvious step distribution within the range. Add sulfuric acid, and quantitative potassium iodide, the amount of potassium iodide is controlled within a certain range, otherwise it will affect the colorimetry. Reducing potassium permanganate in an acidic environment and releasing iodine to make the solution turn yellow. At 420nm wavelength, the absorbance of the solution is inversely proportional to the concentration, and the concentration of oxygen consumption in water can be measured.

Description of drawings

Fig. 1 is flow chart;

Fig. 2 is the schematic diagram of color comparison card in embodiment 1.

Detailed ways

The following are only preferred specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention are all Should be covered within the protection scope of the present invention. The following examples are only used to explain the present invention, but not to limit the present invention, and the protection scope of the present invention should be based on the protection scope of the claims. The following describes the embodiments of the present invention in detail. In order to facilitate the description of the present invention and simplify the description, the technical terms used in the description of the present invention should be interpreted in a broad sense, including but not limited to conventional alternatives not mentioned in this application, and including direct implementation methods and indirect implementation methods.

Example 1

Illustrate the present embodiment in conjunction with Fig. 1 and Fig. 2, a kind of assay method of seawater COD disclosed in the present embodiment, it comprises the steps:

S1. Accurately measure 5ml of seawater sample into the digestion tube, then add 0.5ml of sodium hydroxide solution with a concentration of 3mol/L into the digestion tube, and mix evenly to obtain an alkaline primary mixed solution;

S2. Add 0.5ml of potassium permanganate solution with a concentration of 0.002mol/L to the mixed solution, oxidize, cover and mix evenly, and mix evenly to obtain a secondary mixed solution, which takes about 1 minute;

S3. Place the secondary mixed liquid in a constant temperature digester for digestion. The constant temperature range of the digester is 100-120°C, so that the organic matter in the secondary mixed liquid is oxidized, and the oxygen-consuming substances in the water are oxidized to be digested. solution; this step takes about 15 minutes;

S4. Take out the test tube immediately after the digestion, and cool the water to room temperature, which takes about 2-3 minutes;

S5. Add 0.5ml of sulfuric acid solution to the cooled digestion solution, the concentration of the sulfuric acid solution is 1+3, that is, the sulfuric acid solution obtained after mixing 1 part of 98% concentrated sulfuric acid and 3 parts of ultrapure water, after mixing evenly Then add 0.5ml of potassium iodide solution with a concentration of 2.5g/100ml, cover and mix evenly for about 1 minute;

S6. After standing still for 5 minutes, pour into a cuvette or a cuvette;

S7. Place the cuvette or the colorimetric tube in the spectrophotometer for colorimetric measurement. The detection optical path in the spectrophotometer is 10 mm; the colorimetric measurement is completed within half an hour after being poured into the colorimetric device. Colorimetric determination to obtain the COD value of the seawater sample. The spectrophotometer has a built-in seawater COD standard curve or a built-in curve calibrated on-site for colorimetric determination. This step takes about 1 minute.

This program accurately controls the addition order, concentration and amount of each reagent, so that the concentration of potassium iodide in the finally obtained sample presents an obvious step distribution within the range of COD content of 0-5 mg/L, which is convenient for spectrophotometric analysis. The meter reads the value directly.

The above-mentioned embodiment takes about 25 minutes in total, which saves a lot of time compared with the titration method. The traditional titration method is used in excess, and its color cannot show a step distribution within the range of COD.

In this embodiment, the detection wavelength in the spectrophotometer is 420nm visible light. When chloride ions and certain substances exist together, certain absorption will be produced in the ultraviolet spectral region, but a single chloride ion has no obvious absorption in the ultraviolet-visible spectral region. The reagents in this embodiment coexist with chloride ions, and there is no obvious absorption in the adopted 420nm wavelength visible light, so the chloride ions have no influence on the method during the colorimetric process. The absorbance of the test solution is inversely proportional to the oxygen demand concentration. Comply with the Lambert-Beer law within the measuring range. The concentration of reagents used and the order of addition must be exactly the same.

When this method is implemented, a set of standard curves can be made with standard solutions first, and the standard curves can be built into the water quality analyzer or spectrophotometer, and the measured COD value can be obtained by comparing the readings with reference to the standard curve during actual testing. After the standard curve is obtained, the samples can be processed in batches during the actual measurement. For example, 30 tubes of samples can be processed at the same time. Because the spectroscopic measurement results within 30 minutes after the above-mentioned processing is completed are acceptable, multiple samples can be processed at one time. For example, 3 samples are processed at the same time. After the completion, the spectrophotometer is used to measure one by one. Assuming that the reading of each tube takes 1 minute, 30 tubes take 30 minutes, and 30 tubes are processed at the same time, the total processing time is 25 minutes. Minutes, plus it takes 30 minutes to read all the samples, and the total time is 55 minutes. The average time for each sample is equivalent to less than 2 minutes for the measurement of a single sample, and the efficiency is much higher than the traditional titration method. times.

In this embodiment, the optimal measurement range for seawater oxygen demand is 0-5 mg/L, and the over-range can be diluted for measurement.

data verification

First equip the standard solution to measure the standard curve:

1. Prepare seawater standard solutions with an oxygen demand of 0mg/L, 1mg/L, 2mg/L, 3mg/L, 4mg/L, and 5mg/L. The standard solutions contain 20000mg/L of chloride ions. The above standard solutions After processing according to the aforementioned method steps, the absorbance X was read by a spectrometer, and the results were as follows:

Concentration Y 0mg/L 1mg/L 2mg/L 3mg/L 4mg/L 5mg/L Negative absorbanceX 0 0.104 0.211 0.320 0.427 0.533

Based on the above results, the regression curve equation is:

Y=9.2872X-0.0075

R=0.9998

2. Take six groups of seawater standard solutions with an oxygen demand of 2.5mg/L, respectively containing 1000mg/L, 2500mg/L, 5000mg/L, 10000mg/L, 15000mg/L, 20000mg/L chloride ion concentration, according to the above After the method steps are processed, the absorbance X is read by the spectrometer, and the oxygen demand is calculated based on the regression curve, and the test results are obtained, as follows:

Chloride concentration 1000mg/L 2500mg/L 5000mg/L 10000mg/L 15000mg/L 20000mg/L The measurement results 2.61mg/L 2.49mg/L 2.44mg/L 2.38mg/L 2.61mg/L 2.58mg/L error 4.4% -0.4% -2.4% 4.8% 4.4% 3.2%

The test results are within ±5%.

3. Comparison of offshore and estuary multi-point sampling test with national standard titration test:

Sampling point Offshore 1 Offshore 2 Breeding area 1 Breeding area 2 Estuary 1 Estuary 2 National standard titration method 3.57 5.40 9.09 11.2 3.09 3.21 Spectroscopic method of the present invention 3.40 5.54 8.81 11.6 3.12 3.15 error -4.8% 2.6% -3.1% 3.6% 1% -1.9%

The spectrophotometric measurement of the present embodiment is basically consistent with the national standard titration comparison results.

Judging from the experimental results in this embodiment, this method meets the accuracy and precision requirements for seawater oxygen demand detection. This embodiment does not require a complicated titration process, and it only takes a few seconds to directly measure the water sample result by using the spectrophotometric method, which is greatly improved. The detection time is shortened, which can meet the detection requirements of different environments, and is more suitable for field and emergency detection, as well as mariculture detection. The method reduces the consumption of reagents, lowers the detection cost, and reduces the discharge of detection waste liquid. The method is simple to operate and easier for laboratory personnel to grasp. This method is applicable to professional water quality detectors and ordinary spectrophotometers. The colorimetric method of this method can be applied to various methods such as cuvettes and cuvettes.

Example 2

Illustrate the present embodiment in conjunction with embodiment 1, the assay method of a kind of seawater COD disclosed in the present embodiment, it comprises the steps:

S1. Accurately measure 10ml of seawater sample in the digestion tube, add 1ml of sodium hydroxide (concentration 3mol/L) solution into the digestion tube, and mix evenly to obtain a mixed solution;

S2. Add 1ml of potassium permanganate solution (concentration 0.002mol/L) to the mixed solution for oxidation, cover and mix evenly, and mix again to obtain a secondary mixed solution. This step takes about 1 minute;

S3. Place the secondary mixed solution in a digester after constant temperature treatment for about 15 minutes (± 5 seconds) for regular digestion. The constant temperature condition of the digester is to seal and heat at 95-100°C (select 95°C during operation), so that Oxidize the organic matter in the secondary mixed liquid, oxidize the oxygen-consuming substances in the water, and obtain the digestion liquid;

S4. Take out the test tube immediately after the digestion, and cool the water to room temperature for about 2-3 minutes;

S5. Add 1ml of sulfuric acid solution to the cooled digestion solution, the concentration is 1+3, that is, the sulfuric acid solution obtained after mixing 1 part of 98% concentrated sulfuric acid and 3 parts of ultrapure water, and then add 1ml of potassium iodide after mixing evenly solution (concentration 2.5g/100ml), cover and mix evenly for 1 minute;

S6, after standing still for 5 minutes, pour into a cuvette or a colorimetric tube, the detection optical path in the spectrophotometer is 10mm, and a cuvette or a cuvette can be used;

S7. Put the cuvette or the colorimetric tube in the water quality measuring instrument for colorimetric measurement, measure the absorbance by putting it into the water quality measuring instrument, and read the COD concentration value of the sample.

Claims (7)

1. The method for measuring the COD of the seawater is characterized by comprising the following steps of:

s1, measuring A ml of seawater sample, placing the seawater sample in a digestion tube, adding (A/10) ml of sodium hydroxide solution with the concentration of 3mol/L into the digestion tube, and uniformly mixing to obtain primary mixed solution;

s2, adding (A/10) ml potassium permanganate solution with the concentration of 0.002mol/L into the mixed solution for oxidation, and uniformly mixing again to obtain secondary mixed solution;

s3, placing the secondary mixed solution in a constant-temperature digestion device for closed digestion, so that organic matters in the secondary mixed solution are oxidized, and a digestion solution is obtained;

s4, immediately taking out the test tube after digestion is finished, and cooling the test tube to room temperature;

s5, adding (1+3) sulfuric acid solution (A/10) ml into the cooled digestion solution, uniformly mixing, then adding (A/10) ml potassium iodide solution with the concentration of 2.5g/100ml, and capping and uniformly mixing;

s6, standing for 2-20 minutes, and pouring into a colorimetric device;

s7, placing the colorimetric device in a water quality tester for colorimetric measurement or placing the colorimetric device in a spectrophotometer for reading absorbance for measurement;

s8, obtaining the COD value of the seawater sample based on the corresponding relation between the COD and the standard colorimetric card or the standard curve of the COD and the absorbance.

2. The method for measuring seawater COD according to claim 1, wherein in S1, the seawater sample sampling amount a satisfies: 25ml is more than or equal to A is more than or equal to 5ml.

3. The method according to claim 1, wherein in S3, the temperature of the digestion vessel is 95 to 120 ℃.

4. The method for measuring seawater COD according to claim 1, wherein in S5, the (1+3) sulfuric acid solution is a solution obtained by mixing 1 part by mass of 98% sulfuric acid (ρ=1.84 g/ml) with 3 parts by mass of distilled water.

5. The method according to claim 1, wherein the resting time is 5 minutes in S6.

6. The method according to claim 1, wherein the colorimetric determination is performed within a second half hour from the pouring into the colorimetric device in S7.

7. The method according to claim 1, wherein in S7 the cuvette or cuvette is used as the colorimetric device.

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