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What are the methods of COD testing instruments?

by:BOQU     2022-09-30

1. Spectrophotometry

The COD detection spectrophotometry is based on the classic standard method. Potassium dichromate oxidizes organic matter, and hexavalent chromium generates trivalent chromium. The relationship between the absorbance value of hexavalent chromium or trivalent chromium and the COD value of water samples is determined. COD value of water sample. Using the above principles, the most representative method abroad is the US Environmental Protection Agency EPA.Method0410.4 'Automatic Manual Colorimetry'. American Society for Testing and Materials ASTM: D1252-2000 'Determination of Chemical Oxygen Demand of Water Method B - Sealing Digestion Spectrophotometry' and international standard IS015705-2002 'Small sealed tube method for the determination of chemical oxygen demand (COD) in water quality. Our country is the unified method of the former State Environmental Protection Administration 'rapid closed catalytic digestion method (including spectrophotometry)'.

2. Rapid digestion method

The classic standard method is the reflux 2h method. In order to improve the analysis speed, various rapid analysis methods have been proposed. Compared with the classical standard method, the acidity of sulfuric acid in the digestion system is increased from 9.0mg/l to 10.2mg/l, the reaction temperature is increased from 150℃ to 165℃, and the digestion time is reduced from 2h to 10min-15min. In addition, the traditional method of heating and digestion by thermal conduction radiation is changed, and microwave digestion technology is used to improve the digestion reaction speed. Due to the wide variety of microwave ovens and different powers, it is difficult to test out a unified power and time in order to achieve the best digestion effect. The price of microwave ovens is also high, and it is difficult to develop a unified standard method.

3. Rapid digestion spectrophotometry

The chemical oxygen demand (COD) determination method, whether it is the reflux volume method, the rapid method, or the photometric method, uses potassium dichromate as the oxidant, silver sulfate as the catalyst, and mercury sulfate as the masking agent for chloride ions. COD digestion system-based assay method. On this basis, people have carried out a lot of research work for the purpose of saving reagents, reducing energy consumption, easy operation, fast, accurate, and reliable. The method has the characteristics of small occupied space, low energy consumption, a small amount of reagents, minimal waste liquid, low energy consumption, simple operation, safety, and stability, accuracy and reliable, suitable for large-scale determination, etc., which makes up for the deficiencies of the classical standard method. At present, my country has also issued a national standard for rapid digestion spectrophotometry, HJT399-2007 'Determination of Water Quality Chemical Oxygen Demand Rapid Digestion Spectrophotometry'.

4. Rapid digestion spectrophotometry test steps

Rapid digestion spectrophotometry generally uses a COD analyzer, a digester, and a matching prefabricated reagent digestion tube. Prefabricated reagents are consumables and can be purchased directly from the manufacturer.

(1) Reagents. Preparation of water samples

Each COD prefabricated reagent digestion tube already contains the reagent components required for COD analysis. After unscrewing the cap, add 2 mL of the water sample, and tighten the cap (when using 0-15000mg/L COD prefabricated test tubes, only add 0.2 mL of water sample).

(2) Digestion of water samples

Put the COD prefabricated test tube into which the water sample has been added into the COD digester, and select the appropriate digestion temperature and time according to the requirements of the reagent for heating and refluxing. After the digestion process is over, the digester can be automatically closed. Remove the COD test tube from the digester and cool it to room temperature.

(3) COD colorimetric determination

Turn on the COD analyzer, and select the corresponding COD test program in the main menu, no need to establish a standard curve, just put the test tube into the instrument to read the COD value expressed in concentration units, and record the experimental results.

5. Cost-saving methods

The cost of prefabricated reagents purchased by manufacturers is relatively high. In order to reduce costs, two methods can be considered:

One is to purchase large-capacity prefabricated reagents or third-party compatible reagents.

The second is to configure compatible reagents by yourself and use the original digester and COD analyzer (there are surprises at the end of the article).

6. Precautions

Whether you buy large-capacity prefabricated reagents or configure your own reagents, you need to reuse digestion tubes, so there are some precautions:

(1) The wear of the digestion tube will affect the measurement results. It is necessary to reduce the wear as much as possible, and the digestion tube with serious wear should be eliminated in time.

(2) When cleaning, do not rub the digestion tube with the test tube brush, which will easily cause scratches and affect the colorimetric. It is recommended that after the digestion tube pours the waste liquid into the waste liquid tank, rinse the digestion tube with tap water to clean it, and then clean it with ultrasonic waves. If HgSO4 powder remains on the inner wall of the digestion tube, soak it in dilute hydrochloric acid or dilute sulfuric acid for 2-4 hours, and wipe the residual dirt on the outer wall with alcohol cotton. The cleaned digestion tube was soaked in deionized water for several hours, and then dried in an oven at 60-70°C overnight.

(3) The temperature of the system has an influence on the measurement, and it must be cooled to room temperature before colorimetry. The solution should be measured in time after cooling to room temperature, and the measurement will be affected if it is placed for too long after digestion.

(4) Since this measurement is a colorimetric method, the chromaticity, and turbidity of the water sample have a great influence on the measurement results, so the chromaticity and turbidity of printing and dyeing wastewater, papermaking wastewater, etc. are not suitable for this method. Determination.

(5) The difference in the calibration curve has a great influence on the measurement results. If you use self-configured reagents, it is better to use a self-made standard curve instead of relying on the built-in standard curve in the original program of the instrument. Fluctuations and the effects of different batches of reagents.

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