Introduction to the Five COD Detection Methods and Principles

I. What is COD?

COD( Chemical Oxygen Demand ) refers to the amount of oxidant consumed when treating a water sample with a certain strong oxidant under certain conditions. It reflects the degree of pollution of substances in the water. The larger the chemical oxygen demand, the more serious the organic pollution in the water.

COD indicates mg/L the COD value detected by water quality monitoring instruments. Water quality can be divided into five categories. Categories one and two, COD≤15mg/L basically meet the drinking water standards. Water with values greater than category two cannot be used as drinking water. Category three, COD≤20mg/L Category four, COD≤30mg/L and category five, COD≤40mg/L are considered polluted water. The higher the value, the more serious the pollution. II. 5 COD Detection Methods COD≤15mg/L Dichromate reflux method

Determination principle: In a sulfuric acid medium, potassium dichromate is used as an oxidant, silver sulfate as a catalyst, and mercury sulfate as a masking agent for chloride ions. The sulfuric acid concentration of the digestion reaction solution is The boiling point temperature of

1 148℃±2℃

      is the digestion temperature. The reaction is heated by refluxing with water cooling. 9mol/L After the digestion solution cools naturally, ferrous sulfate solution is titrated with ferrous sulfate solution using ferrous phenanthroline as an indicator, and the water sample is calculated according to the consumption of ferrous sulfate solution. Advantages and disadvantages: The reflux device occupies a large experimental space, consumes a lot of water and electricity, uses a large amount of reagents, is inconvenient to operate, and is difficult to determine in large quantities and quickly. Potassium permanganate method 2h Determination principle: Potassium permanganate is used as an oxidant to determine COD≤15mg/L The result is called the permanganate index (

CODMn

2 ). After the water sample is acidified with sulfuric acid, a certain amount of potassium permanganate solution is added, and the reaction is heated in a boiling water bath.

      The remaining potassium permanganate is reduced by adding an excess of sodium oxalate solution, and then the excess sodium oxalate is back-titrated with potassium permanganate solution. The permanganate index is calculated. COD≤15mg/L Advantages and disadvantages: The advantage of the potassium permanganate method is that the pollution generated during the experiment is less than that of the national standard method. However, the disadvantage is that the experiment requires back titration of excess sodium oxalate, which is time-consuming. In addition, the oxidizing power of the acidic potassium permanganate method is low, and the oxidation is incomplete, so the measured permanganate index is lower than the dichromate index, usually differing from the national standard method by COD≤15mg/L 3-8 times. Therefore, CODCr 30min is mainly aimed at wastewater with a relatively high content of reductive pollutants, while

      is mainly aimed at river water and surface water with relatively low pollution. Spectrophotometry Determination principle: The principle of this method is the same as that of the national standard method. The principle is that in an acidic solution, the reducing substances in the sample react with potassium dichromate to generate trivalent chromium ions. Trivalent chromium ions have a strong absorption capacity for light at a wavelength of The absorbance is related to the concentration of trivalent chromium ions according to the Lambert-Beer law. The amount of trivalent chromium ions is related to the amount of reducing substances in the sample. Therefore, the COD of the sample can be indirectly measured by measuring the absorbance of trivalent chromium. Advantages and disadvantages: Compared with the traditional national standard method, this method effectively saves the time spent on preparing chemical reagents and does not require titration, making it convenient to operate. However, the only drawback is that the digestion process still takes times. Therefore, hours.

3 Rapid digestion method

      The classic standard method is reflux 600nm method. In order to improve the analysis speed, various rapid analysis methods have been proposed. These methods mainly improve the reaction speed by increasing the concentration of oxidant, increasing the sulfuric acid concentration, increasing the reaction temperature, and adding co-catalysts in the digestion reaction system. COD≤15mg/L The result is called the permanganate index (

      Advantages and disadvantages: The sulfuric acid concentration of the digestion system is increased from 2 9.0mg/l to

4 The reaction temperature is increased from

      150℃ 2h to

165℃ The digestion time is reduced from 10.2mg/l to ~ . The disadvantage is that different types of microwave ovens have different power and time. Rapid digestion spectrophotometry ，消解时间由2h减少到10min～15min。缺点为微波炉种类不同，试验的功率和时间均不同。

5、快速消解分光光度法

      Determination Principle: Rapid digestion spectrophotometry refers to a method that uses a sealed tube as a digestion tube. A small amount of water sample and reagent are taken into the sealed tube, placed in a small constant temperature heating plate, digested by constant temperature heating, and then determined by spectrophotometry. COD≤15mg/L The result is called the permanganate index (

      Advantages and Disadvantages: Small footprint, low energy consumption, small reagent usage, minimized waste liquid, low energy consumption, simple operation, safe and stable, accurate and reliable, suitable for large-scale determination.

Three, Common Problems and Methods for COD Detection

1 How to remove the chlorine COD≤15mg/L impact on the detection?

      Answer: Prepare a blank sample containing deionized water and a chlorine component with the same chlorine concentration as your sample. After adding the digestion solution, the blank sample will quickly turn a very dark color, but you can then subtract the effect of chlorine and obtain a detection result applicable to your sample.

2、 COD≤15mg/L Where does the main error in the detection come from?

      Answer: Chlorine is COD≤15mg/L the biggest factor affecting the detection. Every COD≤15mg/L experimental bottle used in the dichromate method contains mercuric sulfate, which can remove chlorine interference. And MnIII method uses a vacuum pretreatment device to remove chlorine with a concentration of 1000mg/l的氯。

3 Can the same COD≤15mg/L blank sample be used for multiple detections?

      Answer: Multiple experiments using the same experimental bottle can reuse the same blank sample. This blank sample must be kept in a dark place. Continuously measure the absorbance of the blank sample over time to check if the results are stable. Zero the instrument using bottled deionized water in absorbance mode, and then detect the absorbance of the blank sample. Record the detection results. When the absorbance changes by more than 0.01 absorbance units, a new blank sample needs to be prepared.

4 How to verify COD≤15mg/L the accuracy of the detection results?

      Answer: Use a COD≤15mg/L standard solution whose concentration falls within the concentration range you are using. Detect this standard solution as if it were your sample. If your detection results are close to the standard concentration, you can be sure that your instrument and reagents are working properly, and your detection is accurate.

5 Can the sample be diluted for determination COD≤15mg/L ？

      Answer: Yes. Dilute the sample, and then add the appropriate amount of diluted sample to the bottle (it can be 2 or 0.2 ml). Multiply the final detection result by your dilution factor.