Determination of COD in water by acid potassium permanganate method
The acid potassium permanganate method is suitable for the determination of chemical oxygen demand (COD) in industrial circulating cooling water, and its measurement range is 2-80mg/L. The principle used in this method is that potassium permanganate exhibits strong oxidizing properties in acidic solutions. Under certain conditions, the reducing substances in the water sample are oxidized, and potassium permanganate is reduced to manganese ions. Excess potassium permanganate can be measured by oxalic acid.
Reagents and instruments used in COD
1. Boiling water bath device
2. Erlenmeyer flask 250mL.
3. 50mL acid burette.
4. Set the clock.
5. Saturated silver sulfate solution.
6. Sulfuric acid solution (1+3).
7. Sodium oxalate standard solution
Accurately weigh 0.6700g of sodium oxalate, dissolve it with a small amount of third-grade reagent water, transfer to a 1000mL volumetric flask, dilute to the mark, and shake well.
8. Potassium permanganate solution
Weigh 3.2g potassium permanganate and dissolve it in 1L tertiary reagent water, boil it on a boiling water bath for about 2h, and place it overnight. Store in a brown bottle. Filter with glass wool before use.
9. Potassium permanganate standard solution
a. Preparation
Use a pipette to draw 50mL potassium permanganate solution (0.1mol/L) in a 500mL volumetric flask, dilute to the mark with tertiary reagent water, and shake well.
b. Calibration
Add 50mL tertiary reagent water in a 250mL beaker, then add 5mL sulfuric acid (1+3), then use a pipette to transfer 10mL sodium oxalate standard solution, heat to 60~80℃, use the potassium permanganate standard solution to be calibrated Titration, the solution turns from colorless to light red as the end point of the titration. Calculate the concentration of potassium permanganate standard solution (c) according to the consumption volume of potassium permanganate standard solution.
COD detection steps
Use a pipette to take 25-100mL on-site water sample in an Erlenmeyer flask, add 50mL tertiary reagent water, 5mL sulfuric acid solution (1 x 3), 5-10 drops of silver sulfate saturated solution, and then pipette 10.00mL high manganese Potassium acid standard solution. Slowly heat to boiling on the electric stove, then boil for 5 minutes. The water sample should be pink or red. If it is colorless, add 10.00mL potassium permanganate standard solution; or reduce the sampling volume and re-boil for 5min according to the above process. Cool to 60-80℃, add 10.00mL sodium oxalate standard solution with a pipette, the solution should be colorless, if it is red, add 10.00mL sodium oxalate standard solution. Use potassium permanganate standard solution to drop to pink as the end point. At the same time, make a blank test. Finally, the COD parameter in the water sample is obtained through the calculation formula.
