Both COD and the permanganate index are significant indicators in monitoring water quality. Then, what is the difference between them? In this chapter, the manufacturer of the water quality online monitoring instrument will join us to explore!
COD represents two methods of indices, while the permanganate index is only one of them. COD encompasses the manganese method and the chromium method, and the permanganate index refers to the detection value of the manganese method. It is mainly utilized for detecting low concentrations of COD and is applied in the testing of drinking water and deep well water. In such cases, where the COD is less than 5mg/l, the chromium method is unable to measure the accurate value. However, for the majority of high concentration COD, the chromium method is employed for detection.
During the analysis process using the CODMn-8000 online analyzer for the permanganate index of drinking water, the method employed by this instrument is the potassium permanganate colorimetric method. After filtration, the sample is sent to the digestion tank. Simultaneously, the water quality analyzer controls the heating device for heating. In the digestion tank, potassium permanganate and concentrated sulfuric acid are added in sequence and undergo digestion at 100°C. The digestion time can be selected based on the quality of the raw water. After digestion, the sample is cooled for an appropriate period and then undergoes optical detection. By measuring the absorbance, the concentration of the permanganate index of the water to be tested is calculated. After measurement, the entire measurement system will be automatically cleaned with pure water to ensure accurate measurement for the next time.
The COD-8000 COD online analyzer utilizes potassium dichromate for high-temperature digestion and colorimetric determination. Under high-temperature and high-pressure conditions, Cr(VI) in the mixed solution composed of the water sample, potassium dichromate, silver sulfate (acting as a catalyst to ensure more complete oxidation of straight-chain aliphatic compounds), and concentrated sulfuric acid is reduced to Cr(III), resulting in a color change of the mixture solution. The extent of the color change is in correspondence with the COD (chemical oxygen demand) concentration of the measured water sample. The instrument can directly determine the COD value of the water sample through photoelectric colorimetry. Chloride ions in the water sample are the main interfering substances. The COD-8000 type COD (chemical oxygen demand) online analyzer can eliminate the interference of chlorine by adding HgCl2 to complex the chloride ions in the water sample.
