The core principle is rapid digestion spectrophotometry, with the potassium dichromate method being the most commonly used.
A known amount of potassium dichromate solution is added to the sample. In a strong sulfuric acid medium, using silver sulfate as a catalyst, the sample is digested at high temperature (usually heated to approximately 175°C). The COD value is then determined spectrophotometrically. When the COD value in the sample is between 100 mg/L and 1000 mg/L, the absorbance of trivalent chromium (Cr3+) produced by the reduction of potassium dichromate is measured at a wavelength of 600 nm ± 20 nm. The COD value in the sample is directly proportional to the increase in the absorbance of trivalent chromium (Cr3+). The absorbance of trivalent chromium (Cr3+) is then converted into the COD value of the sample.
When the COD value in the sample ranges from 15 mg/L to 250 mg/L, the total absorbance of the two chromium ions-unreduced hexavalent chromium (Cr6+) and reduced trivalent chromium (Cr3+)-is measured at a wavelength of 440 nm ± 20 nm. The COD value in the sample is directly proportional to the decrease in absorbance of hexavalent chromium (Cr6+), the increase in absorbance of trivalent chromium (Cr3+), and the decrease in total absorbance. The total absorbance value is then converted into the COD value of the sample.
