At 420 nm, use reagent water as a reference solution. Measure all samples, blanks, andstandards against this solution. The absorption measurement of an undigested blank containingdichromate, with reagent water replacing sample, will give initial dichromate absorption. Anydigested sample, blank, or standard that has a COD value will give lower absorbance because ofthe decrease in dichromate ion. Analyze a digested blank with reagent water replacing sample toensure reagent quality and to determine the reagents’ contribution to the decrease in absorbanceduring a given digestion. The difference between absorbances of a given digested sample and thedigested blank is a measure of the sample COD. When standards are run, plot differences ofdigested blank absorbance and digested standard absorbance versus COD values for eachstandard.
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