The simultaneous colorimetry of chromium(III) and dichromate ions in a single wavelength is proposed for the determination of chemical oxygen demand(COD) in waters, after oxidation of the samples with dichromate and sulfuric acid. The interrelation among COD, chromium(III) and dichromate ions concentrations permits to simplify the numerical resolution of the system, as follow: from a multicomponent system involving chromium(III) and dichromate ions concentrations and absorbances, in at least two wavelengths, to a linear regression involving COD concentration measured in a single wavelength. The procedure is as follows: a) mix 5 mL of standard or sample solution(30 mgL-1 - 300 mgL-1 range, using potassium hydrogenphtalat as standard) with 5 mL of digestion solution(7.000 mmolL-1 dichromate - 0.04 molL-1 mercuric sulfate - 1.5 molL-1 sulfuric acid) and 10 mL of catalytic solution(0.02 molL-1 silver sulfate - 18 molL-1 sulfuric acid); b) heath for two hours at 140 oC-150 oC; c) take absorbances against water, at 430 nm, using 1 cm length cuvettes.
CO; water analysis; simultaneous colorimetry
