An improved determination of chemical oxygen demand in water and wastes by a simplified acid dichromate digestion

Abstract

A simplified and improved chemical oxygen demand (COD) procedure is suitable for water and waste water samples containing up to 1500 mg Cl l −1. Samples are digested with sulfuric acid, potassium dichromate, silver sulfate, mercury(II) sulfate and sulfamic acid in open glass tubes for 2 h at 140 ± 2° C without boiling, and excess dichromate is determined colorimetrically at 440 nm. The use of 49-position digestion racks, a large oven and a spectrophotometer fitted with a 1-cm flow cell allow analysis of ca. 12 samples h −1. A correction for chloride interference is not required in the 0–500 mg Cl l −1 range. The detection limit is 3 mg l −1, and the relative standard deviation at the 112 mg COD l −1 level is 4.3%. Thirty-five waste water samples were analyzed by the standard dichromate reflux method and by the proposed procedure. There was no significant difference between the two sets of data ( P > 0.25). Recovery data for 15 major water pollutants including benzene, toluene and pyridine are presented.