Determination of low chemical oxygen demand values in water by the dichromate semi-micro method

Abstract

A semi-micro modification of the dichromate method for the determination of low concentrations (1–35 mg l^–1) of the chemical oxygen demand is described. It is based on the experimentally established dependencies of the oxidation efficiency on the concentrations of H₂SO₄, K₂Cr₂O₇ and Ag₂SO₄, and on the temperature and time of heating. The procedure involves increasing the concentration of H₂SO₄ and heating in closed test-tubes in a heating block at 170 °C for 40 min and the spectrophotometric determination of dichromate at 455 nm. The interfering effect of chloride, up to a concentration of 100 mg l^–1, is masked by using a combination of HgSO₄ and KCr(SO₄)₂. The modification has been tested on 30 samples of ground, tap, precipitation and surface waters and on 16 chemical compounds, and gives results comparable to those from a standard reflux method.