Feasibility of Mercury-free Chemical Oxygen Demand (COD) Test with Excessive Addition of Silver Sulfate

Abstract

The conventional chemical oxygen demand (COD) test requires mercuric sulfate (HgSO₄) for masking chlorides (Cl⁻) in a sample. Since the use of mercury (Hg) will be strictly restricted under the Minamata Convention on Mercury, the feasibility of Hg-free COD test by closed reflux, colorimetric method with excessive addition of silver sulfate (Ag₂SO₄) was discussed in this research. Although Cl⁻ contamination tended to increase the dichromate consumption even in the conventional COD test, the Ag₂SO₄ dose in sulfuric acid reagent not less than 10.3 g/kg-H₂SO₄ successfully masked Cl⁻ not more than 500 mg-Cl/L. As a result of applying to 14 organic compounds with theoretical oxygen demand of 90 mg-O₂/L, the Hg-free COD test tended to output slightly higher COD than the conventional one; the average degradation efficiency of 96% in the Hg-free test and 94% in the conventional test. The average accuracy for the Hg-free COD test was superior to the conventional one. In the economical viewpoint, the Hg-free COD test successfully cut off the chemical cost of Ag₂SO₄ and HgSO₄ from 13,459 to 7,831 JPY/1,000 samples. Consequently, the Hg-free COD test was thought to be feasible for water samples containing Cl⁻ not more than 500 mg-Cl/L.