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 Ag2SO4dose 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 Ag2SO4and 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.