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

摘要

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.