Discussion
In the COD test of sewage, organic matter is oxidized by \(\mathrm{K_{2}Cr_{2}O_{7}}\) in the presence of :
Answer: B
The oxidation is carried out in the presence of \(\mathrm{H_{2}SO_{4}}\), sulphuric acid.
In the chemical oxygen demand test the sample is refluxed with potassium dichromate in strongly acidic conditions provided by concentrated sulphuric acid, with silver sulphate as catalyst and mercuric sulphate to bind chloride. The dichromate left over is titrated with ferrous ammonium sulphate to find how much was used.
Hydrochloric acid would itself be oxidised by dichromate, nitric acid is an oxidant that would interfere, and hydrofluoric acid is not used in the test.
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