Outline of the method – COD determined by the refluxing the sample with an excess of potassium dichromate in acid condition and estimated by titration to find the amount of dichromate consumed which directly proportion to COD.
Apparatus - COD digester
- Reaction vessels with condensers
Reagents
- 0.25 N Potassium dichromate:-Dissolve 49.036 gm of potassium dichromate in 1 litre distilled water for 1 N solution. Dissolved 12.259 gm of potassium dichromate in 1 litre distilled water for preparation of 0.25 N solution.
- Silver sulphate Sulphuric acid:-Dissolve 10gm of silver sulphate in 2.5 lit of con Sulphuric acid.
- STD ferrous ammonium Sulphates 0.1N:- Dissolve 40g of ferrous ammonium Sulphates in a previously cooled mixture of 40ml of Sulphuric acid & 200ml of water, dilute with water to1000 ml and mix it thoroughly.
- Ferro ion indicator:-Dissolve1.3485g of 1, 10-phenanthroline (monohydrate), together with 0.695g of ferrous sulphate in distilled water and dilute to 100ml.
- Potassium acid phthalates for STD:-Dissolve 425.1 gm of potassium acid phthalate (which is dried for 1 hr in the oven at 1500C ) in 1 lit distilled water. This solution corresponds to 500 ppm COD
Procedure
1. Take 10ml potassium dichromate (0.25N) + 20ml sample + 30ml silver sulphate Sulphuric acid solution + 1 g mercuric sulphate in reaction vessel. Add one or two glass bead in each reaction vessel.
Calculation of COD in ppm = (blanking reading – sampling reading) X N of FAS X 8000/ sample volume
1. Take 10ml potassium dichromate (0.25N) + 20ml sample + 30ml silver sulphate Sulphuric acid solution + 1 g mercuric sulphate in reaction vessel. Add one or two glass bead in each reaction vessel.
Calculation of COD in ppm = (blanking reading – sampling reading) X N of FAS X 8000/ sample volume