Chemical oxygen demand determination, COD analysis

Outline of the method – COD is determined by refluxing the sample with an excess of potassium dichromate in acid condition and estimated by titration to find the amount of dichromate consumed that directly proportions 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:-Β 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 beads in each reaction vessel.

2. Insert the reaction vessel in the holes of the digestion block which has attained 1500C temperature. Put air condensers on the reaction vessels. Reflux the contents for 2 hr. cool the mixture content and transfer them to the 200ml conical flask.

3. Remove all the mixture contents by washing the reaction vessel with distilled water. Add 80ml distilled water in the same flask and titrate the excess potassium dichromate with the Standard ferrous ammonium sulphate (FAS) using 4 to 5 drops of ferric indicator. The endpoint colour changes from yellow to reddish brown (in case of an auto-titration indicator is not required. The endpoint is detected by the electrode.

4. Do the blank the same way.

Calculation of COD in ppm = (blanking reading – sampling reading) X N of FAS X 8000/ sample volume

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