A 25 ml of sewage water sample was refluxed with 10 ml 0f 0.25 N K2Cr2 O7 solution. The untreated dichromate requires 6.5 ml of 0.1 N FAS.10 ml of dichromate solution and 25 ml distilled water , under the same condition as sample required 27 ml of 0.1 n FAS. Calculate the COD of sewage.

COD = V2 –V1 X N X 8 X 1000/V

V2 = volume of FAS for blank titration

V1 = volume of FAS for sample titration

V = volume of sample taken for test

N = Normality of FAS

COD = 27 –6.5 X 0.1X 8 X 1000/25

COD = 656 ppm