Question

20 ml sample of waste water was refluxed with 30 ml of potassium dichromate solution and after refluxing the excess unreacted dichromate required 11 ml of 0.1 N FAS solution . Blank of 20 ml of distilled water on refluxing with 30 ml of dichromate solution required 14 ml of 0.1 N FAS solution . Calculate the COD value of waste water .

Answer 1

Given data : Volume of FAS required for the blank solution =V_B =14ml
Volume of FAS required for estimation of sewage =V_E =11 ml 
V_B – V_E =14-11 =3 ml 
Normality of FAS =N =0.1 , Volume of sewage =20 ml
To find : COD

Solution :
COD = 8 x N x (V_B – V_E ) x 1000/ Volume of sewage mg/lit
COD = 8x 0.1 x 3 x 1000 / 20 = 120 mg/lit .
The COD value of waste water is 120 mg/lit .