Water soluble organic carbon

Estimation of water soluble organic carbon

Principle

A known quantity of water extract of soil is digested with K2Cr2O7 – H2SO4 medium and the excess chromic acid left unused by water soluble organic matter is determined by back titration with ferrous ammonium sulphate using diphenylamine indicator.

Apparatus required

·         Beaker

·         Conical  flask

·         Centrifuge

·         Digestion block

Reagents required

1.      0.07 N potassium dichromate

2.      Concentrated sulphuric acid

3.      Orthophosphoric acid

4.      Diphenylamine indicator

5.      0.035 N ferrous ammonium sulphate

6.      0.4 M sulphuric acid

Procedure

v  Taken 5 g of field moist sample in a beaker

v  Add 10 ml of distilled water and keep for one hour

v  Centrifuge the content at 10,000 rpm for 30 minutes and filter the supernatant solution. From the filtrate take 5 ml in a conical flask and treat with 5 ml of 0.07 N potassium dichromate, 10 ml of conc. Sulphuric acid and 5 ml of orthophosphoric acid

v  Mix carefully and digest at 1500C for 30 minutes using digestion block and cool

v  Titrate the contents against 0.035 N ferrous ammonium sulphate in 0.4 M sulphuric acid using 1 ml diphenylamine indicator

v  Simultaneously run a blank. From the titre values find out the water soluble organic carbon present in the soil sample.

Calculation

Volume of distilled water added =  5 g

Weight of soil taken                   =  10 ml

Volume 0.07 N K2 Cr2O7 used   =   5 ml

Volume of 0.035 N ferrous ammonium sulphate used for blank=  X ml

Volume of 0.035 N ferrous ammonium sulphate used for sample =Y ml

1 ml of  0.07 K2 Cr2O7    =       0.00021 g of ‘C’

Therefore 5 – (5xY/X) ml of 0.07 K2 Cr2O7  =

Therefore in 100 g  =