Biomass carbon

Estimation of Biomass Carbon

Principle

A known quantity of fumigated K2SO4 extract of the soil is digested with K2Cr207 - sulphuric acid mixture. The unused chromic acid left after the oxidation is back titrated against standard ferrous ammonium sulphate solution using diphenylamine indicator.

Apparatus required

1.Digestion tube

2.Block digester

3.250 ml conical flask

Reagents required

1.Mercury II oxide,

2.Sulphuric - orthophosphoric acid mixture (2:1)

3.Phenolphthalein indicator

4.0.4 N K2Cr207

5.0.2 N ferrous ammonium sulphate

Procedure

·         Pipette out 5 ml of the filtrate obtained from the previous .experiment in a Digestion tube

·         Add 2 ml of 0.4 N K2Cr2O7 , 0.07 g of mercury II oxide, 15 ml of sulphuric orthophosphoric acid mixture

·         Digest the contents at 150°C for 30 minutes in a block digester

·         Cool and transfer the contents into a 250 ml conical flask using 25 ml of distilled water

·         Add 2-3 drops phenolphthalein indicator and titrate against 0.2 N Ferrous Ammonium Sulphate, till the colour changes from bluish green to reddish brown occurs.

Calculation

Weight of soil taken                                     = 25 g

Volume of extract taken                              = 5 ml

Volume of 0.4 N K2Cr207 used                    = 2 ml

Volume of 0.2 N ferrous ammonium sulphate used for blank = X ml

Volume of 0.2 N ferrous ammonium sulphate used for sample = Y mL

         1 mL of 0.4 N K2Cr2O7        = 0.0006 g of 'C'

Biomass carbon in sample (ppm)                 = 2 – {2- X/Y) x 0.00021 x 10/5 x 106/5

Biomass carbon in control (ppm)                 = 2 – {2- X/Y) x 0.00021 x 10/5 x 106/5

Biomass carbon present (ppm)                    = (Sample - Blank)