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ORGANIC CARBON IN SOIL - DETERMINATION BY WALKLEY & BLACK METHOD.

TITLE :- Determination of organic carbon in soil by walkley & black method. 

Principle -  

The organic carbon in organic matter is oxidised by known excess of chromic acid .(K2 cr2 O7 + H2 s o4). The excess acid chromic acid not reduced by organic matter is determined by Black titration which standard feso4 solution (redox titration) using ferroin indicator. the organic in soil calculated from the chromic acid utilised (reduced by it)

Reagent 

  1. Potassium dichromate solutions, 

Dissolve 49.04gm of dried reagents grade potassium dichromate, K2cr2O7 in distilled water & dilute it to one liter. 

  1. Ferrous sulphate solutions 0.5N

Dissolve 139gm of reagent grade ferrous sulphate feso4. 7H2o in distilled  water and add 15ml of concentrated H2so4 cool and dilute to one litters or dissolve 196.19gm ferrous  ammonium sulphate feso4(NH4)so4.6H2o and making up-to one litters standardized the solution against potassium add 20 ml concentrated H2so4, before making 1 liter  volume. 

  1. Ferroin indicators ortho-phenanthroline-ferrous complex 0.025N.

Dissolve 1.485gm ortho-phenanthroline monohydrate and 0.695 gm feso4. 7H2o in water dilutes the solution to 100ml. 

PROCEDURE 

  1. Take 1gm of soil in a 500ml conical flask. 

  2. Add 10ml k2cr2o7 & 20 ml concentrated H2so4 

  3. Stir the flask 

  4. Keep it for half an hour. 

  5. Add 200ml distilled water, add 3, 4 drops ferroin indicator. 

  6. Titrate against 0.5N ferrous sulfate till colour changes to brick red. 

  7. Run the blank. 

  8. End point.(brown, green, blue & red)  

OBSERVATION

  1. weight of soil= 1gm

  2. ml of feso4 required blank sample = ------ml

  3. ml of feso4 require for sample =------ml

  4. Normality = 0.5N feso4

FORMULA 

(B-T)

 × normality of ferrous sulphate × 0.003 × 100 × 1.3/weight of soil sample. 

CALCULATION

ORGANIC CARBON =                 (B-T) ×0.5×0.003×100 ×1.3/1

=(B-T)×0.0015×1.3

=(B-T) ×0.195

INTERPRETATION (SIX TIER SYSTEM) 

  1. VERY LOW.                   <0.20%

  2. IOW.                               0.20 TO 40%

  3. MODERATE.                  41 TO 60 %

  4. MODERATELY HIGH     61 TO 80%

  5. HIGH.                             81 TO 100%

  6. VERY HIGH.                  >100%.

RESULTS 
According to %  of organic soil is ------(ratings) in organic carbon containt. 
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