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Soil Analysis-Determination of Available Organic Carbon content in the Soil



Determination of Soil Organic Carbon:


  • Take 1 g of soil in a 500 mL conical flask.
  • Add 10 mL of 1N K2Cr2O7 solution and shake to mix it.
  • Then add 20 mL Con. H2SO4 and swirl the flask 2 or 3 times.
  • Allow the flask to stand for 30 minutes on an asbestos sheet for the reaction to complete. 
  • Pour 200 mL of water to the flask to dilute the suspension. Filter if it is expected that the end point of the titration is not to be clear.
  • Add 10 mL of 85% H3PO4 and 1 mL of Diphenylamine indicator and back titrate the solution with 0.5 N Ferrous Ammonium Sulphate, till the colour flashes from violet through blue to bright green. H3PO4 gives sharper endpoint, by making the colour change, distinct through a flocculating effect.
  • Note the volume of Ferrous Ammonium Sulphate.
  • Carryout blank titration (without soil) in a similar manner.


i. Calculation:


% of Organic Carbon in Soil (R) is,

R=\frac{(V_{1}-V_{2})\times N\times 0.003\times 100}{W}\times C




W - Weight of Sample

V1 - Blank Titre value

V2 - Titre value of the Sample

N - Normality of K2Cr2O7 (Here it is 1N)

C - Correction Factor (1.334, 1.724)



Points to Remember while Performing the Experiment in a Real Laboratory:


  1. Always wear lab coat and gloves when you are in the lab. When you enter the lab, switch on the exhaust fan and make sure that all the chemicals and reagents required for the experiment are available. If they are not available, prepare the reagents using the components for reagent preparation.
  2. Properly adjust the flame of the Bunsen burner. The proper flame is a small blue cone; it is not a large plume, nor is it orange.
  3. Make sure to clean all your working apparatus with chromic acid and distilled water and ensure that all the apparatus are free from water droplets while performing the experiment.
  4. Make sure to calibrate the electronic weigh balance before taking the measurements.
  5. Ensure that the desiccator  has sufficient amount of desiccant; Silica gel
  6. Use chromic acid to clean the crucible, then heat it and make sure to cool it before placing in the desiccators. Ensure that you are handling the crucible, with cleaned tongs or with tissue paper .Never touch it with your hand.
  7. Switch on the oven and adjust the temperature to 1300 C. Make sure to use a cotton glove while working with a hot air oven.
  8. Make sure to clean the Kipp's apparatus tube with water and ensure that it has sufficient solid material; iron sulfinide and acid, H2SO4 for producing H2S gas.
  9. Clean all glassware with soap and distilled water. Once the experiment is completed, recap the reagent bottles. Switch off the light, exhaust fan, hot air oven and gas cylinder before leaving the lab.
  10. Discard the used gloves in a waste bin.



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