Wet Oxidation Soil Organic Matter

Walkley-Black method

PRINCIPLE

  1. In this reaction carbon is oxidized by the dichromate ion. Excess dichromate ion is then back titrated with ferrous ion.
    1. Dichromate ion reacts with carbon as follows:
      Cr2O72- 3 Co+ 16H+ =  4 Cr3+ + 3 CO2 + 8H2O
    2. Ferrous ion reacts with dichromate as follows:
      6 Fe2+ + Cr2O72- + 14 H+ =  2 Cr3+ + 6 Fe3+ + 7 H2O

REAGENTS

  1. Potassium Dichromate: K2Cr2O7
  2. Ferrous Ammonium Sulfate: Fe(NH4)2(SO4)2 · 6 H2O
  3. Sulfuric Acid: H2SO4
  4. Phosphoric Acid: H3PO4
  5. Sodium Fluoride: NaF
  6. Diphenylamine: C6H5NHC6H5

SOLUTIONS

  1. 1N Potassium Dichromate:
    2. 1. Weigh 49.04 g potassium dichromate (previously dried for 2 hours at 100 C) into a 1 liter volumetric flask. Dissolve and dilute to volume with deionized water and mix well.
  2. 0.5N Ferrous Ammonium Sulfate
    1. Slowly add 20 ml. sulfuric acid to a 1 liter volumetric flask containing 800 ml. deionized water.
    2. Add 196.1 g ferrous ammonium sulfate. Dissolve, dilute to volume with deionized water, and mix well.
    3. Prepare daily.
  3. Diphenylamine Indicator:
    1. Dissolve 0.500 g diphenylamine in 20 ml. deionized water.
    2. Slowly add 100 ml. sulfuric acid. Carefully mix with a glass stirring rod.
      CAUTION: this solution is corrosive and can cause sever burns. Proper precautions are given on the MSDS sheet for sulfuric acid.

PROCEDURE

  1. Weigh 1.00 g soil into a 500 ml. erlenmeyer flask.
  2. Add 10 ml. of 1N potassium dichromate solution.
  3. Add 20 ml. sulfuric acid and mix by gentle rotation for 1 minute, taking care to avoid throwing soil up onto the sides of the flask. Let stand for 30 minutes.
  4. Dilute to 200 ml. with deionized water.
  5. Add 10 ml. phosphoric acid, 0.2g ammonium fluoride, and 10 drops diphenylamine indicator.
  6. Titrate with 0.5N ferrous ammonium sulfate solution until the color changes from dull green to a turbid blue. Add the titrating solution drop by drop until the end point is reached when the color shifts to a brilliant green.
  7. Prepare and titrate a blank in the same manner.
  8. Prepare one duplicate sample and one quality control sample with each set of samples analyzed.

CALCULATION

% Organic Matter = 10[1(S-B)] X 0.67
          S = sample titration
          B = blank titration

QUALITY CONTROL

  1. Values on the duplicate samples must agree within 20% of the average of the two values.
  2. Values on the quality control sample must lie within the limits established for this sample.

NOTE: This procedure requires the routine use of sulfuric acid. Sulfuric acid is a corrosive, strong oxidant and should be handled with caution. Refer to the MSDS sheet for proper handling.

REFERENCES

  1. Jackson, M. L. 1958. Soil Chemical Analysis. 214–221.
  2. Walkley, A. 1947. A Critical Examination of a Rapid Method for Determination of Organic Carbon in Soils - Effect of Variations in Digestion Conditions and of Inorganic Soil Constituents. Soil Sci. 63:251–257.
  3. Walkley, A. and I. A. Black. 1934. An Examination of Degtjareff Method for Determining Soil Organic Matter and a Proposed Modification of the Chromic Acid Titration Method. Soil Sci. 37:29–37.
  4. Schollenberger, C. J. 1927. A Rapid Approximate Method for Determining Soil Organic Matter. Soil Sci. 24:65–68.