Wet Oxidation Soil Organic Matter
In this reaction carbon is oxidized by the dichromate ion. Excess
dichromate ion is then back titrated with ferrous ion.
- Dichromate ion reacts with carbon as follows:
Cr2O72- 3 Co+ 16H+ = 4 Cr3+ + 3 CO2 + 8H2O
- Ferrous ion reacts with dichromate as follows:
6 Fe2+ + Cr2O72- + 14 H+ = 2 Cr3+ + 6 Fe3+ + 7 H2O
- Potassium Dichromate: K2Cr2O7
- Ferrous Ammonium Sulfate: Fe(NH4)2(SO4)2 · 6 H2O
- Sulfuric Acid: H2SO4
- Phosphoric Acid: H3PO4
- Sodium Fluoride: NaF
- Diphenylamine: C6H5NHC6H5
1N Potassium Dichromate:
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.
0.5N Ferrous Ammonium Sulfate
- Slowly add 20 ml. sulfuric acid to a 1 liter volumetric flask
containing 800 ml. deionized water.
- Add 196.1 g ferrous ammonium sulfate. Dissolve, dilute to volume
with deionized water, and mix well.
- Prepare daily.
- Dissolve 0.500 g diphenylamine in 20 ml. deionized water.
- Slowly add 100 ml. sulfuric acid. Carefully mix with a glass
CAUTION: this solution is corrosive and can cause sever burns.
Proper precautions are given on the MSDS sheet for sulfuric acid.
- Weigh 1.00 g soil into a 500 ml. erlenmeyer flask.
- Add 10 ml. of 1N potassium dichromate solution.
- 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.
- Dilute to 200 ml. with deionized water.
- Add 10 ml. phosphoric acid, 0.2g ammonium fluoride, and 10 drops
- 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.
- Prepare and titrate a blank in the same manner.
- Prepare one duplicate sample and one quality control sample with
each set of samples analyzed.
% Organic Matter = 10[1(S-B)] X 0.67
S = sample titration
B = blank titration
- Values on the duplicate samples must agree within 20% of the average
of the two values.
- 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.
- Jackson, M. L. 1958. Soil Chemical Analysis. 214–221.
- 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.
- 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.
- Schollenberger, C. J. 1927. A Rapid Approximate Method for
Determining Soil Organic Matter. Soil Sci. 24:65–68.