Wet Oxidation Soil Organic Matter
Tiurin's Method (modified)
Reference: Mebius, L.J. 1960. A Rapid Method for the Determination of Organic Carbon in the Soil. Analytica Chimica Acta. 22: 120-124.
This determination of soil organic matter is by oxidizing it with potassium dichromate (K2Cr2O7). The reaction generates heat when oil of vitriol (sulfuric acid: H2SO4) are added to potassium dichromate.
3 C + 2 K2Cr2O7 + 8 H2SO4 → K2SO4 + 2 Cr2(SO4)3 + 8 H2O + 3 CO2
- Weigh out 0.3 g of soil.
- Transfer the sample into the 100 ml Erlenmeyer flask.
- Pipette 10 ml of 0.4 N potassium dichromate (K2Cr2O7) into the flask.
- Put 2-3 glass boiling pellets into the flask, put funnels on the flasks (reciprocal evaporating dish).
- Put the flask on a heating plate under the ventilating hood and boil for 5 minutes taking care not to overheat the sample (timing from the beginning of the boiling).
- After 5 minutes of boiling, cool the samples to room temperature.
- Transfer the sample quantitatively into a 300 ml Erlenmeyer flask.
- Add 2 ml of 85% orthophosphorus acid (H3PO4) and 5 drops of of N-phenylanthranilic acid dissolved in a 0.2% Na2CO3 solution (phenylenediamine).
- Titrate the sample with 0.1 Mohr's salt [ferrous sulphate - FeSO4⋅(NH4)2] until a green color develops.†
- Conduct the same procedure with the blank.
- Repeat with two more replicates of the sample.
- Subtract blank from sample and determine average.
† 6 Fe(NH4)2SO4 + K2Cr2O7 + H2SO4 → 3 Fe2(SO4)3 + Cr2(SO4)3 + K2SO4 + 7 H2O + 6 (NH4)2SO4
Calculation to percentage of organic carbon (OC) formula:
% OC = (a–b) x 0.0003 x 100 / n
- a = ml of Mohr's salt titrated in blank
- b = ml of Mohr's salt titrated in sample
- n = weight of the sample (g)
presume that 1 ml of 0.1 N Mohr's salt equals 0.0003 g of organic carbon.
To calculate % organic matter (% OM) from % organic carbon (% OC) used the "Van Bemmelen factor" of 1.724.
% OC x 1.724 = % OM