Sunday 31 January 2016

Determination of Organic Matter/Organic Carbon in Soil


    Samples are digested with an excess of acidic potassium dichromate solution. The chromium
    is reduced from the Cr (VI) oxidation state to the Cr (III) state by the oxidisable organic
    carbon present in the sample. The unreacted Cr (VI) can then be titrated with ferrous
    sulfate to determine the amount of Cr (VI) consumed. The oxidisable organic carbon, organic
    matter and total organic carbon can be calculated.


2.1 Underestimation of the organic matter/organic carbon may result when higher oxides of       manganese are present in substantial quantities. The possible effects of these
      interferences should be taken into account in the analysis of some types of poorly aerated

2.2 Over estimation of organic matter/carbon may occur due to the presence of large amounts
      of chloride or metallic or ferrous iron or sulfides in the soil sample.

      The interference due to chlorides may be minimised by either washing the sample with
      water or replacing the concentrated sulphuric acid with COD digestion reagent. The silver
      in the reagent precipitates the chlorides in the sample.

      The sulfides can be destroyed prior to crushing by the addition of dilute sulfuric acid. Add
      acid until no further evolution of hydrogen sulfide occurs. The sample should be dried prior
      to crushing.


3.1 Sulphuric acid (H2SO4), 98%, AR

3.2 Silver sulphate (Ag2SO4), AR

3.3 Sulfuric acid reagent: refer to QWI-EN/EP026 or prepare as below: add 25.3 g of silver       sulphate (3.2) to a 2.5 L Winchester of sulphuric acid (3.1). Let stand for 24 hours
      to dissolve. Mix.

3.4 Potassium dichromate (K2Cr2O7), AR

3.5 Potassium dichromate, 0.1667 M: Accurately weigh 49.035 g of K2CR2O7 (5.4) and dissolve in       water. Make up to 1 L in a volumetric flask. Verify by running x 200 on ICP-AES.
      The expected Cr result I 17320 mg/L. For acceptance/rejection criteria refer to QWI-EN/50.

3.6 Sulfuric acid, 1 N: Carefully add 28 mL of sulfuric acid (3.1) to 800 mL of reagent grade       water. Allow to cool and make to 1 L. This may be done in a beaker.

3.7 Sulfuric acid, 0.1 N: Carefully add 100 mL of 1 N sulfuric acid (3.1) to 800 mL of reagent       grade water. Allow to cool and make to 1 L.

3.8 Ferrous sulphate heptahydrate (FeSO4.7H2O), AR

3.9 Standard ferrous sulfate titrant, ~0.5 M: Dissolve 140 g FeSO4.7H2O (3.8) in ~800 mL of       water. Carefully add 14 mL of H2SO4 (3.1), cool and dilute to 1 L. Standardise on use.

3.10 Sodium diphenylamine-sulfonate, AR

3.11 Indicator solution: Dissolve 0.25 g of sodium diphenylamine-sulfonate (3.10) in 100 mL of       water.

3.12 Phosphoric acid, 85%, AR

3.13 Organic Matter Standard – Garden soil. In-house.


4.1 Mortar and pestle

4.2 Drying oven, 104 +/- 10oC

4.3 Balance (0.0001 g accuracy)

4.4 Volumetric flask, 100 and 1000 mL

4.5 Beaker, capable of holding 1 L

4.6 Pipettes, 1 and 10 mL

4.7 Measuring cylinders, glass, 20 and 200 mL

4.8 Erlenmeyer flasks, 250 or 500 mL

4.9 Burette, 25 Ml, 0.1 mL increments

4.10 Hotplate or similar heat insulating surface.


5.1 Sample Pre-treatment

      5.1.1 Dry sample at 104 +/- 10oC and crush using a mortar and pestle.

      5.1.2 If sample contains sulfides or chlorides, the following procedure is to be employed.
               Carefully (wearing glasses, gloves and laboratory coat) wash about 10 g of sample
               with COD digestion 0.1 N sulphuric acid reagent (5.6) until all effervescence ceases.                The sample is then washed with reagent grade water, dried and crushed again.

5.2 Weigh 0.2-5.0 g (+/-0.01 g)* of sample and standard (5.13) into a 250 mL conical flask.

      Note: The size of sample for chemical analysis will vary with the amount of organic matter
      present in the soil. The most suitable size of sample to be used is that it is giving a total of
      5 mL to 8 mL dichromate solution reduced.


Soil Type Appropriate Amount of Soil
Organic horizon* 0.1-0.2 g
Heating rate 5 to 6°C/min
Surface Soils 0.5 g
Subsoils 2.0 g

* Surface layer of decomposing material not significantly mixed with the mineral soil.

5.3 Run 10 mL of the 0.1667 M K2Cr2O7 solution (3.5) into the conical flask containing the       sample, standard or blank from a burette and, very carefully, add 20 mL sulphuric acid
      reagent (3.3) using a measuring cylinder.

      Note: Standardise FeSO4 by running 3 blanks through steps 5.3-5.9.

5.4 Swirl the mixture thoroughly for about 1 min and then stand it on a heat insulating surface       for 30 minutes to allow oxidation of the organic matter to proceed.

5.5 Add 200 mL of water followed by 10 mL of phosphoric acid (3.12) and 1 mL of the indicator       (3.11) to the flask, shake the resultant mixture thoroughly. If the indicator is absorbed by
      the soil, add a further 1 mL of the solution.

      Note: The heat transfer affects the extent and rate of reaction. Therefore it is crucial that
      the flask is cooled prior to the titration in order to obtain uniformity of results.

5.6 Titrate with 0.5 M of FeSO4 (3.9) in 0.5 mL increments, swirling the contents of the flask       until the colour of the solution changes from blue to green.

      Note: If the sample background is high, filter the suspension through an acid-washed glass
      fibre filter.

5.7 Add a further 0.5 mL of K2Cr2O7 solution (3.5) changing the colour back to blue.

5.8 Titrate with 0.5 M of FeSO4 (3.9) drop by drop, with continued swirling until the colour       changes from blue to green.

5.9 Record the total volume of FeSO4 titrant to the nearest 0.05 mL.


6.1 Calculation Equation for Organic Matter and TOC

                     V (mL) x 0.67
          OM% = -------------------
                      Wt. Sample (g)

          TOC% = OM% x 0.58 OR

                     V (mL) x 0.39
          TOC% = -------------------
                      Wt. Sample (g)

      OM%   =    organic matter present in the oven dry sample, in percent
      TOC%  =    total organic carbon present in the oven dry sample, in percent
      V       =    volume of K2Cr2O7 needed to oxidise the organic matter in the soil (mL)
               = 10.5 x (1-V2/V1)
      Wt     =    mass of sample soil used in the test (oven-dried) (g)

6.2 Relationship Between Organic Matter (OM%), Total Organic Carbon (TOC%) and Oxidisable
      Organic Carbon (OOC%)

      The procedure is based on the determination of the oxidisable organic carbon (OOC)       content of the soil and assumption that:
      (a) Soil organic matter contains an average of 58% of carbon by mass.
      (b) 77% of the carbon in the organic material is oxidised.
      (c) 1 mL of volume of K2Cr2O7 consumed is equivalent to 3 mg of carbon (Piper, 1943, Soil
      and Plant Analysis).

      The percentage of organic matter (OM) present in the oven-dried sample is calculated

      OM% = TOC% / 0.58

      OM% = ---------------
                  0.58 x 0.77

      TOC% = OM% x 0.58
                = OOC% x 1.3

      Because the Oxidisable Organic Carbon in the oven-dried sample is calculated below:

                    (V1-V2) x NFeSO4 x 0.003 (g/mL) x 100%
      OOC% = -------------------------------------------------
                                Wt. Sample (g)

                    10.5 x (1-V2/V1) x 6 x 0.1667 x 0.3
                = -------------------------------------------------
                                Wt. Sample (g)

                = V x 0.3 / Wt. Sample (g)


                  V (mL) x 0.67
      OM% = ---------------------
                  Wt. Sample (g)

                  V (mL) x 0.39
      TOC% = ---------------------
                  Wt. Sample (g)

      OM%    =   organic matter present in the oven dry sample, in percent
      TOC%   =   total organic carbon present in the oven dry sample, percent
      OOC%   =   oxidisable organic carbon present in the oven dry sample, in percent
      V      =   volume of K2Cr2O7 required to oxidise the organic matter in the soil (mL)
              = 10.5 x (1-V2 / V1)
      Wt   =   mass of sample soil used in the test (oven-dried) (g)
      N     =   normality of FeSO4 (N)
      V2    =   total volume of FeSO4 used in the sample (mL)
      V1    =   average of total volume of FeSO4 used in blank (mL)

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