REAGENTS

  1. Potassium Dichromate: K2Cr2O7
  2. Sulfuric Acid: H2SO4
  3. Ferrous Ammonium Sulfate: Fe(NH4)2(SO4)2·6H2O
  4. Ferrous Sulfate: Fe2SO47 H2O
  5. 1,10 Phenanthroline

SOLUTIONS

  1. 0.267 N Potassium Dichromate in Sulfuric Acid:
    1. Dissolve 13.092 g potassium dichromate in 400 mL deionized water.
    2. Slowly add 550 mL of concentrated sulfuric acid. Caution - Exothermic reaction.
    3. Let cool, transfer to a 1 liter volumetric flask.
    4. Dilute to volume with deionized water and mix well.
  2. 0.2N Mohr's Salt:
    1. Weigh 78.415 g ferrous ammonium sulfate into a 1 liter volumetric flask containing 500 mL deionized water.
    2. Dissolve and slowly add 50 mL of concentrated sulfuric acid and let cool.
    3. Dilute to volume with deionized water and mix well.
    4. Prepare daily.
  3. Ferroin Indicator Solution:
    1. Weigh 3.71 g of 1,10 Phenanthroline into a 250 mL volumetric flask.
    2. Add 1.74 g ferrous sulfate.
    3. Dissolve and dilute to volume with deionized water and mix well.

PROCEDURE

  1. Weigh 0.1 - 0.5 g soil (depending on expected organic carbon content) into a 250 mL erlenmeyer flask.
  2. Add 15 mL of 0.267N potassium dichromate solution.
  3. Connect to a reflex condenser and boil for 15 minutes.
  4. Let cool.
  5. Wash down the condenser and flask with deionized water.
  6. Add 10 drops of ferroin indicator solution.
  7. Titrate with Mohr's salt solution.
  8. The color of the solution at the beginning is yellow-orange to dark green, depending on the amount of unreacted dichromate. As the end point approaches, the color will shift to a turbid gray. At the end point the color will be a dark red.
  9. A blank consisting of 15 mL. 0.267 N potassium dichromate solution is titrated in exactly the same manner.

CALCULATIONS

  1. % organic carbon = [(A-B) × .003 × 1.15 × 100] ÷ g soil A = mL of potassium chromate solution × normality of potassium dichromate solution. B = mL of Mohr's solution × normality of Mohr's solution.
  2. For the conditions specified in the procedure,
    % organic carbon = [1.3817 (0.69 × mL titration)] ÷ g soil
  3. The blank is used to determine the normality of the Mohr's salt solution. Normality Mohr's solution = (mL. K2Cr207) ÷ mL. Mohr's solution titration
  4. If the normality of the Mohr's solution is not 0.2 then equation 1 will have to be used to calculate the % organic carbon.

REFERENCE

  1. Mebius, L. J., 1960, A rapid method for the determination of organic carbon in soil. Anal. Chem. Acta. 22:120-124.