Organic Carbon

Walkley-Black for mineral soils and Loss on Ignition (LOI) for organic soils.

from the Soil Survey Methods Manual:

Ball-mill one teaspoon of soil for 5 minutes in the ball-mill. Weigh out 0.250 (+/-) 0.001 g of soil into 250mL Erlenmeyer flasks. Add 10mL of potassium dichromate solution. Under the hood with the exhaust fan on, add 20mL concentrated sulfuric acid to each sample. (Be sure and wear a lab coat and safety glasses when adding the acid.) Let the samples cool for 30 minutes in the hood. Add about 200mL of water to each sample. Add 5 to 8 drops ferrous sulfate complex indicator to each sample. Titrate to a dark green end point with ferrous sulfate. Run two blanks first. Blanks should be within 0.1mL of each other. Samples run after the blanks should all be less than the blanks.

Potassium dichromate (K2Cr2O7)
Ferrous sulfate (FeSO4.6H20)
Sulfuric acid (H2SO4)
1, 10-Phenanthroline-ferrous sulfate complex

Preparation of Reagents
1.0N Potassium dichromate
Dissolve 49.04g per liter in deionized water using volumetric flask and make to volume.

0.5N Ferrous sulfate
Dissolve 138.30g plus 40mL concentrated sulfuric acid per liter in deionized water and make to volume.

Sample Preparation and Procedures
Grind four ½-teaspoon scoops of soil in ball-mill for 3 minutes. Sample weight is based on estimated organic matter content as follows:

1.000g for light colored soils
0.500g for gray to brown soils
0.250g for most soils
0.100g for black soils

Weigh in duplicate, ground soil into 25mL Erlenmeyer flasks. Add exactly 10.0mL of 1.0N potassium dichromate and swirl gently to thoroughly wet soil. Place samples in fume hood. Carefully add 20mL concentrated sulfuric acid down the side of the flask into soil solution and swirl gently to thoroughly mix. Allow to stand for at least 1 hour. Include blanks which also serve as standards.

To digest samples, carefully add about 200mL water, washing sides of flasks. Add 5 drops of ferrous sulfate complex indicator to each flask and titrate with 0.5N ferrous sulfate. The endpoint is a sharp change from orange to green to reddish-brown.

Sample Calculations
Normality FeSO4 = [(N K2Cr2O7) (mL)] divided by (mL blank titer)
Percent organic carbon = {[(mL blank)-(mL sample titer)] divided by g of sample}(Norm. FeSO4)(0.3/0.77)
Note: Ferrous sulfate FeSO4 has a valence of +2
69.15g/L = 0.25 N
138.30 g/L = 0.50 N

Soil Survey Methods Manual. August 1992.  p.346.  U.S. Department of Agriculture.