The harmful content of soil such as organic matter adversely affect its characteristics and behavior and therefore should be within 2%.
- Take an air dried soil sample and determine the moisture content in it. Using the second sample, produce approximately 100 g of clay passing through a 10 mm IS sieve. Powder this sample to pass through a 425 µm IS sieve.
- If the soil contains a high percentage of sulfides or chlorides it can be neutralized with dilute sulfuric acid or washed with distilled water to remove the salts.
- Take 5 grams of clay from a well-mixed clay filtered through a 425 micron sieve and keep it in a glass bottle. The weight of the sample should be low (up to 0.2 g) for soils with a high content of organic matter.
- Pour 10 ml of potassium dichromate solution with burette into the conical flask and add 20 ml of concentrated sulfuric acid. Swirl the mixture for about a minute and place it on an heatproof surface for 30 minutes to allow oxidation.
- Mix 200 ml of distilled water with 10 ml of orthophosphoric acid and 1 ml of sodium fluoride as the indicator. Shake the mixture vigorously. If the indicator is absorbed, add another 1 ml of the indicator.
- Add ferrous sulphate solution to the second burette until the color of the solution changes from blue to green. To change the color from green to blue add another 0.5 ml of potassium dichromate.
- Add the solution of ferrous sulphate drop by drop by swirling continuously until the color of the solution changes by a drop. Note the total volume of ferrous sulphate solution used in the experiment.
The total amount of potassium dichromate used for the oxidation of organic matter in soil is given by the following formula:
V = 10.5 [1.0 – (Y/X)]
Y = total volume of ferrous sulfate used in the test
X = Total amount of ferrous sulfate used in the standardization test
The percentage of organic matter present in the oven-dried sample can be calculated with the following formula:
Organic matter, percentage by weight = (0.67*W.)2*v) / (w.)1*And3,
woo2 = Weight on the oven-dried base of the soil sample passed through a 10 mm sieve
V = total volume of potassium dichromate used in the oxidation of organic substances
woo1 = weight on an oven-dried basis of the total soil sample before filtration
woo3 = Weight on oven-dried basis of the soil sample used in the test
IS 2720 (Part 22) – 1972