DETERMINATION OF ORGANIC MATTER IN SOILS ( IS: 2720 – Part – 22)

In civil engineering, several studies showed that organic soils may show considerably low bearing capacities, and considered them as soft soils due to their high settlement values indeed under relatively applied loads. Consequently, there’s a tendency in the field of construction to avoid executing any projects above this type of problematic soils.
when exposed to structure’s loads, especially that wide areas of the area are covered with fine granulated soils containing different rates of organic matter. It is very necessary to know whether soil is organic or not. However, organic matter can also decrease the efficiency of compaction because the organic matter absorbs some of the energy transmitted to the sample. As per MORT& H specification , organic soil can not be used in road construction. Here is procedure how to find out the organic percentage in soil.

Determination of total Organic matter in soils.

Sample Preparation:

1. Total weight of original soil sample (Oven dried) – W1
2. Sieve the sample on 10 mm IS sieve and weighed the passing material – W2
3. Then sieve the sample on 425 micron IS sieve and take the soil sample for the test – Approximately 5 grams – W3 grams.

Preparation of Reagents:

1. Potassium Dichromate Normal solution: Dissolve 49.035 grams of Potassium Dichromate in one liter of distilled water.
2. Ferrous Sulphate 0.5 N solution: Dissolve 140 grams of Ferrous Sulphate in 0.5 N Sulphuric acid to make one liter of solution (Add 14 ml of concentrated Sulphuric acid to distilled water to make one liter of solution for 0.5 N Sulphuric acid).
3. Concentrated Sulphuric acid: Gr 1.83.
4. Ortho Phosphoric acid: Gr 1.70 to 1.75.
5. Indicator: 25 grams of Sodium Diphemylamine-Sulphonate dissolved in 100 ml of distilled water.

Standardization of Ferrous Sulphate Solution:

1. Take 10 ml of Normal Potassium Dichromate solution in to 500 ml conical
2. Add 20 ml concentrated Sulphuric acid and swirled and allowed to cool for some
3. Add 200 ml of distilled water, 10 ml of Ortho Phosphoric acid and 1ml of the Indicator and the mixture shall be shaking thoroughly.
4. Ferrous Sulphate solution added through burette in 0.5 ml increments, up to the solution changes from blue to
5. Add 5 ml Potassium Dichromate, then solution changing the color back to blue.
6. And then Ferrous Sulphate added drop by drop until the color of the solution changes from blue to Measure the total volume of Ferrous Sulphate solution in ml and recorded it as ‘X’.

Procedure:

1. Take 5 grams of soil sample of 425 micron IS sieve passing in 500 ml conical
2. Add 10 ml of Potassium Dichromate
3. Add 20 ml of concentrated Sulphuric acid and allowed to 30 minutes on a heat insulating surface like asbestos
4. Add 200 ml distilled water, 10 ml of Ortho Phosphoric acid, 1 ml of the Indicator and the mixture shall be shake
5. Ferrous Sulphate solution added through burette in 0.5 ml increments, up to the solution changes from blue to
6. Add 5 ml Potassium Dichromate, then solution changing the color back to blue.
7. And then Ferrous Sulphate added drop by drop until the color of the solution changes from blue to Measure the total volume of Ferrous Sulphate solution in ml and recorded it as ‘Y’.

Calculations:

1. The Volume of Potassium Dichromate used to oxidize organic, V= 5(1 – Y/X).
2. Percentage of Organic Matter in soil = (0.67W2V) / (W1W3)