Estimation of Organic Carbon and Calcium Carbonate | Soil Analysis

In this article we will discuss about the estimation of organic carbon and calcium carbonate for analysing soil.

Estimation of Organic Carbon:

Principle:

Soil is digested with chromic acid (K₂Cr₂O₇ + H₂SO₄) making use of the heat evolved in the mixture. The excess chromic acid that is not reduced by the organic matter in the soil is determined by titrating with  standard fer­rous sulphate or ferrous ammonium sulphate solution using diphenyl amine as indicator.

Requirements:

Reagents:

1. 1 (N) Potassium dichromate Dissolve 49.04 g of K₂Cr₂Oy and make up the volume to 1000 ml.

2. 0.5(N) Ferrous ammonium sulphate (196 g of hydrated salt in one litre) containing 1.25% silver sulphate.

3. Orthophosphoric acid (85%) or sodium flouride.

4. H₂SO₄ containing 1.25% silver sulphate.

5. Indicator: Diphenyl amine.

6. Soil sample.

7. Sieve.

8. Titration apparatus.

9. Distilled water.

10. Conical flasks (500 ml).

11. Pipettes.

12. Asbestos sheet.

Procedure:

1. Sieve the soil through 0.2 mm sieve, weigh one gram and place it at the bot­tom of a dry 500 ml Erlenmeyer conical flask.

2. Pipette out 10 ml of 1(N) K₂Cr₂0₇ into the flask swirling it a little and keep it on an asbestos sheet.

3. Add 20 ml of H₂S0₄ containing 1.25% silver sulphate. Swirl the flask two to three times and allow the flask to stand for 30 minutes.

4. Add 200 ml of distilled water.

5. Incorporate 10 ml phosphoric acid or 0.5 g of Sodium fluoride and 1 ml of diphenylamine indicator.

6. Titrate with 0.5(N) ferrous ammonium sulphate till colour flashes from blue to violet to green.

7. Run a blank also without soil.

Repeat the process if more than 7 ml of dichromate solution is consumed, with lesser quantity of soil.

Calculation:

Where, B = the volume of ferrous am­monium sulphate required for blank.

T = Volume of soil sample.

Organic matter = % of organic carbon X 1.784.

Estimation of Calcium Carbonate:

Approximate amount of CaCO₃ in soil can be obtained by rapid titration method.

Requirements:

1. 0.1 (N) HCl (175 ml of con. HCl diluted to 2 litres with distilled water. No need to standardize).

2. Standard decinormal 0.1 (N) NaOH.

3. Phenolphthalein.

4. Soil sample.

5. Titration apparatus.

6. Flasks.

Procedure:

1. Weigh 5.0 g soil sample and place it in a 250 ml flask.

2. Pipette out 50 ml of 1 (N) HCl into it and stir vigorously several times during a pe­riod of one hour and allow to settle.

3. Filter and take 5.0 ml of filtrate into an­other flask.

4. Add 3-5 drops of phenolphthalein.

5. Titrate against standard 0.1 (N) NaOH (if colour of indicator fades nearing end point, add some more indicator).

6. Run a blank without soil to obtain titre value of HCl (with 5 ml HCl).

Calculation:

% of CaCO₃ in soil= (Blank titre value – actual titration value) × Normality of NaOH.