SOIL CLASSIFICATION AND SOIL TESTING (Part - 3)

CONTENTS 

 

13.	DELETERIOUS CONTENTS OF SOILS.
I	DETERMINATION OF TOTAL SOLUBLE SULPHATES (IS : 2720 – Part – 27)
II	DETERMINATION OF ORGANIC MATTER (IS : 2720 – Part – 22)

  

13. DELETERIOUS CONTENTS OF SOILS.

I. Determination of Total Soluble Sulphates (IS : 2720 – Part – 27)

 A. By Precipitation Method:

 Object:

 

Determination of total soluble sulphate content of soil by precipitation method.

 

Apparatus:

 

1) Analytical Balance – sensitive to 0.001 g ,

2) Glass Beaker – 250 ml capacity,

3) Glass Funnel – 50 mm diameter,

4) Glass Bottle – 250 ml capacity with a rubber cork,

5) Crucible – 50 ml capacity,

6) Pipette – 25 ml,

7) Burette – 50 ml with 0.1 ml graduation,

8) Filter paper – Whatman No.42,

9) Heating equipment, Muffle furnace, Mechanical shaker, Drying oven and A Mortar with rubber-covered pestle.

 

Reagents:

 

1. Phenolphthalein Indicator Solution: Dissolve 0.1 g of phenolphthalein in 60 ml of

rectified spirit and dilute with distilled water to 100 ml.

 

2. Concentrated Hydrochloric Acid: Specific gravity 1.18 (conforming to IS:265-1976)

 

3. Barium Chloride Solution: 5 percent. Dissolve 5 g of barium chloride in 100 ml of distilled water.

 

4. Silver Nitrate Indicator Solution: 0.5 percent. Dissolve 500 mg of silver nitrate in 100 ml of distilled water.

 

Procedure:

 

a) Soil Sample: The soil sample shall be brought to a state in which it may be crumbled if necessary, by drying it in an oven maintained at 105 to 1100C. The aggregations of particles shall be broken up in mortar with rubber-covered pestle or the mechanical device. The sample shall be thoroughly mixed and then sub-divided by quartering.

 

b) Procedure: Take 10 g of soil from the sample prepared as above in a 250 ml bottle with 100 ml of distilled water. Give occasional shaking for 2 hours by means of the mechanical shaker. Allow the soil suspension to stand overnight. Filter and take 25 ml of filtrate in a beaker and add concentrated hydrochloric acid to just neutralize the solution if it is found alkaline to phenolphthalein indicator. Add further 4 ml concentrated hydrochloric acid to make the solution acidic. Boil the soil suspension and add barium chloride filter it. The precipitation shall be wash free from chloride ions. The filteration mass also done through a preheated with sintered glass crucible. In the case of filter paper after drying ashing shall be done on a low flame and the precipitate then ignited over a burner or in a muffle furnace at 600 to 7000C for half an hour cool in a desiccator weigh and note the weight of the residue. This is the weight of barium sulphate. A corresponding weight of sodium sulphate should be calculated and thus its percentage determined.

 

Note: To check whether the residue is free of chloride ions, collect the washings in a separate test tube at different time intervals and a drop of 0.5 percent silver nitrate solution to it. The formation of white cloudy precipitate shows the presence of chloride ions in the precipitate. Continue washing until the white precipitation is not formed in the washings by the addition of silver nitrate solution. Hot water may be used for washing.

 

Calculations:

       W1

Sulphates (as SO4), by mass = 41.15 -------

       W2

 

  W1

Sulphates (as Na2 S04), by mass = 60.85 -------

   W2

 

Where,

W1 = mass in g of the precipitation and

W2 = mass in g of the soil contained in the solution taken for precipitation.

 

B. Volumetric Method:

 

Reagents:

 

1. Barium Chloride Solution: N/4. Dissolve 30.54 g of barium chloride in one litre of distilled water.

2. Potassium Chromate Solution: N/4. Dissolve 24.275 g of potassium chromate in a small amount of distilled water. Add a few drops of silver nitrate solution to it to remove any chloride, filter and dilute to 250 ml.

 

3. Silver Nitrate Indicator Solution: 0.5 percent. Dissolve 500mg of silver nitrate in 100 ml of distilled water.

 

4. Dilute Solution of Ammonium Hydroxide: Sp.Gr.0.888. Mix ammonium hydroxide and distilled water in the ratio of 1:2.

 

5. Concentrated Hydrochloric Acid: Sp.Gr.1.11 (conforming to IS:265-1976).

 

Procedure:

 

Weigh 10 g of the soil specimen obtained by the method specified in above, in a beaker and add about 50 ml water. Stir well, allow to decant, filter, wash the soil on filter paper with a small quantity of water and make the filtrate to 100-ml. Pipette out 10 ml of the water extract in a conical flask, make it slightly acidic by adding concentrated hydrochloric acid and heat to boiling. While boiling, add barium chloride solution (N/4) from the burette till the precipitation is complete and barium chloride solution is in slight excess.

 

Neutralize the solution with ammonium hydroxide and titrate the excess of barium chloride against potassium chromate solution (N/4). The end point may be confirmed, if considered necessary, by using silver nitrate solution as an external indicator.

 

Calculations:

 

Sulphates as sodium sulphate in soil, percent by mass = 0.0177 X 100 (x – y)

 

Where,           x = volume of N/4 barium chloride added in ml.

y = volume of N/4 potassium chromate solution used in back titration in ml.

 

II. DETERMINATION OF ORGANIC MATTER (IS : 2720 – Part – 22)

 

Apparatus:

1) Oven – thermostatically controlled (105 to 1100C),

2) Balance – sensitive to 0.001 g,

3) Volumetric Flask – 2 nos of one litre capacity,

4) Burette – 2 nos of 25 ml with 0.1 ml graduation,

5) Pipette – 10 ml,

6) Conical Flask – 2 nos of 500 ml capacity,

7) Graduated Measuring Cylinders – 200 ml & 25 ml capacities,

8) Desiccator – with any desiccating agent other than sulphuric acid,

9) Glass Weighing Bottle – 25 mm diameter & 50 mm height fitted with a ground glass stopper,

10) Wash Bottle,

11) Sieves – 10mm & 425 micron.

 

Reagents:

 

1. Potassium Dichromate Normal Solution: Dissolve 49.035 g of potassium dichromate in one litre of distilled water.

 

2. Ferrous Sulphate Solution: 0.5 N – Dissolve approximately 140 g of ferrous sulphate in 0.5 N sulphuric acid to make one litre of solution (add 14 ml of concentrated sulphuric acid to distilled water to make one litre of solution for 0.5 N sulphuric acid).

 

3. Sulphuric Acid, Concentrated: sp.gr. 1.83 (conforming to IS : 266-1977).

 

4. Orthophosphoric Acid, 85 percent: sp.gr. 1.70 to 1.75.

 

5. Indicator Solution: 0.25 g of sodium diphemylamine-sulphonate dissolved in 100 ml of distilled water.

 

Soil sample preparation:

 

The portion of the air-dried sample selected and weighed (W1). It shall then be sieved on a 10 mm IS sieve and all particles other than stones crushed to pass through the sieve.

 

The equivalent weight on oven-dry basis of the material passing 10 mm IS sieve (W2) shall be calculated and recorded to the nearest 0.1 percent of its total weight. A sample weighing approximately 100 g shall be obtained from the material passing 10 mm IS sieve by quartering. This sample shall then be pulverized so that it passes the 425 micron IS sieve.

 

Standardization of ferrous sulphate solution:

 

10 ml of the normal potassium dichromate solution will be run from a burette into a 500 ml conical flask. 20 ml concentrated sulphuric acid shall then be added carefully and the mixture swirled and allowed to cool for some minutes. 200 ml of distilled water shall then be added to the mixture followed by 10 ml of phosphoric acid and 1 ml of the indicator and the mixture shall be shake thoroughly. Ferrous sulphate solution shall then be added from the second burette in 0.5 ml increments, the contents of the flask being swirled, until the color of the solution changes from blue to green. A further 0.5 ml of potassium dichromate shall then be added, changing the color back to blue. Ferrous

sulphate solution shall then be added drop by drop with continued swirling until the color of the solution changes from blue to green after the addition of a single drop. The total volume of ferrous sulphate solution used (x) shall be noted to the nearest 0.05 ml (one ml ferrous sulphate solution is equivalent to 10.5/x ml potassium dichromate).

Procedure:

A 5-gram soil sample shall be taken from the thoroughly mixed portion of the material passing the 425-micron IS sieve and used for the test.

 

The sample shall be placed in a glass weighing bottle and weighed to 0.001 g. A small quantity, from 5 g to 0.2 g depending on the organic content shall be transferred to a dry 500 ml conical flask, the weighing bottle reweighed (W3).

 

10 ml of N potassium dichromate solution shall be run into the conical flask from a burette, and add 20 ml of concentrated sulphuric acid very carefully from a measuring cylinder. The mixture shall be thoroughly swirled for about one minute and allowed to stand on a heat-insulating surface, such as asbestos or wood for 30 min to allow oxidation of the organic matter to proceed. During this period, the flask shall be protected from draughts. Distilled water, 200 ml, shall then be added along with 10 ml of orthophosphoric acid and one ml of the indicator. The mixture shall be shaken vigorously. If the indicator is absorbed by the soil, a further one ml of the solution shall be added. Ferrous sulphate solution shall then be added from the second burette in 0.5 ml increments, the contents of the flask being swirled, until the color of the solution changes from blue to green. A further 0.5 ml of potassium dichromate shall then be added, changing the color of the solution back to blue. Ferrous sulphate solution shall then beadded drop by drop with continued swirling until the color of the solution changes from blue to green after the addition of a single drop. The total volume of the ferrous sulphate solution used (y) shall be noted to the nearest 0.05 ml.

 

Calculations:

 

The volume (V ml) of potassium dichromate used to oxidize the organic matter in the soil

is given by the following formula:

 

V = 10.5 (1 – y/x)

 

Where,           x = total volume of ferrous sulphate used in the standardization test.

y = total volume of ferrous sulphate used in the test.

 

The percentage of organic matter (OM) present in the oven-dried sample shall be

 

calculated from the following formula:

 

Percentage of Organic Matter (OM) = (0.6 W2 V) / (W1 W3)

 

Where,           W1 = total weight of original sample.

W2 = weight of soil passing 10 mm IS sieve.

W3 = weight of dry soil specimen used in the test.

V = volume of potassium dichromate solution used to oxidize organic.