Determination Of Total Soluble Sulphates In Soils (Volumetric Method)

STANDARD

  • IS: 2720 (Part 27) 1977.

OBJECTIVE

  • To determine the total soluble sulphates in soils.

APPARATUS

  • Analytical balance of sensitivity 0.001 gram.
  • Glass beakers 2 nos of 150 ml capacity.
  • Glass funnel of 6 cm diameter.
  • Measuring flasks one of 100 ml and two of 500 ml capacity.
  • Burette 2 nos of 25 ml with 1/20 ml graduation.
  • Conical flasks 6 nos of 150 ml capacity.
  • Pipette 10 ml.
  • Filter papers.
  • Heating equipment.
  • Thermostatically controlled oven of capacity up to 250°C. 
  • Reagents
  • Barium Chloride solution (N/4) – Dissolve 30.54 gm of barium chloride in one liter of distilled water.
  • potassium Chromate (N/4) – Dissolve 24.275 gm of potassium chromate in a small amount of distilled water and  add a few drops of Silver Nitrate solution to remove any chloride, filter and dilute to 250 ml.
  • Silver Nitrate Indicator solution (0.5%) – Dissolve 500 mg of silver nitrate in 100 ml of distilled water.
  • Dilute solution of Ammonium Hydroxide Specific Gravity 0.888).
  • Mix Ammonium Hydroxide and distilled water in the ratio of 1: 2 (One part of Ammonia and two parts of    distilled water).
  • Concentrated Hydrochloric acid (Specific Gravity 1.11).

PROCEDURE

  • Take exactly 10 grams of oven dried soil sample well pulverized in a mortar with rubber-covered pestle or in a mechanical device.
  • Transfer the soil sample in to the beaker and add 50 ml of water
  • Stir well, allow to decant, filter, wash on a filter paper with small quantity of water and make the filtrate to 100 ml.
  • Pipette out 10 ml of water extract in to a conical flask.
  • Make it slightly acidic by adding concentrated Hydrochloric acid and heat to boiling.
  • While boiling add Barium Chloride solution (N14) from the burette till the precipitation is complete and Barium Chloride solution is in light excess.
  • Neutralize the solution with ammonium hydroxide and titrate the excess of Barium Chloride against Potassium Chromate solution (NI 4).
  • If considered necessary the end point can be further conformed by using Silver Nitrate solution as an external indicator.
  • A brick red colour is obtained when a drop of Silver Nitrate is added to a drop of the chromate solution.

CALCULATIONS

  • Sulphates as Sodium Sulphate in soil, percent by mass = 0.0177 x 100 (X-Y).
    X=Volume of N/ 4 barium chloride added.
    Y = Volume of N/4 Potassium Chromate solution used in back titration
    X-Y = N/4 barium chloride actually used for perceptibly Sulphate.

REPORT

  • Report the result obtained as the percentage of total soluble sulphates in the soil.

PRECAUTIONS

  • Pure chemicals and distilled water shall only be used in the test.
  • Pure chemicals shall mean chemicals that do not contain impurities that affect the results of analysis.

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