Methods of test for soils, Part 27: Determination of total soluble sulphates
1977 Edition

IS 2720 Part 27 (1977) prescribes three main techniques for determining total soluble sulphates in soils: (1) Gravimetric or precipitation method where sulphates are precipitated as barium sulphate and weighed, serving as the standard method; (2) Volumetric titration involving back titration of excess barium chloride with potassium chromate; and (3) Colorimetric or turbidimetric methods, which estimate sulphate concentration by measuring the turbidity or color intensity of the precipitate. These approaches help assess soil volume stability and engineering suitability.

The precipitation method outlined in IS 2720 Part 27 requires the use of N/4 barium chloride solution to precipitate sulphates as barium sulphate and N/4 potassium chromate solution for back titration of excess barium chloride. Hydrochloric acid is employed to acidify samples, and a 0.5% silver nitrate solution is used to detect chloride ions in washings. Essential equipment includes burettes for titration, test tubes for collecting washings, filtration setups for separating precipitates, and wash bottles for rinsing the precipitate with hot water. The sulphate content is calculated using the formula: Sulphates (%) = 0.0177 × 100 × (x - y), where x and y are volumes of titrants.

In the volumetric titration method as per IS 2720 Part 27, excess barium chloride is added to the sulphate-containing solution, causing insoluble barium sulphate to form. The surplus barium chloride is then back-titrated with potassium chromate until a yellow endpoint appears, confirmed by a brick-red precipitate upon adding silver nitrate. In contrast, the precipitation method involves filtering, drying, and weighing the barium sulphate precipitate. While the volumetric method offers quicker results with less equipment, the precipitation method is more precise but time-intensive.

The colorimetric or turbidimetric technique serves as a supplementary method under IS 2720 Part 27 for determining soluble sulphate content. It involves measuring the turbidity or color intensity of barium sulphate precipitate formed in the soil extract using a photoelectric colorimeter with a blue filter or a turbidimeter. This approach is rapid and sensitive, especially effective for detecting low sulphate concentrations, and complements the gravimetric and volumetric methods by providing a convenient alternative.

To guarantee precise sulphate analysis per IS 2720 Part 27, soil samples should be collected avoiding contamination and moisture loss, then air-dried at ambient temperature (avoiding oven drying to prevent sulphate alteration). The dried sample should be pulverized to pass through a 425-micron sieve for uniformity. An aqueous extract is prepared by mixing a known mass of soil with distilled water, followed by filtration to remove particulates. The filtered extract is then used for sulphate determination by precipitation, volumetric, or colorimetric methods. Proper storage in airtight containers is essential to maintain sample integrity.