INTRODUCTION

Sulphates is largely distributed in nature and may be present in natural waters in concentration ranging from few hundred to several thousand mg/l. It occurs naturally in frequent minerals, including barite, epsomite and gypsum. These dissolved minerals subsidize to the mineral content of drinking-waters. Acid Mine Drainage (AMD) may give large amounts of sulphates through pyrite oxidation. It is the second most abundant anion in seawater. Its high concentration owes to the high to reasonable solubility of the salts that it forms with the foremost cations in seawater, namely, Na, Mg²⁺ and Ca²⁺.

Environmental Significance :

• Sulphates are of substantial concern because they are indirectly liable for two serious problems frequently related with handling and treatment of wastewater. They are odour and sewer corrosion problem result from the reduction of sulphates to hydrogen sulphide under anaerobic conditions.
• The amount of sulphates in wastewater is a feature of concern in defining the magnitude of problems that can rise from reduction of sulphates to hydrogen sulphide. For instance, knowledge of the sulphates content of the sludge or waste fed to digestion units provides a means of estimating the hydrogen sulphide content of the gas produced. From this information, the design engineer can determine whether scrubbing facilities will be needed to remove hydrogen sulphide and size of the units required.

Principle :

The turbidimetric method of measuring sulphates is based upon the fact that barium sulphates tend to precipitate in a colloidal form of uniform size and that this tendency is enhanced in presence of a sodium chloride, hydrochloric acid and glycerol.

SO₄^2- + BaCl₂ → BaSO₄ + 2Cl^-

The absorbance of the barium sulphates formed is measured by a spectrophotometer at 420nm and the sulphates ion concentration is determined by evaluation of the reading with a standard curve.

Relevant Indian Standard for Sulphate Test :

1. IS 3025 (Part 24)-1986: Method of Sampling and Test (Physical and Chemical) for Water and Wastewater : Sulphates, First Revision.