Sulphate

ESTIMATION OF SULPHATE

Principle

         Sulphate in water sample is precipitated as Barium Sulphate by the addition of Barium Chloride in acid medium. The precipitate is filtered, washed free of chloride, ignited and weighed as Barium Sulphate.

Reaction

         Na2SO4 + BaCl2  →  BaSO4 + 2 NaCl

Reagents required

1.      Dilute HCl,

2.      Solid NH4Cl,

3.      10% BaCl2 solution,

4.      AgNO3 solution.

Procedure

         Pipette out 50 ml of water sample into a 400 ml beaker. To that add 10 ml of dilute hydrochloric acid and 1 gm of solid NH4Cl. The content of the beaker is boiled and 10 ml of Barium chloride is added drop by drop with simultaneous stirring. The boiling is continued for another 2-3 minutes and the precipitate is allowed to settle. The completion of precipitation is tested by adding small quantity of Barium chloride along the sides of the beaker. The beaker is kept on a sand bath for half an hour to promote granulation. The precipitate is filtered through Whatman No.42 filter paper and washed with hot water till the filtrate runs free of chloride. Transfer the filter paper along with the precipitate to a weighed silica basin. Then dry the contents in an electric oven and ignite over the flame. Cool the silica basin with the ash. The ignition is continued followed by cooling and weighing till a constant weight is attained.

Calculation

Volume of the water sample taken          =               50 ml

Weight of empty silica basin                    =               W1

Weight of silica basin + ash                     =                  W2

Weight of BaSO4 alone                                 =         W2–W1

233.3 g of BaSO4 contains 96.06 g of Sulphate

Therefore (W2-W1) g of BaSO4 contains  = 96.06 / 233.4 x (W2-W1) g of BaSO4

This is present in 50 ml of the sample

Therefore 1 million ml of sample contains (ppm)     =96.06/*(w2-w1)*1000*1000/233.4*50

     In terms of milli equivalent per litre =           ppm / 48.03