The test is not applicable to water containing an appreciable amount of calcium and magnesium as these are also titratable with EDTA. The test is generally applicable to water hardness less than 20mg/l as CaCO3.
Outline of the method – a measured excess of Std barium chloride solution is added to the sample and the excess of barium chloride estimated by titrated against EDTA.
Reagents
1. Dil HCL acid – 0.5N
2. Std barium chloride sol (0.04N ) – dissolve 4.886g of barium chloride dihydrate in distilled water and make up to 1 litre. One millilitre of the solution is equivalent to 1.92mg of sulphate (as SO4)
3. Buffer solution – dissolve 67.5f of ammonium chloride in 570ml of ammonium hydroxide (Sp Gr 0.92) and dilute with distilled water to 1 lit.
4. Erichrome black T indicator solution.
5. Std EDTA sol 0.02N
Procedure –
Pipette 10ml of the sample into a porcelain dish and acidify with 1ml of dilute HCl. Add exactly 10ml of standard barium chloride acid. Add exactly 10ml of Std barium chloride solution and stir well. Add 1ml buffer solution and several drops of Erichrome black T. Titrate with Std EDTA solution to a blue colour which does not change on addition of further drops of indicator solution.
Calculation
Sulphates (as SO4), mg/l = 192(10-V/2)
Where
V = volume of Std EDTA solution