2. Limit Tests for Chloride ($Cl^-$) & Sulphate ($SO_4^{2-}$)

The Limit Test for Chloride is based on the chemical precipitation reaction between soluble chloride ions and Silver Nitrate ($AgNO_3$) in the presence of dilute Nitric Acid ($HNO_3$) to form an insoluble white opalescent precipitate of Silver Chloride ($AgCl$). Dilute $HNO_3$ is added to prevent the precipitation of other interfering silver salts (e.g. silver carbonate, silver phosphate). The Limit Test for Sulphate is based on the reaction between soluble sulphate ions and Barium Chloride ($BaCl_2$) in the presence of dilute Hydrochloric Acid ($HCl$) and alcohol (Barium Sulphate Reagent) to form an insoluble white precipitate of Barium Sulphate ($BaSO_4$). Dilute $HCl$ prevents precipitation of barium carbonate/phosphate, while the trace alcohol in barium sulphate reagent prevents supersaturation and promotes uniform colloidal turbidity.