1 Test for sulfates, carbonates and ammonium / PAG4

Testing for Negative ions (anions) 3.1.4 Qualitative analysis Fizzing due to CO2 would be observed if a carbonate was present Testing for Presence of a carbonate Add any dilute acid and observe effervescence. Bubble gas through limewater to test for CO2 – will turn limewater cloudy 2HCl + Na2CO3
2NaCl + H2O + CO2 Testing for Presence of a sulphate Acidified BaCl2 solution is used as a reagent to test for sulfate ions If Barium Chloride is added to a solution that contains sulphate ions a white precipitate forms Ba2+ (aq) + SO4 2-(aq)
BaSO4 (s). Other anions should give a negative result which is no precipitate forming The acid is needed to react with carbonate impurities that are often found in salts which would form a white Barium carbonate precipitate and so give a false result Sulphuric acid cannot be used to acidify the mixture because it contains sulphate ions which would form a precipitate. The sequence of tests required is carbonate, sulfate then halide. (This will prevent false results of as both BaCO3 and Ag2 SO4 are insoluble.) Testing for positive ions (cations) Test for ammonium ion NH4 + , by reaction with warm NaOH(aq) forming NH3 Ammonia gas can be identified by its pungent smell or by turning red litmus paper blue.