RP4 - testing for cations and anions

Barium chloride + 10 drops NaOH + excess NaOH

Colourless solution => Colourless solution => Colourless solution

Magnesium chloride + 10 drops NaOH + excess NaOH

Colourless solution => Slightly white precipitate => White precipitate

Calcium nitrate + 10 drops NaOH + excess NaOH

Colourless solution => Slightly white precipitate => Slightly white precipitate

Strontium chloride + 10 drops NaOH + excess NaOH

Colourless solution => Slightly white precipitate => Slightly white precipitate

Barium Chloride + 10 drops H2SO4 + excess H2SO4

Colourless solution => White precipitate => White precipitate

Calcium nitrate + 10 drops H2SO4 => excess H2SO4

Colourless solution => Slightly white precipitate => Slightly white precipitate

Magnesium chloride + 10 drops H2SO4 + excess H2SO4

Colourless solution => Slightly white precipitate => Colourless solution

Strontium chloride + 10 drops H2SO4 + excess H2SO4

Colourless solution => White precipitate => White precipitate

Potassium chloride + nitric acid and silver nitrate + dilute ammonia solution + concentrated ammonia

Colourless solution => White precipitate => Colourless solution => Colourless solution

Potassium Bromide + nitric acid and silver nitrate + dilute ammonia + concentrated ammonia

Colourless solution => Cream precipitate => Cream precipitate => Colourless solution

Potassium Iodide + nitric acid and silver nitrate + dilute ammonia + excess ammonia

Colourless solution => Yellow precipitate => Yellow precipitate => Yellow precipitate

What is the purpose of adding the nitric acid in the tests for halide ions?

To remove carbonate ions which would present as a white precipitate, therefore giving a false positive

What is the purpose of adding the hydrochloric acid in the test for sulfate ions?

To remove carbonate ions which would present as a white precipitate, therefore giving a false positive

In a test for sulfate iond, why is sulfuric acid not used to acidify the solution?

Sulfuric acid contains sulfate ions and will give a white precipitate of barium sulphate, a false positive

In the test for halide ions, why is hydrochloric acid not used to acidify the solution?

Hydrochloric acid contains chloride ions which will give a white precipitate therefore a false positive