3.1.4 Qualitative Analysis

Test for cations

\(NH_4^+(aq) + OH^-(aq) \rightarrow NH_3(g) + H_2O(l)\)
Aqueous sodium hydroxide added to a solution of ammonium ion and warm the solution
Ammonia gas is produced (unlikely to see bubbles as it is very soluble in water)
Ammonia can be smelled / turns damp red litmus paper blue

Add dilute nitric acid (not \(HCl\) or \(H_2SO_4\) if doing sulfate or halide test later) to the solid or solution being tested
If there are bubbles then the compound could be a carbonate
- \(2H^+(aq) + CO_3^{2-}(aq) \rightarrow CO_2(g) + H_2O(l)\)
Check the gas produced by bubbling it through lime water (calcium hydroxide)
Lime water will turn cloudy if the gas is carbon dioxide as a white precipitate of calcium carbonate forms
- \(CO_2(g) + Ca(OH)_2(aq) \rightarrow CaCO_3(s) + H_2O(l)\)

Add barium chloride (barium nitrate if halide test needed later) to the solution of the compound
If white precipitate forms then \(SO_4^{2-}\) is present
- \(Ba^{2+}(aq) + SO_4^{2-}(aq) \rightarrow BaSO_4(s)\)
- \(BaSO_4\) is insoluble in water

Add aqueous silver nitrate (\(AgNO_3\)) to an aqueous solution of a halide
Silver halide precipitates as different colour
- \(Ag^+(aq) + X^-(aq) \rightarrow AgX(s)\)
- \(AgCl\) = white
- \(AgBr\) = cream
- \(AgI\) = yellow
Add aqueous ammonia to test the solubility of the precipitate (colours can be difficult to tell apart)
- \(AgCl\) = soluble in both dilute and concentrated \(NH_3(aq)\)
- \(AgBr\) = soluble in concentrated \(NH_3(aq)\) only, insoluble in dilute \(NH_3(aq)\)
- \(AgI\) = insoluble in dilute and concentrated \(NH_3(aq)\)

Neither sulfate / halide produce bubbles with dilute acid \(\rightarrow\) cannot have incorrect conclusion
\(BaCO_3\) is white and insoluble in water so it gives a false positive result on carbonates as well in sulfate tests
\(Ag_2CO_3\) and \(Ag_2SO_4\) both insoluble in water and form precipitates in the test so it gives a false positive result for carbonates and sulfates in halide tests