Qualitative analysis

Qualitative analysis finds out which elements an organic compound contains.

Carbon and hydrogen: heat the compound with copper(II) oxide. Carbon is oxidised to CO₂, which turns lime water milky; hydrogen is oxidised to water, which turns white anhydrous copper sulphate blue.

Nitrogen, sulphur and halogens are bonded covalently, so they do not give ionic tests directly. In Lassaigne's test, the compound is fused with sodium metal. This converts them into ionic sodium compounds: N into NaCN, S into Na₂S, and halogens into NaX. The fused mass is boiled with water to give the 'sodium fusion extract', which is then tested:

• Nitrogen: add iron(II) sulphate, then acidify. A Prussian blue colour, from Fe₄[Fe(CN)₆]₃, shows nitrogen.
• Sulphur: with lead acetate (and acetic acid), a black precipitate of PbS; with sodium nitroprusside, a violet colour.
• Nitrogen and sulphur together: they form NaSCN, which gives a blood-red colour with Fe³⁺.
• Halogens: first boil the extract with nitric acid to destroy any CN⁻ and S²⁻ (which would also react with silver). Then add silver nitrate: a white precipitate (AgCl, which dissolves in ammonia) shows chlorine; pale yellow (AgBr) shows bromine; yellow (AgI) shows iodine.
• Phosphorus: oxidise it to phosphate, then add ammonium molybdate: a yellow precipitate.

This is why the sodium fusion step matters: carbon tetrachloride, CCl₄, does not give AgCl with silver nitrate directly, because its chlorine is covalently bonded, not present as Cl⁻ ions.