B. Identification of basic radicals (cations)

• This is harder than anion analysis because there are many cations, they interfere with each other, and one metal may have several oxidation states.
• Cations are divided into six analytical groups, each with its group reagent.
• The classification depends on the different solubilities of their salts.
• For example, the first group (Ag⁺, Hg₂²⁺, Pb²⁺) chlorides are sparingly soluble in water and are precipitated as chlorides by the group reagent, dilute HCl.

A usual summary of the six groups (for orientation only; you are examined on the examples below):

We study examples from several groups.

Second analytical group — example Cu²⁺. Precipitated as sulphides in acidic medium: dissolve the salt in water, acidify with dilute HCl and pass H₂S. CuSO₄ + H₂S → H₂SO₄ + CuS↓ (black copper(II) sulphide, soluble in hot nitric acid: 3CuS + 8HNO₃ → 3Cu(NO₃)₂ + 2NO + 3S + 4H₂O).

Third analytical group — Al³⁺, Fe²⁺, Fe³⁺. Precipitated as hydroxides by ammonium hydroxide (group reagent) when no other interfering cations are present.

Fifth analytical group — example Ca²⁺. Precipitated as carbonates by ammonium carbonate (group reagent).

• CaCl₂ + (NH₄)₂CO₃ → 2NH₄Cl + CaCO₃↓ (white; soluble in dil. HCl: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂; and in water containing CO₂: CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂).
• Confirmatory: dilute H₂SO₄ gives a white ppt. of CaSO₄: CaCl₂ + H₂SO₄ → 2HCl + CaSO₄↓.
• Flame test: calcium salts colour the Bunsen flame brick red.