Master Salt Analysis Like a Pro

Salt analysis (qualitative inorganic analysis) is the systematic procedure used to identify the cation and anion present in an unknown inorganic salt. It is one of the most important practicals in CBSE Class 12 Chemistry and a foundation topic for JEE and NEET inorganic chemistry.

The original solution (O.S.) is a solution of the salt prepared in a suitable solvent — usually water, dilute HCl, dilute HNO₃, or aqua regia, depending on the solubility of the salt. The original solution is used for systematic cation analysis through Groups 0 to VI.

Dilute HCl precipitates only the cations whose chlorides are insoluble — Pb²⁺, Ag⁺, and Hg₂²⁺. The dilute concentration provides a controlled chloride ion concentration so that the chlorides of Group II–VI cations (which are soluble) remain in solution and can be analysed in subsequent groups.

Cations are identified by passing the original solution through Groups 0 to VI in fixed order. Group 0 (NH₄⁺) is tested first using NaOH. Group I uses dilute HCl. Group II uses H₂S in acidic medium. Group III uses NH₄Cl + NH₄OH. Group IV uses H₂S in alkaline medium. Group V uses (NH₄)₂CO₃. Group VI tests for Mg²⁺. Once the cation is precipitated in a group, a confirmatory test pinpoints the exact ion.

Anions are identified in two stages. The dilute H₂SO₄ test detects carbonate, sulphide, sulphite, nitrite and acetate by gas evolution. The concentrated H₂SO₄ test detects chloride, bromide, iodide, nitrate and oxalate. The anion is then confirmed using specific tests — silver nitrate test for halides, brown ring test for nitrate, BaCl₂ test for sulphate, and ammonium molybdate test for phosphate.

Dry tests are performed on the solid salt before it is dissolved. They include noting the colour and smell of the salt, heating the salt in a dry test tube to observe colour changes and any gas evolved, the flame test using a clean platinum wire dipped in concentrated HCl, and the borax bead test for transition metal cations. Dry tests give early hints about the cation and anion present.

H₂S is not passed in Group I because the goal of Group I is to precipitate only Pb²⁺, Ag⁺, and Hg₂²⁺ as their insoluble chlorides using dilute HCl. If H₂S were passed first, sulphides of Group II cations would also precipitate together, making it impossible to separate Group I from Group II cleanly.

The principle is selective precipitation based on the Solubility Product (Ksp) of insoluble salts. By controlling the concentration of the precipitating ion (Cl⁻, S²⁻, OH⁻, CO₃²⁻) and the pH of the medium, only the cations of one group exceed their Ksp and precipitate, while cations of later groups remain in solution. The Common Ion Effect is used to fine-tune ion concentrations during the procedure.

The basic apparatus includes test tubes and a test tube stand, test tube holder, Bunsen burner, platinum or nichrome wire for the flame test, watch glass, glass rod, dropper bottles for reagents (dilute HCl, dilute and concentrated H₂SO₄, NaOH, NH₄OH, NH₄Cl, H₂S source, BaCl₂, AgNO₃, FeSO₄, (NH₄)₂CO₃), a centrifuge or filter paper for separating precipitates, and a wash bottle.

Always test for NH₄⁺ in the original solution before adding any ammonium reagent. Use cold dilute HCl in Group I to fully precipitate PbCl₂. Pass H₂S only in a fume hood — it is toxic. Keep the medium acidic for Group II and alkaline for Group IV. Centrifuge or filter completely before moving to the next group. Confirm each cation and anion with at least one specific confirmatory test.