Salt Analysis: Anion Tests
Loading simulation…
flagWhat you'll discover
- arrow_forwardTest for carbonate with dilute acid and lime water
- arrow_forwardTest for chloride with acidified silver nitrate and confirm with ammonia
- arrow_forwardTest for sulphate with acidified barium chloride
- arrow_forwardPerform and interpret the brown ring test for nitrate
- arrow_forwardWrite the ionic equations behind each positive observation
Gases first: the dilute acid test
Systematic anion analysis begins with dilute acid, because the easiest anions to find are those that escape as gases. Add dilute H₂SO₄ (or HCl) to the solid salt: brisk effervescence of a colourless, odourless gas that turns lime water milky proves carbonate. The chemistry: CO₃²⁻ + 2H⁺ → H₂O + CO₂, then CO₂ + Ca(OH)₂ → CaCO₃ (the white milkiness) + H₂O.
A subtle classic: pass the gas through lime water for a long time and the milkiness disappears, because excess CO₂ converts insoluble CaCO₃ into soluble calcium hydrogencarbonate, Ca(HCO₃)₂. If nothing fizzes with cold dilute acid, carbonate is absent and you move to the precipitation tests.
Precipitation tests: silver for chloride, barium for sulphate
Chloride test: acidify the salt solution with dilute HNO₃, then add AgNO₃. A curdy white precipitate of AgCl that is insoluble in HNO₃ but dissolves in NH₄OH confirms chloride: Ag⁺ + Cl⁻ → AgCl↓. The acidification step matters — it destroys carbonate and sulphite, which would otherwise also give white silver precipitates and a false positive.
Sulphate test: acidify with dilute HCl, then add BaCl₂. A heavy white precipitate of BaSO₄, insoluble in the acid, confirms sulphate: Ba²⁺ + SO₄²⁻ → BaSO₄↓. Again the acid is the safeguard: BaCO₃ is also white but dissolves in HCl with fizzing, so only sulphate survives the acid test. In both tests the logic is identical — precipitate plus acid-insolubility equals confirmation.
The brown ring test for nitrate
Nitrate is the awkward anion: almost all nitrates are soluble, so no simple precipitation test exists. The classic identification is the brown ring test. To the salt solution add freshly prepared FeSO₄ solution, then pour concentrated H₂SO₄ carefully down the inside of the tilted tube. The dense acid forms a separate bottom layer, and at the junction of the two layers a brown ring appears.
The chemistry happens at that interface: the acid makes the medium strongly acidic, nitrate oxidises Fe²⁺ to Fe³⁺ and is itself reduced to NO (NO₃⁻ + 3Fe²⁺ + 4H⁺ → NO + 3Fe³⁺ + 2H₂O); the NO then binds a remaining Fe²⁺ to form the brown complex [Fe(H₂O)₅NO]²⁺. Shake the tube and the ring vanishes — the layers mix, the local conditions are destroyed. The FeSO₄ must be fresh because air slowly oxidises Fe²⁺ to Fe³⁺, which cannot form the complex.