Mastering Confirmatory Tests For Anions in Chemistry

An anion is the negatively charged ion in a salt — the partner of the positively charged metal cation. When you receive an unknown salt in qualitative analysis and must identify it, finding the anion is half the battle, because the anion decides which specific confirmatory test will give a unique and unmistakable result.

A confirmatory test is a reaction that proves the presence of one particular anion beyond doubt. Preliminary tests, such as heating the dry salt or adding dilute acid, only narrow down the possibilities. The confirmatory test uses a reagent that produces a characteristic precipitate, gas, or colour that no other common anion produces under the same conditions.

Key Concepts

• Carbonate (CO3^2-): with dilute HCl it gives brisk effervescence of CO2, which turns lime water milky.
• Chloride (Cl-): with silver nitrate solution gives a white precipitate of AgCl, which dissolves in ammonium hydroxide solution.
• Sulphate (SO4^2-): with barium chloride solution gives a white precipitate of BaSO4, which does not dissolve in dilute HCl.
• Nitrate (NO3-): the brown-ring test — ferrous sulphate solution plus concentrated sulphuric acid gives a brown ring at the junction of the two liquids.
• Nitrite (NO2-): acidified solution turns starch-iodide paper blue because iodine is liberated.

Formulas

• Chloride test: AgNO3(aq) + NaCl(aq) → AgCl(s, white) + NaNO3(aq). The white AgCl dissolves in NH4OH, forming the soluble complex [Ag(NH3)2]+. Assumption: no bromide or iodide present, since AgBr (pale yellow) and AgI (yellow) also precipitate.
• Sulphate test: BaCl2(aq) + Na2SO4(aq) → BaSO4(s, white) + 2NaCl(aq). Molar mass of BaSO4 is 233.39 g/mol. The precipitate is insoluble in dilute HCl, which rules out carbonate and sulphite.
• Brown-ring test: 6FeSO4 + 2HNO3 + 3H2SO4 → 3Fe2(SO4)3 + 4H2O + 2NO, followed by FeSO4 + NO → [Fe(H2O)5(NO)]SO4, the brown ring. The acid must be added slowly down the side of the test tube so the layers stay separate.

Worked Example

How much silver chloride precipitates when excess silver nitrate is added to 25.0 mL of 0.100 mol/L sodium chloride solution?

1. Moles of NaCl = concentration × volume = 0.100 mol/L × 0.0250 L = 0.00250 mol.
2. The reaction Ag+ + Cl- → AgCl is 1:1, so moles of AgCl formed = 0.00250 mol.
3. Molar mass of AgCl = 107.87 + 35.45 = 143.32 g/mol.
4. Mass of AgCl = 0.00250 mol × 143.32 g/mol = 0.358 g, about 0.36 g of white precipitate.
5. To confirm it is chloride and not bromide or iodide, add ammonium hydroxide: AgCl dissolves, AgBr dissolves only slightly, and AgI does not dissolve.

Common Misconceptions

• A white precipitate with silver nitrate always means chloride. Wrong. Silver bromide gives a pale-yellow precipitate and silver iodide a yellow one, and both also form. The solubility of the precipitate in ammonium hydroxide is the real confirmation for chloride.
• Effervescence with dilute acid always means carbonate. Wrong. Sulphites also fizz, giving SO2 gas which smells like burning sulphur. Only CO2 turning lime water milky confirms carbonate.

Real-World Applications

• Water quality testing: municipal labs test drinking water for chloride, sulphate, and nitrate, since excess nitrate is harmful to infants and excess sulphate causes laxative effects.
• Food safety: sulphite preservatives in dried fruits and wines are monitored with confirmatory colour tests because some people are sensitive to them.
• Soil and fertilizer testing: nitrate and sulphate levels guide how much fertilizer a field needs.

Confirmatory tests turn a guess into proof: a dissolving white precipitate for chloride, an acid-insoluble one for sulphate, a brown ring for nitrate — always checking the known interferences before declaring the result final.