Anion Tests — Dilute Acid Group

Why a Dilute Acid Group

Qualitative analysis works through reactions that the senses can register directly — a precipitate forming, a colour changing, or a gas escaping. The dilute acid anion group is built entirely on that third signal. When a salt of carbonate, sulphide, sulphite, nitrite or acetate is treated with dilute sulphuric acid (dilute HCl serves the same purpose), the weak conjugate acid that forms is unstable and breaks down, liberating a characteristic gas at room temperature or on gentle warming.

Because all five respond to one reagent, the dilute acid test is the very first wet test performed on the salt, before any confirmatory chemistry. The NCERT manual is explicit that preliminary tests with dilute H₂SO₄ — alongside the later concentrated acid test — give a good early indication of the acid radical present and should always precede confirmatory work. Anions that resist dilute acid (sulphate, phosphate, the halides, borate) are deliberately left for the concentrated acid group and the sulphate–phosphate–borate tests.

The Gas-Evolution Test Setup

The procedure is fixed: take about 0.1 g of the salt in a test tube, add 1–2 mL of dilute sulphuric acid, and observe at room temperature. If nothing escapes, warm the tube gently. Any gas that comes off is then led through a delivery tube to the appropriate detector — lime water, lead acetate paper, dichromate paper or KI–starch paper — exactly as in Fig. 7.1 of the manual. The apparatus below is the standard arrangement.

Carbonate — CO₃²⁻

Carbonate is the most conspicuous member of the group. Adding dilute H₂SO₄ to a carbonate produces brisk effervescence of a colourless, odourless gas. Passing that gas through lime water turns it milky because insoluble calcium carbonate precipitates.

$$\ce{Na2CO3 + H2SO4 -> Na2SO4 + H2O + CO2 ^}$$

$$\ce{Ca(OH)2 + CO2 -> CaCO3 v + H2O}$$

A subtle confirmatory cue: if CO₂ is bubbled in excess, the milkiness disappears as the insoluble carbonate redissolves to soluble calcium hydrogen carbonate. This reversal is itself diagnostic of CO₂.

$$\ce{CaCO3 + CO2 + H2O -> Ca(HCO3)2}$$

Confirmation of carbonate uses the solid salt or its aqueous solution directly — never a sodium carbonate extract, which would itself carry carbonate ions and give a false positive.

Sulphide — S²⁻

With warm dilute H₂SO₄ a sulphide gives hydrogen sulphide, a colourless gas with the unmistakable smell of rotten eggs. Held over the mouth of the tube, a strip of filter paper moistened with lead acetate turns black as black lead sulphide forms.

$$\ce{Na2S + H2SO4 -> Na2SO4 + H2S ^}$$

$$\ce{(CH3COO)2Pb + H2S -> PbS v + 2CH3COOH}$$

The confirmatory wet test is the sodium nitroprusside test. Make the salt solution alkaline with ammonium hydroxide (or use the sodium carbonate extract for water-insoluble salts) and add a drop of sodium nitroprusside: a purple to violet colour appears from the complex formed.

$$\ce{Na2S + Na2[Fe(CN)5NO] -> Na4[Fe(CN)5NOS]}$$

Sulphite — SO₃²⁻

On warming with dilute H₂SO₄ a sulphite evolves sulphur dioxide — suffocating, pungent, with the smell of burning sulphur. SO₂ is a reducing gas, and its reducing action drives both confirmatory tests.

$$\ce{Na2SO3 + H2SO4 -> Na2SO4 + H2O + SO2 ^}$$

SO₂ turns paper moistened with acidified potassium dichromate green by reducing chromium(VI) to chromium(III):

$$\ce{K2Cr2O7 + H2SO4 + 3SO2 -> K2SO4 + Cr2(SO4)3 + H2O}$$

With barium chloride, a sulphite solution gives a white precipitate of barium sulphite that dissolves in dilute HCl, re-evolving SO₂; the same precipitate decolourises acidified potassium permanganate, again through reduction.

$$\ce{Na2SO3 + BaCl2 -> 2NaCl + BaSO3 v}$$

$$\ce{BaSO3 + 2HCl -> BaCl2 + H2O + SO2 ^}$$

$$\ce{2KMnO4 + 5SO2 + 2H2O -> K2SO4 + 2MnSO4 + 2H2SO4}$$

Nitrite — NO₂⁻

A nitrite warmed with dilute H₂SO₄ gives reddish-brown fumes of NO₂. The chemistry runs through unstable nitrous acid, which disproportionates; the colourless NO formed is oxidised by air to the brown NO₂.

$$\ce{2NaNO2 + H2SO4 -> Na2SO4 + 2HNO2}$$

$$\ce{3HNO2 -> HNO3 + 2NO ^ + H2O}$$

$$\ce{2NO + O2 -> 2NO2 ^}$$

The classic detector is potassium iodide–starch paper acidified with acetic acid. Nitrous acid liberates iodine, which strikes the deep blue starch–iodine complex.

$$\ce{2HNO2 + 2KI + 2CH3COOH -> 2CH3COOK + 2H2O + 2NO + I2}$$

The Griess–Ilosvay test is the formal confirmation: in a very dilute, acetic-acid-acidified solution, sulphanilic acid is diazotised by the nitrous acid and couples with 1-naphthylamine to give a red azo-dye. The manual stresses the solution must be dilute, since concentrated solutions stall at diazotisation.

Acetate — CH₃COO⁻

Acetate betrays itself by the smell of vinegar when treated with dilute H₂SO₄, as acetic acid vapours escape and turn blue litmus red. Two confirmatory tests seal the identification.

Ester (fruity-odour) test

Heat the salt with ethanol and a few drops of concentrated H₂SO₄. Acetic acid esterifies to ethyl acetate, whose fruity odour confirms acetate.

$$\ce{2CH3COONa + H2SO4 -> Na2SO4 + 2CH3COOH}$$

$$\ce{CH3COOH + C2H5OH ->[H2SO4] CH3COOC2H5 + H2O}$$

Neutral ferric chloride test

Add neutral ferric chloride to the salt solution: a deep blood-red colour appears from an iron(III)–acetate complex, which on boiling decomposes to a brown-red precipitate of iron(III) dihydroxyacetate.

$$\ce{6CH3COO^- + 3Fe^3+ + 2H2O -> [Fe3(OH)2(CH3COO)6]^+ + 2H+}$$

$$\ce{[Fe3(OH)2(CH3COO)6]^+ + 4H2O ->[\Delta] 3[Fe(OH)2(CH3COO)] v + 3CH3COOH + H+}$$

Carbonate vs Sulphite vs Bicarbonate

Three close confusions trip students. CO₂ and SO₂ both turn lime water milky, so milkiness alone cannot separate carbonate from sulphite. The split is by smell — CO₂ is odourless, SO₂ is pungent — reinforced by the dichromate test, which only SO₂ (a reducing gas) passes by turning the paper green.

Carbonate vs bicarbonate is the second pair. Both effervesce with dilute acid to give CO₂, but a normal carbonate gives a precipitate with magnesium sulphate in the cold, whereas a bicarbonate does not until the solution is boiled. The dilute acid gas test cannot tell them apart on its own.

Anion Decision Map

The whole group reduces to a short decision tree driven by what the nose and eye report first, then a single confirming detector. The schematic below condenses the routing.

Observation → Inference Table

The lab-manual record sheet pairs each observation with its inference. This is the format examiners reproduce in match-the-column and assertion–reason items, so internalise the exact wording.