Salt Analysis
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Direct answer
Qualitative salt analysis identifies the ions in a salt by systematic precipitation. Cations are separated into six groups by adding group reagents in fixed order — dilute HCl, H2S in acid, NH4OH with NH4Cl, H2S in base, ammonium carbonate — each group precipitating while the rest stay dissolved. Anions are tested separately by gas-evolution and precipitation tests, including the brown ring test for nitrate and the chromyl chloride test for chloride.
What you must remember
- Group I, dilute HCl: Ag+, Pb2+, Hg2^2+ precipitate as chlorides; AgCl dissolves in NH4OH and returns with dilute HNO3.
- Group II, H2S in dilute HCl: Cu2+, Bi3+, Cd2+, Hg2+, As3+, Sb3+, Sn2+ precipitate as acid-insoluble sulphides — the acidic medium keeps S^2- low.
- Group III, NH4OH with NH4Cl buffer: Fe3+, Al3+, Cr3+ precipitate as hydroxides; Fe3+ confirms as Prussian blue Fe4[Fe(CN)6]3 with potassium ferrocyanide.
- Group IV, H2S in ammoniacal medium: Zn2+, Mn2+, Ni2+, Co2+ sulphides precipitate as the basic medium raises S^2-.
- Group V, (NH4)2CO3 with NH4Cl: Ba2+, Sr2+, Ca2+ carbonates; Group VI: Mg2+ with disodium hydrogen phosphate.
- Flame colours: sodium golden yellow, potassium lilac (viewed through blue cobalt glass), calcium brick red, strontium crimson, barium apple green, copper bluish green.
- Brown ring test for nitrate: fresh FeSO4 layered under concentrated H2SO4 forms a brown ring of [Fe(H2O)5NO]SO4.
- Chromyl chloride test for chloride: salt with K2Cr2O7 and concentrated H2SO4 evolves red-brown CrO2Cl2 fumes; lead acetate then gives yellow PbCrO4.
- Anion quick tests: carbonate fizzes with dilute HCl (limewater milky); sulphide blackens lead acetate paper; sulphite's SO2 turns acidified dichromate paper green; bromide colours the organic layer orange-brown with chlorine water; iodide violet; sulphate gives BaSO4 insoluble in concentrated HCl; phosphate with ammonium molybdate gives canary yellow.
Common confusion
The recurring confusion is why one reagent (H2S) precipitates Group II but not Group IV: in acid, H+ suppresses S^2- so only the least soluble sulphides drop; making the medium basic floods S^2- and precipitates the rest. Students likewise miss why NH4Cl precedes NH4OH — the buffer keeps OH- low so Group IV hydroxides do not precipitate prematurely. In anion tests, the trap is specificity: a BaCl2 precipitate must survive concentrated HCl before it counts as sulphate.
Exam-focused takeaway
JEE Main frames salt analysis as single-line recall: match ion with confirmatory test, precipitate colour or flame. JEE Advanced (steady weight through the practical-chemistry portion) prefers sequence and reasoning — which ions remain in the filtrate after a step, why a reagent's medium matters, balanced equations for the brown ring and chromyl chloride tests. Learn the scheme as a flow diagram you can redraw; disjointed facts fail exactly when the paper asks for the next step.
Frequently asked questions
Why is H2S passed in acidic medium for Group II?
Acid suppresses S^2- through the common ion effect, so only the least soluble sulphides precipitate, leaving the rest for later groups.
Why is NH4Cl added before NH4OH in Group III?
It buffers OH- low, so only the very insoluble hydroxides (Fe3+, Al3+, Cr3+) precipitate while others remain dissolved.
What is the brown ring test?
For nitrates: FeSO4 solution with the sample, layered under concentrated H2SO4, forms a brown ring of [Fe(H2O)5NO]SO4 at the junction.
What does the chromyl chloride test detect?
Chloride: heating with K2Cr2O7 and concentrated H2SO4 evolves red CrO2Cl2 fumes — a response chromates and sulphates cannot duplicate.
Why is potassium viewed through blue cobalt glass?
The glass absorbs sodium's golden yellow flame, which would otherwise mask potassium's delicate lilac.
Which carbonates precipitate in Group V, and how are they separated?
Ba2+, Sr2+ and Ca2+ with ammonium carbonate; differential solubility of their chromates and sulphates separates them from barium to calcium.