JEE Chemistry Qualitative Salt Analysis Notes Set 01

Preliminary Tests

1. Flame test: Na golden yellow; K lilac (through blue glass); Ca brick red; Sr crimson; Ba apple green; Cu blue-green; boron green-edged.
2. Borax bead test: the metal oxide combines with boric anhydride to give a coloured metaborate. Cu blue (green when hot), Fe yellow-brown (hot) to yellow (cold), Cr green, Mn violet (oxidising flame), Co blue, Ni brown.
3. Charcoal cavity (cobalt nitrate) test: Al gives blue (Thenard's blue), Zn green (Rinmann's green), Mg pink.
4. Dilute acid test: carbonate gives colourless odourless \( \ce{CO2} \) (turns lime water milky); sulphite gives pungent \( \ce{SO2} \) (turns acidified dichromate green); sulphide gives rotten-egg \( \ce{H2S} \) (blackens lead acetate paper); nitrite gives brown fumes.
5. Concentrated \( \ce{H2SO4} \) test: chloride gives colourless HCl fumes, bromide gives reddish-brown bromine vapour, iodide gives violet iodine vapour, nitrate gives brown \( \ce{NO2} \) with copper turnings.

Wet Tests for Acid Radicals

Analysis of Basic Radicals

Why the reagents are chosen: dilute HCl supplies the common ion H⁺ in group II, which lowers the sulphide ion concentration so that only the very insoluble sulphides (CuS, PbS, HgS, Bi₂S₃, CdS, As₂S₃) precipitate. In group IV the ammonia makes the medium basic, so sulphide ion concentration rises and the more soluble sulphides (ZnS, NiS, CoS, MnS) separate. In group III, ammonium chloride suppresses the ionisation of ammonia so that only the least soluble hydroxides precipitate, leaving Mg²⁺ and group IV ions in solution.

Key Confirmatory Tests

1. \( \ce{Fe^{3+}} \): potassium ferrocyanide gives Prussian blue; thiocyanate gives blood red. \( \ce{Fe^{2+}} \): potassium ferricyanide gives Turnbull's blue.
2. \( \ce{Cu^{2+}} \): excess ammonia gives deep blue \( \ce{[Cu(NH3)4]^{2+}} \); potassium ferrocyanide gives a chocolate brown precipitate; potassium iodide gives white CuI and iodine.
3. \( \ce{Ni^{2+}} \): dimethylglyoxime in ammoniacal solution gives a rose red precipitate.
4. \( \ce{Zn^{2+}} \): white ZnS with \( \ce{H2S} \) in basic medium; white precipitate soluble in excess NaOH.
5. \( \ce{Al^{3+}} \): white gelatinous Al(OH)₃ soluble in excess NaOH; Thenard's blue in the charcoal cavity test.
6. \( \ce{Mn^{2+}} \): fuses with sodium carbonate and nitre to give green manganate; lead dioxide with conc. nitric acid gives pink permanganate.
7. \( \ce{Pb^{2+}} \): white PbCl₂ soluble in hot water; yellow lead chromate; yellow lead iodide (golden spangles).
8. \( \ce{Ag^+} \): white AgCl soluble in ammonia; black Ag₂S with \( \ce{H2S} \).
9. \( \ce{Hg2^{2+}} \): calomel turns black with ammonia; \( \ce{Hg^{2+}} \): with excess tin(II) chloride gives white then grey precipitate.
10. Alkali metals and ammonium: flame tests for Na and K; ammonium gives ammonia gas with NaOH and brown precipitate with Nessler's reagent.