JEE Chemistry Qualitative Salt Analysis Notes Set 03

Dry Heating Test

Heat a little salt in a dry test tube held slightly inclined, and note the gas, sublimate and residue.
Colourless odourless gas that turns lime water milky: carbonate (CO2). Rotten egg smell that blackens lead acetate paper: sulphide (H2S). Suffocating smell that turns acidified dichromate green: sulphite (SO2). Reddish brown gas that turns ferrous sulphate solution brownish black: nitrite or nitrate (NO2). Violet vapours with black sublimate: iodide. White sublimate: ammonium salt.
Residue colours: yellow when hot and white when cold indicates Zn2+; brown when hot and yellow when cold indicates Pb2+; a blue salt turning white indicates hydrated CuSO4; a coloured salt turning black or brown indicates Cu2+, Ni2+, Co2+, Mn2+, Fe2+ or Cr3+.

Flame Test

The salt is made into the volatile chloride by moistening with conc. HCl and is held in a non-luminous flame. Colours: Li crimson red, Na golden yellow, K pale violet (seen through cobalt glass), Ca brick red, Sr crimson, Ba apple green, Cu green with blue centre. Be and Mg give no flame colour.
Barium sulphate and phosphate give a poor colour because they are insoluble and are not easily converted into chlorides. Borate gives a green flame (the Cu2+ and Ba2+ ions interfere because they also give green).

Borax Bead Test

Borax on heating forms a glassy bead of sodium metaborate and boron trioxide: \( \ce{Na2B4O7.10H2O -> Na2B4O7 -> 2NaBO2 + B2O3} \). The coloured metal oxide dissolves in B2O3 to form a coloured metaborate, so the test is given by d-block ions with partly filled d orbitals. Zn2+ (d10) gives a colourless bead.
Cu: blue when cold in the oxidising flame (green hot); colourless in the reducing flame (forming CuBO2) and then red opaque (metallic copper). Fe: yellow or brown in the oxidising flame and bottle green in the reducing flame. Cr: green in both flames. Co: blue in both flames. Mn: amethyst (violet) in the oxidising flame and colourless in the reducing flame. Ni: brown in the oxidising flame and grey in the reducing flame.

Charcoal Cavity and Cobalt Nitrate Tests

Carbonates formed on the charcoal block decompose to oxides: ZnO (yellow when hot, white when cold), PbO (brown when hot, yellow when cold, with a grey bead of lead that marks paper), CuO reduced to red scales of copper, and white residues from Ca, Ba and Mg oxides.
If the residue is white, moisten it with cobalt nitrate and heat strongly: Zn gives green (Rinmann's green), Al gives blue (Thenard's blue), Mg gives pink and Sn gives bluish green.

Solubility Rules for Anions

Carbonates, sulphites and phosphates are insoluble except those of alkali metals and ammonium. Sulphides of alkali metals, ammonium and (slowly) alkaline earths dissolve; most others are insoluble. Nitrates and nitrites are soluble (silver nitrite sparingly). Acetates are soluble except that Ag and Hg(I) acetates are sparingly soluble.
Chlorides, bromides and iodides are soluble except those of Ag, Hg(I), Pb (soluble in hot water) and Cu(I). Sulphates are soluble except those of Ba, Pb and Sr; Ca and Ag sulphates are slightly soluble.

Sodium Carbonate Extract

Boil the salt with about three times its mass of solid sodium carbonate in water for ten minutes, cool and filter. The filtrate holds the sodium salts of the anions and the cations are removed as insoluble carbonates. It is used when the salt is insoluble in water or the cations interfere or colour the solution.
It contains excess carbonate, so it must be acidified (with HCl, HNO3 or H2SO4, depending on the test) before testing for anions, and it cannot be used to test for carbonate or bicarbonate itself.

Anions Identified with Dilute Acid

Carbonate: brisk effervescence of CO2, which turns lime water milky and clears again with excess. Sulphite: SO2 with a burning-sulphur smell, turns acidified dichromate green and decolourises acidified permanganate. Sulphide: H2S with rotten egg smell, black PbS with lead acetate, yellow CdS with cadmium carbonate, violet colour with sodium nitroprusside in alkaline medium (the iron stays +2). Nitrite: brown NO2, brown colour with ferrous sulphate (brown ring in acetic acid), blue with starch iodide. Acetate: vinegar smell, red colour with neutral ferric chloride.
Zinc with dilute H2SO4 reduces sulphite to H2S, while sulphate is unaffected.

Anions Identified with Conc. Sulphuric Acid

Chloride: colourless pungent HCl that gives white fumes with ammonia and a white AgCl precipitate soluble in ammonia. The chromyl chloride test (heat with solid K2Cr2O7 and conc. H2SO4 in a dry tube; deep red CrO2Cl2 vapours that give a yellow chromate with NaOH and yellow lead chromate with lead acetate) is positive only for ionic chlorides.
Bromide: red-brown Br2 vapour, pale yellow AgBr that is partly soluble in ammonia, orange-red starch paper, and no chromyl test. Iodide: violet I2 vapour, yellow AgI insoluble in ammonia, and a violet organic layer when chlorine water and CHCl3 are added. Nitrate: brown NO2 that intensifies with copper turnings, and the brown ring test with freshly prepared FeSO4 and conc. H2SO4 poured down the side of the tube, giving \( \ce{[Fe(H2O)5NO]^2+} \).

Anions with Specific Precipitation Reactions

Sulphate: white BaSO4 with BaCl2, insoluble in dilute HCl and HNO3 (sulphite also gives a white BaSO3, but it dissolves in HCl and decolourises acidified KMnO4). Phosphate: canary yellow ammonium phosphomolybdate with conc. HNO3 and ammonium molybdate on warming, and yellow silver phosphate that dissolves in both dilute HNO3 and ammonia.
Borate: green-edged flame of triethyl borate when heated with conc. H2SO4 and ethanol. Chromate and dichromate: yellow lead chromate with lead acetate, and reduction to green Cr3+ by SO2 or H2S.