JEE Chemistry Qualitative Salt Analysis Notes Set 02

Group Classification of Cations

In systematic analysis the basic radicals are separated into groups by adding group reagents one after another. A group is precipitated only when the ionic product of its salts exceeds the solubility product under the conditions of that step; ions of later groups stay in solution because their solubility products are not reached.

Yellow ammonium sulphide is \( \ce{(NH4)2S_x} \). Arsenic, antimony, tin, cadmium, cobalt and nickel are outside the JEE Advanced syllabus, so the emphasis below is on the other ions.

Why Each Group Reagent Is Used

1. Group I chlorides (AgCl, PbCl2, Hg2Cl2) have lower solubility products than the chlorides of all other metals, so dilute HCl removes them first. PbCl2 is appreciably soluble in water, so lead is never removed completely here; the remainder is precipitated as PbS in group II.
2. Group II sulphides have very low solubility products. HCl supplies H+, which suppresses the ionisation of H2S through the common ion effect, so the sulphide ion concentration is only enough to precipitate group II. Sulphuric acid cannot replace HCl because it would precipitate BaSO4, SrSO4 and CaSO4. Nitric acid cannot be used because it oxidises H2S to sulphur, which looks like CdS or As2S3.
3. Group III: NH4Cl suppresses the ionisation of NH4OH, lowering [OH-] so that only the hydroxides of Fe, Cr and Al (very low Ksp) are precipitated. Ammonium sulphate cannot be used because sulphate would precipitate BaSO4 and SrSO4. Too much NH4Cl is also harmful (Mn may separate as MnO2.H2O, and Cr stays in solution). NaOH cannot replace NH4OH because Al(OH)3 and Cr(OH)3 dissolve in excess alkali.
4. Before group III the group II filtrate is boiled to expel H2S and a drop of concentrated HNO3 is added so that any Fe2+ is oxidised to Fe3+; Fe(OH)2 has a higher Ksp and would otherwise be only partly precipitated and interfere with group IV. If the medium is still acidic the hydroxides do not form at all.
5. Group IV: NH4OH removes H+ from H2S (the H+ combines with OH- to give water), which raises [S2-] enough to precipitate ZnS, MnS, CoS and NiS. In neutral solution precipitation is incomplete because the HCl formed (for example \( \ce{MnCl2 + H2S -> MnS + 2HCl} \)) holds back the ionisation of H2S.
6. Group V: ammonium carbonate must be added in neutral or alkaline solution containing ammonium salts; without NH4+ present, magnesium would also be precipitated as a basic carbonate.

Preparation of the Original Solution

The original solution (O.S.) is used to detect every basic radical except \( \ce{NH4+} \) (which is tested on the solid with NaOH). The salt or mixture is tried first in water; if insoluble, in dilute HCl; if still insoluble, in concentrated HCl. A small quantity is dissolved in the first solvent that works and the solution is filtered.

Group I: Lead, Mercury(I) and Silver

To separate the three: hot water dissolves PbCl2 (confirmed with K2CrO4 or KI); the residue is treated with ammonia, where Hg2Cl2 turns black and AgCl dissolves (reprecipitated white by HNO3).

Group II A: Mercury(II), Copper, Bismuth, Lead and Cadmium

Group III: Aluminium, Iron(III) and Chromium

Solid NH4Cl is added in good quantity (otherwise Zn, Mn and Mg hydroxides may separate) but a large excess must be avoided (Cr may stay in solution and Al may form a colloid). The mixture is boiled until it only faintly smells of ammonia so that the Al(OH)3 sol coagulates.

Group IV: Manganese, Zinc, Cobalt and Nickel

Dimethylglyoxime in ammoniacal medium also gives a red colour with Fe(II), a yellow precipitate with bismuth and a brown colour with cobalt. Anhydrous Co(II) salts are blue and hydrated ones pink; NiCl2 residue is yellow and turns green with water.

Group V: Barium, Strontium and Calcium

The carbonates are dissolved in acetic acid and the solution is split in three parts. Tests must be done in the order barium, strontium, calcium (BSC), because Ba2+ answers all three confirmatory reagents and Sr2+ answers two of them.

Calcium carbonate dissolves in water rich in carbon dioxide (as hydrogen carbonate) and reappears on boiling. Potassium hexacyanidoferrate(II) with ammonium chloride gives a white mixed salt with calcium only (not with Sr, Ba or Mg).

Group VI and Zero Group: Magnesium and Ammonium

1. Magnesium: the group V filtrate is first freed of calcium with ammonium oxalate. Na2HPO4 with NH4Cl and ammonia gives white crystalline \( \ce{Mg(NH4)PO4} \) (NH4Cl prevents Mg(OH)2). The solid forms slowly from dilute solution; scratching the tube helps. In neutral solution white MgHPO4 forms.
2. Mg(OH)2 (from ammonia) dissolves in ammonium salts because NH4+ removes OH- as weak NH4OH; NaCl cannot do this.
3. Ammonium carbonate gives a precipitate with Mg only when NH4+ salts are absent, because ammonium ion converts carbonate to hydrogen carbonate and the Ksp of MgCO3 (about 1 x 10-5) is high.
4. Titan yellow is adsorbed on Mg(OH)2, giving a deep red colour or precipitate.
5. Ammonium: warming with NaOH liberates ammonia, recognised by its smell, white fumes with HCl, blackening of paper wet with mercury(I) nitrate, deep blue colour on paper wet with CuSO4, and a brown colour on paper wet with MnCl2 and H2O2 in ammonia.
6. Nessler's reagent (alkaline \( \ce{K2[HgI4]} \)) gives a brown precipitate or colour (the iodide of Millon's base). Sodium hexanitritocobaltate(III), chloroplatinic acid and sodium hydrogen tartrate give yellow, yellow and white precipitates respectively.

Quick Distinctions to Remember

1. Cu from Cd: CuS is insoluble in hot dilute H2SO4; copper-cyanide complex is not attacked by H2S.
2. Zn from Al: Zn gives white precipitate with K4[Fe(CN)6]; Al does not. Both dissolve in excess NaOH, but only Zn forms a soluble ammine.
3. Hg(I) from Hg(II): HgO from Hg(II) is insoluble in excess NaOH but dissolves readily in acids; Hg(I) gives green Hg2I2 and black precipitates with ammonia.
4. Fe(III) from Al and Cr: Fe(OH)3 does not dissolve in excess NaOH.
5. Co from Ni: Co gives a blue thiocyanate colour in amyl alcohol and green cobalticarbonate; Ni gives red DMG precipitate and black Ni2O3 with bromine water and bicarbonate.
6. Mn from the rest of group IV: MnS dissolves in acetic acid, and oxidation to purple permanganate (NaBiO3 or PbO2 with HNO3) is characteristic.