Cation Group III — Fe³⁺, Al³⁺, Cr³⁺ (NH₄OH + NH₄Cl)

Where Group III sits in the scheme

Systematic cation analysis precipitates ions group by group in a fixed order, each group defined by a reagent that selectively throws down only the cations whose salts are insoluble under those exact conditions. Group I uses dilute $\ce{HCl}$, Group II uses $\ce{H2S}$ in acidic medium, and Group III is reached only after both have been removed. By the time the analyst arrives here, the chlorides and acid-stable sulphides are gone, and the solution is ready for the trivalent hydroxide-forming ions.

The cations of analytical Group III are the trivalent ions $\ce{Fe^3+}$, $\ce{Al^3+}$ and $\ce{Cr^3+}$. Their defining chemical property is that their hydroxides have extremely small solubility products, so they precipitate even when the hydroxide-ion concentration is kept deliberately low. That low $\ce{OH-}$ concentration is what protects the next group — Group IV — from being lost, and it is engineered by the choice of reagent.

The group reagent: NH₄OH + NH₄Cl

The Group III group reagent, exactly as listed in NCERT Table 7.11, is ammonium hydroxide in the presence of ammonium chloride. The two reagents are not interchangeable in role: $\ce{NH4OH}$ is the source of the precipitating hydroxide ion, while $\ce{NH4Cl}$ is a buffering, suppressing additive. Together they create a medium in which $\ce{OH-}$ is present, but only in a tiny, carefully limited amount.

Operationally, the original solution is first treated with $\ce{NH4Cl}$ (solid) and then with an excess of $\ce{NH4OH}$ until the solution smells distinctly of ammonia. A brown or white gelatinous precipitate at this stage signals the presence of Group III cations. The hydroxides formed are:

The precipitation reactions themselves are straightforward double-displacement steps with the weak base:

$$\ce{FeCl3 + 3NH4OH -> Fe(OH)3 v + 3NH4Cl}$$

$$\ce{AlCl3 + 3NH4OH -> Al(OH)3 v + 3NH4Cl}$$

$$\ce{CrCl3 + 3NH4OH -> Cr(OH)3 v + 3NH4Cl}$$

Why NH₄Cl is added first — the common-ion effect

This is the conceptual heart of Group III and the part NEET most often probes indirectly. Ammonium hydroxide is a weak base that dissociates only partially:

$$\ce{NH4OH <=> NH4+ + OH-}$$

Ammonium chloride is a strong electrolyte that floods the solution with $\ce{NH4+}$ ions. By Le Chatelier's principle, this large excess of the common ion $\ce{NH4+}$ pushes the equilibrium above to the left, suppressing the ionisation of $\ce{NH4OH}$ and sharply lowering the concentration of free $\ce{OH-}$.

A low $\ce{OH-}$ concentration is exactly what is wanted. Precipitation occurs only when the ionic product exceeds the solubility product $K_{sp}$. The Group III hydroxides have such minuscule $K_{sp}$ values that even this suppressed $\ce{OH-}$ is enough to throw them down. The Group IV hydroxides — those of $\ce{Co^2+}$, $\ce{Ni^2+}$, $\ce{Mn^2+}$, $\ce{Zn^2+}$ — have much higher $K_{sp}$ values, so at this low $\ce{OH-}$ their ionic product stays below saturation and they remain in solution, to be precipitated later as sulphides.

Precipitation: the three gelatinous hydroxides

All three Group III hydroxides are gelatinous — a soft, jelly-like, high-surface-area form rather than dense crystals. This texture matters in the laboratory because gelatinous precipitates trap mother liquor and adsorb ions readily; the latter property is exploited in the lake test for aluminium. The colours, however, are the quickest discriminators on inspection: a reddish-brown mass points to iron, a white mass to aluminium, and a green mass to chromium.

On the macro scale the chemistry is identical for each — replacement of three chloride ligands by three hydroxide groups — but the downstream confirmatory behaviour diverges sharply because of the differing acid–base character of the three hydroxides. $\ce{Fe(OH)3}$ is purely basic, $\ce{Al(OH)3}$ and $\ce{Cr(OH)3}$ are amphoteric, and that single difference drives every confirmatory test that follows.

The conc. HNO₃ pre-oxidation step

Before precipitating Group III, the NCERT manual directs that 2–3 drops of concentrated $\ce{HNO3}$ be added to the original solution and the mixture heated for a few minutes. The purpose is to ensure that iron is present entirely as $\ce{Fe^3+}$ and not $\ce{Fe^2+}$:

$$\ce{2FeCl2 + 2HCl + [O] -> 2FeCl3 + H2O}$$

This step is essential because the hydroxide of $\ce{Fe^2+}$, namely $\ce{Fe(OH)2}$, has a higher solubility product than $\ce{Fe(OH)3}$. At the low $\ce{OH-}$ used in Group III, ferrous hydroxide may not precipitate completely and some iron would slip through into later groups. Oxidising all the iron to the ferric state guarantees that it appears here, as $\ce{Fe(OH)3}$, where it belongs.

Confirmatory tests for Fe³⁺

The reddish-brown precipitate of $\ce{Fe(OH)3}$ is first dissolved in dilute $\ce{HCl}$ to regenerate ferric chloride, and the resulting solution is split for two independent confirmations.

$$\ce{Fe(OH)3 + 3HCl -> FeCl3 + 3H2O}$$

Potassium thiocyanate (KSCN) — blood-red

A drop of potassium thiocyanate gives an intense blood-red colouration of the thiocyanato-iron(III) complex, the single most diagnostic test for $\ce{Fe^3+}$:

$$\ce{Fe^3+ + SCN- -> [Fe(SCN)]^2+}$$

Potassium ferrocyanide — Prussian blue

With potassium ferrocyanide, $\ce{Fe^3+}$ forms the deep Prussian-blue precipitate of ferric ferrocyanide:

$$\ce{4FeCl3 + 3K4[Fe(CN)6] -> Fe4[Fe(CN)6]3 v + 12KCl}$$

With excess ferrocyanide a soluble form, $\ce{KFe[Fe(CN)6]}$ (soluble Prussian blue), develops as a colloid that cannot be filtered — a detail the NCERT manual notes explicitly.

Confirmatory tests for Al³⁺

Aluminium is signalled by a white gelatinous precipitate, and confirmed by exploiting the amphoteric nature of $\ce{Al(OH)3}$ and its adsorptive capacity.

Dissolution in excess NaOH

$\ce{Al(OH)3}$ first forms with $\ce{NaOH}$ and then re-dissolves in excess to give soluble sodium meta-aluminate — the white precipitate appears and then vanishes on adding more alkali:

$$\ce{AlCl3 + 3NaOH -> Al(OH)3 v + 3NaCl}$$

$$\ce{Al(OH)3 + NaOH -> NaAlO2 + 2H2O}$$

Lake test

In the lake test, the aluminium salt solution is made acidic (turning blue litmus red), then $\ce{NH4OH}$ is added drop by drop. As the solution turns alkaline, $\ce{Al(OH)3}$ precipitates and adsorbs the blue dye from the litmus, forming an insoluble adsorption complex called a "lake". The visible signature is a blue mass floating in a colourless solution, which confirms $\ce{Al^3+}$.

Confirmatory tests for Cr³⁺

Chromium gives a green gelatinous $\ce{Cr(OH)3}$, which like aluminium is amphoteric. Adding excess $\ce{NaOH}$ dissolves it as the green chromite ion:

$$\ce{Cr(OH)3 + NaOH -> NaCrO2 + 2H2O}$$

The confirmation rests on oxidising this green chromite to yellow chromate. Warming the alkaline chromite with hydrogen peroxide oxidises chromium(III) to chromium(VI), giving a yellow chromate solution:

$$\ce{2NaCrO2 + 3H2O2 + 2NaOH -> 2Na2CrO4 + 4H2O}$$

The green-to-yellow change — from chromite to chromate — is the diagnostic transition for $\ce{Cr^3+}$. Acidifying the yellow chromate then converts it to orange dichromate, a further corroboration of chromium.

Observation → inference table

Worked confirmation