• Almost all cations form atoms in a flame test. The colour of the flame in the flame test is associated with elements possessing low energy gaps.

Cations identified by precipitation

• The precipitates of the cations having d⁷, d⁸, d⁹ and d¹⁰ electronic configurations are soluble in excess ammonia and form the respective stable complex ions.
M²⁺(aq) + X^2-(aq) → MX(s)

(d⁷) [Co(NH₃)₆]²⁺ – Yellow brown

(d⁸) [Ni(NH₃)₆]²⁺ – Deep blue

(d⁹) [Cu(NH₃)₄]²⁺ – deep blue

(d¹⁰) [Zn(NH₃)₄]²⁺ – colourless

(d¹⁰) [Ag(NH₃)₂]⁺ – colourless

(d¹⁰) [Cd(NH₃)₄]²⁺ – colourless

Identification of NH₄+

• Ammonium salts give ammonia gas with solutions of alkali(NaOH, KOH, Ca(OH)₂ etc).
eg. NH₄Cl(s) + NaOH(aq) → NH₃(g) + Na⁺(aq) + Cl^–(aq) + H₂O(l)

The evolved NH₃ gas can be tested with Nessler’s reagent or moist red litmus paper.
NH₃(g) + Nerssler’s reagent → brown precipitate/colouration

Separation procedure of a mixture of cations

• Qualitative analysis of a mixture of cations involves the separation of them to five groups. The scheme of qualitative analysis is based on the principle of selective precipitation. The precipitation of cations from a solution one at a time is called selective precipitation.

Group I
• Cold, excess dilute HCl is added to a solution containing a mixture of cations. Only Ag⁺, Pb²⁺ and Hg₂ ²⁺ will be precipitated as insoluble chlorides (AgCl, PbCl₂, Hg₂Cl₂).

Group II
• After the separation of the insoluble chlorides in Group I, the filtrate is still acidic. When H₂S is passed through the solution, only insoluble sulphides get precipitated.
• The concentration of sulphide ion is relatively low because of the higher concentration of H⁺  ions. Other cations such as Mn²⁺ , Zn²⁺ , Ni²⁺ and Co²⁺ with higher Ksp values of their respective sulphides will remain in the solution.

Group III
• The filtrate from Group II is boiled for a few minutes to expel all the dissolved H₂S. Then boil the filtrate for a few minutes with conc.HNO₃ to oxidise Fe²⁺ to Fe³⁺ . The solution is treated with NH₄Cl and NH₄OH.
Fe³⁺ (aq) → Fe(OH)₃(s) (reddish brown)
Al³⁺ (aq) → Al(OH)₃(s) (white gelatinous)
Cr³⁺ (aq) → Cr(OH)₃(s) (green)

Group IV
• The filtrate from Group III contains OH^– ions and is basic. H₂S is passed through this solution in the presence of OH^– ions. Then H⁺ ions produced from H₂S are neutralised by hydroxyl ions.

• The above equilibrium shifts to the right and the concentration of S^2- ions increases

Group V
• Boils off H₂S from Group IV filtrate and add a little amount of NH₄Cl and NH⁴OH in excess. Heat the solution, then add (NH₄)₂CO₃ solution. Here Ca²⁺, Sr²⁺ and Ba²⁺ ions are precipitated as carbonates.