Identification of metallic and non-metallic ions
Francine Taylor-Campbell, Contributor

IMPORTANT POINTS

Metallic ions or cations are identified based on the colour and solubility of the hydroxides formed from their reaction with aqueous ammonia and sodium hydroxide.

Non-metallic ions or anions are generally identified based on the evolution and identification of gases produced when the compounds are heated or reacted with acids. The colour and solubility of precipitates formed on reaction with barium ions and ammonia (for the halide ions) are also used.

Let us summarise the reactions of the metallic ions: Pb²⁺, Al³⁺, Ca²⁺, Zn²⁺, Fe³⁺, Fe²⁺, NH⁴⁺ and Cu²⁺ with aqueous ammonia and sodium hydroxide.

• Fe²⁺ produces a dirty-green precipitate, which is insoluble in excess aq NH₃ and NaOH
• Fe³⁺ produces a rusty-brown precipitate, which is insoluble in excess aq NH²⁺ and NaOH
• Cu²⁺ forms a blue precipitate, which is insoluble in excess aq NaOH, but soluble in excess aq NH₃.

These cations can clearly be identified based on their colours alone.

3 Al³⁺ and Pb²⁺ both form white precipitates when reacted with aq NaOH and NH₃.

In aq NaOH, the precipitates are soluble in excess, while in aq NH₃ they are insoluble in excess. As can be seen, Al³⁺ and Pb²⁺ give the same results. So, to differentiate between them, other reactions must be taken into consideration. Pb²⁺ will form precipitates with I^-, Cl^- and SO₄^2- ions (PbI₂ (yellow), PbC2 (white) and PbSO₄ (white)) while Al³⁺ will not.

Two other cations form white precipitates. Zn²⁺ forms a white precipitate, which is soluble both in excess aq NaOH and NH³ Ca²⁺ forms a white precipitate which is insoluble in excess NaOH but does not form a precipitate when treated with ammonia.

The NH₄⁺ ion can be identified based on the evolution of ammonia (NH₃) gas when warmed with aq NaOH.

The identification of the anions is a lot more complicated. The ones we are concerned with are CO₃^2-, SO₄^2-, SO₃^2-, NO₃^-, Br^-, Cl^- and I^-.

One way to do this is to heat the compound and identify the gas produced.

CO₃^2- =(heat) produces the gas CO₂

NO₃- ===== (heat) gives off O₂ and/or NO₂

SO₃^2- ===== (heat) gives off SO₂

These gases can be identified by their characteristic tests.

• CO₂ is weakly acidic and turns lime water (Ca(OH)₂) milky
• O₂ is neutral and relights a glowing splint
• NO₂ is a red-brown/ yellow-brown acidic gas
• SO₂ is an acidic gas which turns acidified potassium manganate (VII) solution from purple to colourless and changes acidified potassium dichromate (VI) from orange to green.
• NH₃ is an alkaline gas which forms dense white fumes with hydrogen chloride gas.

Some anions react with dilute and concentrated acids (if they are added to the solid).

CO₃^2- reacts with dilute and concentrated acids to release CO₂.

SO₃^2- forms SO₂ gas on reaction with dilute and concentrated acids.

If conc H₂SO₄ acid is added to a solid containing the halide ions (I^-, Cl^-, and Br^-), the following gases can be formed: HI(g), which decomposes to I₂, HCl(g) and HBr, which decomposes to Br₂.

The halide ions, in solution, can be identified differently, based on their reaction with silver nitrate solution (AgNO₃) and the solubility of the precipitate formed in ammonia.

Cl^- forms a white precipitate with AgNO₃ solution, which is soluble in aq NH3.

Br^- forms a cream precipitate with AgNO3, which is partially soluble in aq NH3.

I^- forms a light-yellow precipitate, which is insoluble in aq NH3.

Finally, some anions form precipitates with a solution of Ba²⁺ ions

CO₃^2- forms a white precipitate, which dissolves on addition of acid to form CO2.

SO₃^2- forms a white precipitate, which dissolves on addition of acid to give SO2

SO₄^2- forms a white precipitate, which is insoluble in dilute acid

In the next lesson we will discuss some questions which require identification of anions and cations. Review the reactions and the observations given above so that you can follow in next week's lesson.

Francine Taylor-Campbell is an independent contributor.