L2 - Cation Qualitative Ion Tests

objectives

conduct qualitative investigations – using flame tests, precipitation and complexation reactions as appropriate – to test for the presence in aqueous solution of the following ions:
- cations:
  - barium (Ba2+)
  - calcium (Ca2+)
  - magnesium (Mg2+)
  - lead(II) (Pb2+)
  - silver ion (Ag+ )
  - copper(II) (Cu2+)
  - iron(II) (Fe2+)
  - iron(III) (Fe3+)

Flame Tests for Cations

A little of the metal salt or other compound is mixed with a few drops of concentrated hydrochloric acid and a sample of the mixture is heated strongly in a bunsen flame on the end of a cleaned nichrome wire (or platinum if you can afford it!).
In this simple flame test, many metals ions give characteristic flame colours and the chloride salts tend to be the most volatile giving a stronger colour, hence the use of conc. HCl(aq).
The nichrome/platinum wire should be cleaned in conc. hydrochloric acid and heated in the hottest part of the flame to make sure there is no contaminating flame colours.

All colours are due to electronic excitations to a higher electronic energy levels.
You see the light emitted as the electrons return to lower more stable electronic energy levels.
This is the basis of atomic emission and absorption spectroscopy.

Precipitation Reactions

Some metal ions give coloured hydroxide precipitates that can be used as a simple identification test.
Some metal ions give a white precipitate and others no precipitate at all.
Adding excess sodium hydroxide or ammonia solution can sometimes give useful extra observations.

Using Dilute Sodium Hydroxide

Dilute sodium hydroxide(NaOH) solution is added to a solution containing the suspected ion.
Both the precipitate formed and the effect of excess alkali are important observations.
The test should be repeated with aqueous ammonia solution (NH₃, 'ammonium hydroxide').
The observations with ammonia solution are usually similar, but not always, the same and the differences can be important clues as to the identity of the metal ion.
ppt. = precipitate.

calcium ion: Ca²⁺_(aq) + 2OH^–_(aq) ==> Ca(OH)₂_(s) gives a white precipitate
of calcium hydroxide with sodium hydroxide IF the concentration of calcium ion is high. It is not soluble in excess of NaOH. No precipitate is formed with ammonia solution.
magnesium ion: Mg²⁺_(aq) + 2OH^–_(aq) ==> Mg(OH)₂_(s) gives a white precipitate
of magnesium hydroxide with ammonia or sodium hydroxide, which is not soluble in excess of either NH₃ or NaOH. You could distinguish Mg from Ca with a flame test or ammonia test above.
copper(II) ion: Cu²⁺_(aq) + 2OH^–_(aq) ==> Cu(OH)_2(s) gives a blue/turquoise ppt.
of copper(II) hydroxide with ammonia or sodium hydroxide, which dissolves in excess ammonia to give a deep blue solution of an ammine complex, but copper(II) hydroxide is NOT soluble in excess NaOH.
Cu(OH)_2(s) + 4NH_3(aq) ==> [Cu(NH₃)₄]²⁺_(aq) + 2OH^–_(aq)
iron(II) ion: Fe²⁺_(aq) + 2OH^–_(aq) ==> Fe(OH)₂_(s) gives a dark green precipitate
of iron(II) hydroxide with ammonia or sodium hydroxide, which is not soluble in excess of NH₃ or NaOH. Darkens in air due to oxidation to Fe(OH)₃.
iron(III) ion: Fe³⁺_(aq) + 3OH^–_(aq) ==> Fe(OH)₃_(s) forms a brown precipitate
of iron(III) hydroxide with ammonia or sodium hydroxide, which is not soluble in excess of NH₃ or NaOH.
Another test for iron(III) ions is to add a few drops of potassium/ammonium thiocyanate solution and a blood–red coloured compound is formed.

Iron (III) Hydroxide PPT

Iron (II) Hydroxide PPT

Copper (II) Hydroxide

Magnesium Hydroxide

Using Aqueous NaCl

Adding Cl- ions can also form PPT's for two of our ions - Silver and Lead

Lead Chloride PPT

Silver Chloride PPT

These both look the same!?! So how do we tell the difference?

Adding Potassium Iodide to Lead ions

When we add Potassium Iodide to Lead ions it forms an intensely Yellow PPT

This can be used to distinguish between the Silver and Lead (Note - the Silver also produces a yellow PPT but is much creamier) - only the silver PPT dissolves when AMMONIA is added

Lead Iodide PPT

Silver Iodide PPT

The Sulphate ion test

We usually use this to test for Sulphate ions but the converse can work for testing for Barium ions.

The Barium Hydroxide PPT is soluble and so the hydroxide test does not work for this ion - in this case we can use Sulphate ions (from Sulphuric acid or Sodium Sulphate).

These form a White PPT which is completely insoluble.

Complexation Reactions

When cations dissolve in certain substances those that are small and have a high charge (a high charge density) can attract molecules/ions towards them called ligands.
We do not need to know much about ligands - or explain these complexes are or how they come about. We only need to be able to identify 3 different cation complexes.
Complexes are all solutions as they have an overall ionic charge

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