Testing for Cations
Identification of Positive Ions (Cations)
(CIE IGCSE Chemistry)
Identification of Cations
The identification of metal cations in a water-based solution can be inferred from the color of the precipitate that manifests upon the introduction of sodium hydroxide and ammonia.
Most transition metals produce hydroxides with distinctive colours
The initial addition of a small volume of NaOH typically results in the precipitation of the corresponding metal hydroxide from the solution.
In excess NaOH is added instead of small volume some of the precipitates may re-dissolve (become soluble) and precipitate disappear
A few drops of NaOH is added at the beginning and any colour changes or precipitates formed are noted.
The next step is that the NaOH is added in excess and the reaction is observed again.
The steps are then repeated for the test using ammonia solution . (first few drops then excess NH3 solution)
Results:
The table below contains the results for each of the cations included in the syllabus
If a precipitate is formed from either NaOH or aqueous ammonia then it means that the hydroxide is insoluble in water
Zinc chloride, for example, reacts as such:
ZnCl2 (aq) + 2NaOH (aq) → Zn(OH)2 (s) + 2NaCl (aq)
Ca2+ ions can be distinguished from Zn2+ and Al3+ as calcium hydroxide precipitate does not dissolve in excess NaOH but both zinc hydroxide and aluminium hydroxide do
Zn2+ ions can be distinguished from Al3+ ions as Zn(OH)2 dissolves in excess aqueous ammonia but Al(OH)3 does not
The ammonia or sodium hydroxide solution must be added very slowly. If it is added too quickly and the precipitate is soluble in excess, then you run the risk of not noticing the formation of the initial precipitate, which dissolves as quickly as it forms if excess solution is added.
Be sure to distinguish between the term “colorless” and “clear”. A solution that loses its colour has become colorless. A clear solution is one that you can see through such as water. Solutions can be clear and have colour eg. dilute copper sulphate.
Flame Test :
The flame test is used to identify the metal cations by the colour of the flame they produce
Ions from different metals produce different colours
Dip the loop of an unreactive metal wire such as nichrome or platinum in concentrated acid and then hold it in the blue flame of a Bunsen burner until there is no colour change
This is an important step as the test will only work if there is just one type of ion present
Two or more ions means the colours will mix, making identification erroneous
This cleans the wire loop and avoids contamination
A small sample of the compound is placed on an unreactive metal wire loop such as nichrome or platinum (it must be unreactive metal to make sure it does not interfere with the result color)
Dip the loop into the solid sample / solution and place it in the edge of the blue Bunsen flame
Avoid letting the wire get so hot that it glows red otherwise this can be confused with a flame colour
The colour of the flame is observed and used to identify the metal ion present:
Diagram showing the colours formed in the flame test for metal ions
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Identification of Ions & Gases
