Inorganic Chemistry (ICH602S)
Practical 1: Detection of Cations
Date: 21 July 2015
1. Introduction
Cations in an aqueous Solution can be Identified by using sodium hydroxide solution and
aqueous ammonia solution. All hydroxides are insoluble except Group I metal Hydroxides
and
Calcium hydroxide (limewater–sparingly soluble). Therefore, when an aqueous metal (non
group I) cation reacts with a hydroxide anion (OH--‐--‐ the hydroxide anion can come from
sodium
Hydroxide or aqueous ammonia), an insoluble metal hydroxide is formed, leading to the
observation of a precipitate. The reaction can be represented as follows:
Mn+(aq) + nOH- (aq) → M(OH)n (s)
On addition of excess reagent, certain metal hydroxides can react with either more
hydroxide anions or ammonia to form a soluble metal complex. Therefore, the precipitate
dissolves.
For example, Zn2+: Addition of sodium hydroxide / aqueous ammonia dropwise
Zn2+ (aq) + 2OH‐ (aq) → Zn(OH)2 (s)
Observation: white ppt forms
In summary, the unknown cation(s) can be identified by noting
a) The colour of the precipitate produced (if any);
b) If the precipitate is soluble in excess of the reagent.
2. Practical steps
a. Take between 1 – 2 cm test--tube height of the unknown solution containing the
metal
cation.
b. Add sodium hydroxide solution or aqueous ammonia dropwise, with shaking.
c. Observe the formation of any precipitate.
d. If there is a precipitate, note the colour of the precipitate.
e. Then add the sodium hydroxide solution to excess.
f. Observe if the precipitate (if any) dissolves. If it does, note the colour of the solution
formed.