Cation Groups and Separation Methods

Study Notes

Cations are classified into six groups based on their reactivity with specific reagents.
Each group reagent separates cations from a solution for further analysis.
Group 1: Hydrochloric acid (HCl)
Group 2: Hydrogen sulphide (H2S) in an acidic medium
Group 3: Ammonium chloride (NH4Cl) and Ammonium hydroxide
Group 4: Hydrogen sulphide (H2S) in a basic medium
Group 5: NH4Cl + NH4OH + Ammonium carbonate (NH4)2CO3
Group 6: Has no specific reagent

Barium is first precipitated as chromate.
Strontium can be separated from Calcium by treating their acetate salts with excess concentrated ammonium sulfate ((NH4)2SO4) in a faintly ammoniacal solution.
Calcium forms the soluble (NH4)2[Ca(SO4)2]
Alternatively, a saturated solution of calcium sulfate (CaSO4) can be used.
Strontium sulfate (SrSO4) precipitates as a white solid.

Iron must be present as Fe3+.
If Fe2+ is present, it must be oxidized to Fe3+ by boiling with concentrated HNO3.
The Fe3+ solution is precipitated as hydrous oxide using a slight excess of NH4OH.
The precipitate is then washed with NH4NO3, dried, ignited, and weighed as Fe2O3.

Avoid washing Fe2O3.xH2O with NH4Cl, as it converts some Fe2O3 to FeCl3, which is lost during drying.
Iron can be precipitated using organic precipitants like cupferron, ignited, and weighed as Fe2O3.
It can also be precipitated by 8-hydroxyquinoline (oxine) as oxinate.
Organic hydroxy compounds (citrate or tartarate) must be absent, as they form soluble complexes with iron.

Aluminum ion (Al3+) is precipitated as Al(OH)3 using ammonia in the presence of NH4Cl or a methyl red (M.R.) indicator..
The precipitated Al(OH)3 is washed with either NH4NO3 or NH4Cl, filtered, dried, ignited and weighed as Al2O3.

NH4Cl or M.R. is used to control the pH of the medium and prevent the dissolution of amphoteric Al(OH)3 in excess NH4OH.
Al3+ can be precipitated as aluminum oxinate at pH 4-10 using 8-hydroxyquinoline.

Treat the mixture with excess NaOH. Non-amphoteric Fe(OH)3 precipitates, while the amphoteric Al(OH)3 dissolves as AlO2-.
The Fe(OH)3 residue is determined as described in the "Determination of Iron" section.
Al3+ is determined in the combined filtrate and washing after acidification, where Al3+ is produced and determined as described in the "Determination of Al3+" section.