Separation and Identification of the Group C Cations -...

Experiment 32 Separation and Identification of the Group C Cations
(Ba
2 +
, Ca
2 +
, Sr
2 +
, and, Co
2 +
)
Objectives :
To use a qualitative analysis scheme to separate and identify the Group
C cations.
Theory :
The solution containing the Group C cations is first treated to remove any Group A
cations as insoluble chlorides. The Group B cations are then precipitated as oxides and
hydroxides, with the addition of aqueous ammonia. These steps have been previously
discussed in Experiments 30 and 31. The Group C cations are then precipitated by adding
ammonium oxalate solution.
The cobalt (II) ion is the only Group C cation that reacts prior to the addition of the
ammonium oxalate. Cobalt (II) reacts with NH
3
as shown in Reaction (1). The formation
of the hexamine-cobalt (II) ion prevents the precipitation of cobalt (II) hydroxide in
ammonia solution. When ammonium oxalate solution is added, cobalt (II) precipitates as
cobalt (II) oxalate as shown in Reaction.
is light pink in color, and the other oxalates are white. In order
to prevent the oxalate in the precipitate from interfering at a later stage, the oxalate must
be destroyed. This is accomplished by transferring the precipitate to a crucible and
heating to dryness. Aqueous nitric acid, HNO
3
, is then added, and the solution is taken to
dryness again. Oxalate is thereby converted to CO
2
and the ammonium ions are oxidized
to nitrous oxide (N
2
O). The following reactions occur:

Aqueous HCl is added to the residue (oxides of the Group C cations). The
residue dissolves, yielding a solution containing the Group C cations. The so-
lution may be blue due to the presence of tetrachlorocobaltate(II) ions, CoCl
2+
.
Separation and Confirmation of Group C Cations
Cobalt(II)
Cobalt (II) ion is separated and identified in one step. The addition of nitrite ion to an
acidified solution containing cobalt (II) ions results in the oxidation of cobalt (II) to
cobalt (III), which precipitates as the insoluble yellow com- pound potassium
hexanitritocobaltate (III), K
3
Co(NO
2
)
6
(see Equations 9 and 10). This compound is used
as a means of quantitative analysis for cobalt (or potassium), due to its extreme
insolubility.