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Experiment 32Separation and Identification of the Group C Cations (Ba2+, Ca2+, Sr2+, and, Co2+)Objectives: To use a qualitative analysis scheme to separate and identify the GroupC 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 addingammonium 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 NH3 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 orderto 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, HNO3, is then added, and the solution is taken to dryness again. Oxalate is thereby converted to CO2 and the ammonium ions are oxidized to nitrous oxide (N2O). The following reactions occur:
Aqueous HCl is added to the residue (oxides of the Group C cations). Theresidue dissolves, yielding a solution containing the Group C cations. The so-lution may be blue due to the presence of tetrachlorocobaltate(II) ions, CoCl2+.Separation and Confirmation of Group C CationsCobalt(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), K3Co(NO2)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.