6.1: Separating group IV cations
After removing chloride insoluble salts as the group I, and sulfide insoluble salts as group II and group III, the cations that may still be present in the solution from the initial mixture include \(\ce{Ca^{2+}}\), \(\ce{Ba^{2+}}\), \(\ce{Na^{+}}\), and \(\ce{K^{+}}\). Group IV comprises \(\ce{Ca^{2+}}\) and \(\ce{Ba^{2+}}\) that are separated from the other two ions based on the insoluble ions rule#2 described in chapter 1 which states “Carbonates (\(\ce{CO3^{2-}}\)), phosphates (\(\ce{PO4^{3-}}\)), and oxide (\(\ce{O^{2-}}\)) are insoluble with the exception of alkali metals and ammonia.” Carbonate ion is introduced as ammonium carbonate (\(\ce{(NH4)2CO3}\)):
\[\ce{(NH4)2CO3(s) + 2H2O(l) -> 2NH4^{+}(aq) + CO3^{2-}(aq)}\nonumber\]
Addition of \(\ce{(NH4)2CO3}\) solution cause precipitation of \(\ce{Ca^{2+}}\) and \(\ce{Ba^{2+}}\) as white precipitates \(\ce{CaCO3}\) and \(\ce{BaCO3}\), as shown in Figure \(\PageIndex{1}\):
\[\ce{Ca^{2+}(aq) + CO3^{2-}(aq) <=> CaCO3(s, white)(v)}\quad K_{sp} = 6\times10^{-9}\nonumber\]
\[\ce{Ba^{2+}(aq) + CO3^{2-}(aq) <=> BaCO3(s, white)(v)}\quad K_{sp} = 3\times10^{-9}\nonumber\]
The precipitates of group IV cations are separated by centrifugation and decantation. The precipitate is used to separate and confirm group IV cations and the supernatant is kept for the analysis of group V cations.