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Separation and identification of cations into analytical groups is based on solubility product principle and common ion effect. In addition to that gravimetric estimation of certain cations and anions is also dependent on such principles. In general any procedure that involves precipitation follows these principles.
In the general group separation of cations difference in solubility product values of various salts are exploited. For Group I cations (Pb2+, Hg2(2+), Ag+), Chlorides have low solubility product values, so they are precipitated first as chlorides by adding HCl. Sulfides of certain cations (Pb2+, Hg2+, Cu2+, Cd2+, As3+, Sb3+, Sn2+/4+) have low solubility product values, that is why they are precipitated as sulfides by passing H2S in acid solution in Group II. The acid solution keeps S2- concentration at a low value through common ion effect of H+, otherwise many other cations would have been precipitated here. Hydroxides of Group IIIA cations (Fe3+, Cr3+, Al3+, Mn2+) are precipitated next by adding NH4OH in presence of NH4Cl, the latter keeping the OH- concentration in check, for similar reasons. Sulfides of some cations (Co3+, Ni2+, Mn2+, Zn 2+) are precipitated next by passing H2S in alkaline solution in Group IIIB, alkaline condition helps in attaining high concentration of S2- in solution, necessary to exceed the relatively higher solubility product values of these cations. Group IV cations (Ca2+, Ba2+, Sr2+) are precipitated later as carbonates. Biphosphate salt of Mg2+ has a relatively low solubility product value, hence it is precipitated by adding NaHCO3 in group V. Thus entire process is guided by the solubility product principle and common ion effect.
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