Solubility equilibria ions, selective precipitation

Sulfides with widely different solubilities and solubility products can be selectively precipitated by adding S2 ions to the solution removed from the chlorides in the first step (see Fig. 11.20). Some metal sulfides (such as CuS, HgS, and Sb2S3) have extremely small solubility products and precipitate if there is the merest trace of S2" ions in the solution. Such a very low concentration of S2 ions is achieved by adding hydrogen sulfide, H2S, to an acidified solution. A higher hydronium ion concentration shifts the equilibrium... 

The solubility product is the equilibrium constant for the dissolution of a solid salt into its constituent ions in aqueous solution. The common ion effect is the observation that, if one of the ions of that salt is already present in the solution, the solubility of a salt is decreased. Sometimes, we can selectively precipitate one ion from a solution containing other ions by adding a suitable counterion. At high concentration of ligand, a precipitated metal ion may redissolve by forming soluble complex ions. In a metal-ion complex, the metal is a Lewis acid (electron pair acceptor) and the ligand is a Lewis base (electron pair donor). 

When solutions of soluble ionic compounds are mixed, an insoluble compound will precipitate if the ion product (IP) for the insoluble compound exceeds its fCsp. The IP is defined in the same way as /equilibrium concentrations. Certain metal cations can be separated by selective precipitation of metal sulfides. Selective precipitation is important in qualitative analysis, a procedure for identifying the ions present in an unknown solution. 

The solubility products and reactions used here as a guideline to saturation states are given in Table VI. The results of the calculations for phosphate compounds are plotted as -log lAP (lAP is the ion activity product) as a function of depth at each station (Figs. 49 and 50). Only data from box cores collected during 1975-1976 and some selected horizons from the gravity cores are shown. Hydroxyapatite was supersaturated by a factor of lO -lO at all stations and is not plotted precipitation of this phase is known to be kinetically hindered in seawater (Martens and Harriss, 1970). Bray (1973) and Norvell (1974) inferred likely equilibrium of pore waters with whitlockite [Ca3(P04)2] in Chesapeake Bay and anoxic lake sediments, respectively. Long Island Sound pore waters also tend to have activity products close to those predicted for saturation with respect to whitlockite, although distinct undersaturation is found for most sediment intervals at NWC. 

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