Intrinsic, Ionic, and Total Solubilities; Solubility Product and Precipitation

Abstract

The precipitation phenomenon is commonly used in chemical analysis. For example, in immediate analysis, it allows impurities to be eliminated from the studied medium by precipitating them as poorly soluble derivatives. Contrary to this methodology, in some other cases, the compound of interest may be isolated in a pure state by precipitation. In quantitative analysis, the precipitation phenomenon is the basis for gravimetry and for the titrations of some ions. The precipitation of a product occurs when its solubility is reached. Its solubility is governed by its intrinsic solubility and also, when it is ionizable, by its solubility product. 32.1 Solubility Product and Intrinsic Solubility Let's consider the substance MX and suppose that it can ionize into the cation M + and into the anion X − according to the equilibrium MX M + + X −. The substance MX is said to belong to the group of 1,1-electrolytes since it gives two monocharged ions by ionization. The equilibrium is governed by the thermodynamic dissociation constant K d : K d = a M + × a X − a MX or K d a MX = a M + × a X − When the solution is saturated in MX, that is, when we are in the presence of the following equilibria: MX (solid) MX (solution) M + + X − , we can demonstrate via thermodynamic reasoning (see Sect. 32.3) that the activity a MX(solution) is constant. Hence, we can write K d a MX = constant (at saturation)