Some Aspects of the Spontaneous Solubility Equilibria of Solid Carbonates

In order to adequately carry out the laboratory procedures usually taught in a general analytical chemistry course, students must master the subject of multiple equilibria in aqueous solution, both conceptually and mathematically. The use of a computer program to solve the concentrations of all species at equilibrium with a slightly soluble compound is frequently recommended in the chemical education literature, due to its simplicity and quickness. But, when this calculation procedure fails to consider other important reactions aside from the solubility equilibrium itself, significant differences in the numerical results may be obtained. For several metal carbonates, the usual procedure to calculate their solubilities is to consider carbonate ion hydrolysis as the only additional reaction.In this paper it is shown numerically and graphically that for magnesium and silver carbonates, the precipitation reaction of metal (hydr)oxides would occur at the pH value calculated from the carbonate dissolution plus the anion hydrolysis equilibria. An analytical procedure to solve these complex multiple equilibria system is presented. The final solution to the problem is obtained through a high-order equation that is easily solved by a simple iterative method. The correct solubility-pH curve for the metal carbonates is calculated, and the pH values for the metal hydroxide precipitation onset for several important representative and transition metal carbonates are shown. It is concluded that the overall procedures to solve these multiple equilibria problems are not beyond the comprehension of any chemistry student with a acceptable background in general, inorganic, and analytical chemistry, and that by focusing on tasks such as these, important critical-thinking skills can be obtained.