In 2D chemical potential diagrams, the stoichiometric saturation of a specific composition of a binary solid solution is represented by a straight line; the total saturation curve is the envelope of the stoichiometric saturation lines; it corresponds to the overall thermodynamic equilibrium of a system containing a solid. For a given state of the aqueous solution (a point in the diagram), the fluid is potentially saturated with respect to a given range of compositions of the solid, and undersaturated with respect to another range. This property may be predicted geometrically by drawing the tangents from the fluid point to the total saturation curve: different portions are thus delimitated. Along the stoichiometric lines, metastable equilibria are achieved. The affigraphy concept (Guy and Pla, 1997) gives a frame allowing to correlate the saturation/undersaturation indices for the different compositions of the solid and the stable/metastable properties of the envisaged equilibria. These considerations are useful to discuss the composition of the solid that precipitates from a given composition of the fluid. The solid composition is the result of the competition between the potential dissolution of some range of the solid and the potential precipitation of another range, i.e. by the competition of the different kinetics corresponding to the different (dissolution or precipitation) reactions for all the solid compositions. The chemical affinities of these reactions are proportional to the lengths of the line segments drawn from the point representing the aqueous fluid, perpendicular to the stoichiometric lines for the corresponding solids. If all other parameters (kinetics, surface parameters etc.) are equal for the different solids having different compositions, the problem is then to compute the appropriate sum of all the affinity segments. An application of the preceding method is presented. It is in good connection with that for solids with fixed composition.
E-mail: guy@emse.fr