Co-Precipitation in Solid Solutions

Consider a two-phase system with components AY and BY in which some of the BY(s) becomes dissolved in AY(s) BY is the solute and AY the solvent. If the resulting solid solution is homogeneous, that is, it contains no concentration gradient, the equilibrium distribution of A and B between the liquid and solid is 

Assuming as a first approximation /ay //by = 1 (an ideal solid solution) and that the activity ratio of the species in solution can be replaced by the concentration ratio, then 

As an example, consider a solid solution of 5 % ZnCOs in MgC03 (95 %) in equilibrium with Mg +,Zn + and [COs ] = 10 M (realistic conditions for many submerged soils). We have 

Assuming the solubility of MgC03 is little altered, then [Mg +] = 3.47 x 10 M and from Equation (3.49) 

This compares with [Zn +] = 10 M for equilibrium with pure ZnC03. 

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