Solutions incongruently saturated

A congruently saturated solution is one from which the solid phases are continuously deposited during isothermal evaporation to dryness, whereas in the case of incongruently saturated solutions, at least one of the solid phases disappears during the process of evaporation. 

With reference to the solution represented by the point it may be remarked that it is an incongruently saturated solution (p. 246) it lies, as we see from Fig. 154, in the triangular area, the points on which represent solutions the composition of which can be expressed in terms of the three salts NaHCOg, NaCI, NH Cl, although the solid phases with which the solution is in equilibrium are NaHCOg, NH4HCO3, NH4CI. If sodium chloride is added to this solution, the composition... 

The dotted curves EQ, HP, and KM represent incongruently saturated solutions. 

Initial dissolution of primary silicates is typically incongruent that is, the stoichiometric ratio of elements released to solution is not the same as that found in the bulk phase of the mineral. An excellent example of incongruent dissolution (Fig. 7-13) is presented by Chou and Wollast (1984). They reacted albite with aqueous solutions in a fluidized bed reactor and maintained solution concentrations of the reaction products below saturation for potential secondary products. Even so, the molar ratio of Na/Si initially released to solution was almost an order of magnitude higher than that of the bulk albite. Dissolution incongruence has posed a particularly difficult theoretical problem for researchers working on mineral dissolution problems. 

Another, more recent example by Rodriguez-Hornedo and co-workers elegantly displayed how micellar additives, such as surfactants, can bring thermodynamic stability to unstable co-crystal phases. An incongruently dissolving carbamazepine salicyclic acid co-crystal was tested and it was found that by introducing various concentrations of aqueous solutions of sodium lauryl sulfate (surfactant), a congruently saturated system resulted. 

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