Solutions, congruently saturated

Since the salts were originally present in equimolccular proportions, the final result of evaporation will be the pure double salt. If when the solution has reached the point E the salt A which had separated out is removed, double salt only will be left as solid phase. At a given temperature, however, a single solid phase can exist in equilibrium with solutions of different composition. If, therefore, isothermal evaporation is continued after the removal of the salt A, double salt will be deposited, and the composition of the solution will change in the direction EF. At the point F the salt B will separate out, and on evaporation both double salt and the salt B will be deposited. In the former case (when the salt A disappears on evaporation) we are dealing with an incoftgruently saturated solution but in the latter case, where both solid phases continue to be deposited, the solution is said to be congruently 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. 

Lastly, if the temperature lies outside the transition interval, isothermal evaporation of an unsaturated solution of the composition X (Fig. 127) will lead to the deposition of pure double salt from beginning to end. If a solution of the composition Y is evaporated, the component A will first be deposited and the composition of the solution will alter in the direction of E, at which point double salt will separate out. Since the solution at this point contains relatively more of A than is present in the double salt, both the double salt and the single salt A will be deposited on continued evaporation, in order that the composition of the solution shall remain unchanged. In the case of solution Z, first component B and afterwards the double salt will be deposited. The result will, therefore, be a mixture of double salt and the salt B (congruently saturated solutions). 

Primary saturation is the first state reached during the congruent dissolution of a solid-solution, for which the aqueous-solution is saturated with respect to a secondary solid-phase (JJ., J 4, Glynn and Reardon, Am. J. ScL, in press). This secondary solid will usually have a composition different from that of the dissolving solid. At primary saturation, the aqueous phase is at thermodynamic equilibrium with respect to this secondary solid but remains undersaturated with respect to the primary dissolving solid. The series of possible primary-saturation states for a given SSAS system is represented by the solutus curve on a Lippmann diagram. 

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. 

Now consider the dissolution of mineral Cao.85 Mgo.15 CO3 in pure water with one atmosphere CO2. If this mineral acted like a solid solution phase, it would dissolve congruently along reaction path AA until it reached at A saturation with a calcium-rich magnesian calcite less soluble than itself. We will see later in this... 

Thorstenson and Plummer (1977), in an elegant theoretical discussion (see section on The Fundamental Problems), discussed the equilibrium criteria applicable to a system composed of a two-component solid that is a member of a binary solid solution and an aqueous phase, depending on whether the solid reacts with fixed or variable composition. Because of kinetic restrictions, a solid may react with a fixed composition, even though it is a member of a continuous solid solution. Thorstenson and Plummer refer to equilibrium between such a solid and an aqueous phase as stoichiometric saturation. Because the solid reacts with fixed composition (reacts congruently), the chemical potentials of individual components cannot be equated between phases the solid reacts thermodynamically as a one-component phase. The variance of the system is reduced from two to one and, according to Thorstenson and Plummer, the only equilibrium constraint is IAP g. calcite = Keq(x>- where Keq(x) is the equilibrium constant for the solid, a function of... 

This equilibrium expression represents congruent dissolution of Na-feldspar but the solution quickly becomes saturated with respect to the solid phase Al(OH)3 (gibbsite K p = 10 ) and so the secondary mineral, gibbsite, is formed. 

The dissolntion of the CA j layer and the formation of saturated solution in relation to this snrface layer is the next stage of reaction [6]. The internal layers of CA are exposed and their hydroxylation occnrs immediately. Simultaneously, some amount of AH3 gel is precipitated and therefore the C/A molar ratio in the solution increases insignificantly to the valne abont 1.1. According to Bertrandie [123], this corresponds to the intersection of CA j solnbility cnrve with minimum instability curve (m in Fig. 3.46). This minimnm instability cnrve, from the chemical point of view, corresponds to the transformation of AH3 gel into the C2AHg phase [6, 124]. The CA j dissolntion is congruent and accompanied by the following reaction ... 

Iron(lll) chloride forms various solid hydrates, all of which melt congruently. Table 13.2 on the next page lists the temperatures t of aqueous solutions of various compositions that are saturated with respect to a solid phase. 

Figure 13.4 (a) is a stoichiometric compound with congruent fusion the metastable eutectic between A and B is not represented. Conversely, (b) shows a non-stoichiometric compound with congruent fusion (note that the composition of the melting point does not necessarily correspond to a stoichiometric composition) the metastable eutectic between A and B is not represented, (c) is a stoichiometric compound in the isothermal section of the ternary system A-B-C. (d) < AB> is a non-stoichiometric compound in the isothermal section of the ternary system A-B-C. The thin segments, representing the so-called tie lines, connect the compositions of the two phases in equilibrium crystals and their saturated liquid phase, u.s.s. is an unsaturated solution. 

A. Al Cheikh and M. Murat, Kinetics of non-congruent dissolution of E-glass fiber in saturated calcium hydroxide solution , Cem. Conor. Res. 18,1988, 943-950. 

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