Thermodynamic analysis of solubility data on hellyerite

As pointed out in Section V.7.1.1.1.1.3 solubility measurements of metal carbonates provide an important method to determine the thermodynamic quantities. For hellyerite, however, these data can be unambiguously assigned only when the stoichiometric composition is known. As formula and crystal structure are strongly interdependent, they should be determined simultaneously. So far neither has been established definitively and the thermodynamic quantities derived from solubilities must be used with caution. 

With hellyerite prepared by a new method, solubility measurements were carried out at different temperatures and at / = 1.0 m (Na)C104 [2001GAM/PRE]. In addition, the solubility was also determined at 25°C and different ionic strengths [2002WAL/PRE], In either case the pH variation method was used to study the dissolution reaction according to Reaction (V.109)  

Thermodynamic auxiliary data for H20(l) and C02(g) were taken from Table 

IV-1 (7 = 298.15 K) and from CODATA [89COXAVAG] Tt 298.15 K). Data for Ni were used as selected in this review (see Section V.2.1). Nickel hydroxo and carbonate complexes are negligible [76BAE/MES], [2003BAE/BRA] in the pH ranges where the experiments of [2001GAM/PRE] and [2002WAL/PRE] were carried out. 

Thus the selected standard Gibbs energy and standard enthalpy of formation have been derived from solubility measurements at various temperatures and ionic strengths. 

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