The synthesis and solubility of the copper hydroxyl nitrates: gerhardtite, rouaite and likasite

Abstract

AbstractSyntheses for the three members of the copper hydroxyl nitrate family – the polymorphs rouaite and gerhardtite, and likasite – are presented along with powder diffraction data and unit-cell parameters. The solubilities, determined in 0.05 M KNO3 solution after equilibration at 25°C for 10 days were used to calculate activity-based solubility product constants. The Gibbs energies of formation, obtained from the solubility products, are –653.2±0.7 kJ/mol, –655.1±1.2 kJ/mol and –1506.4±1.1 kJ/mol, for rouaite, gerhardtite, and likasite (Cu3NO3(OH)5·2H2O), respectively. The values for the polymorphs rouaite and gerhardtite validate the observations of Oswald that gerhardtite is the most stable polymorph at room temperature and that the preparation of predominantly rouaite in syntheses carried out at room temperature must be due to the metastability and low rate of conversion to the more stable gerhardtite.