The relative stabilities of the copper hydroxyl sulphates

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Mineralogical Magazine

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The literature contains considerable disagreements on the relative stabilities of the members of the copper hydroxyl sulphate family. Titration of copper sulphate with sodium hydroxide is claimed by some to produce only brochantite, while other reports indicate that antlerite and a dihydrate of antlerite are produced in the titration. Most stability field diagrams show that antlerite is the more stable stoichiomer at pH 4 and sulphate activity of 0.05-1. We have reexamined this stoichiometric family by titration of aqueous copper sulphate with sodium hydroxide and sodium carbonate, reverse titration of sodium hydroxide with copper sulphate and simultaneous addition of copper sulphate and sodium hydroxide at a variety of mole ratios, concentrations, temperatures and reaction times. We have also explored the reaction of copper hydroxide with copper sulphate and the reaction of weak bases, such as sodium acetate, sodium carbonate and urea, with copper sulphate. Our work indicates that: (1) antlerite is not formed in reactions of 0.05 to 1.2 m CuSO4 with 0.05-1.0 m NaOH or Na2 CO3 at room temperature; (2) antlerite is formed in the addition of small concentrations of base (≤0.01 M) to 1 M CuSO4 at 80°C, but not at room temperature or with 0.01 M CuSO4 at 80°C; (3) the formation of Cu5 (SO4)2 (OH)6.4 H2O occurs at large Cu2+ to base mole ratios; (4) the compound described in the literature as antlerite dihydrate is actually Cu5 (SO4)2 (OH)6.4 H2O; (5) at mole ratios of Cu2+, to OH- ranging from 2:1 to 1:2 the predominant product is brochantite; and (6) brochantite and Cu5 (SO4)2 (OH)6.4 H2O are converted to antlerite in the presence of 1 M CuSO4 (the latter requires temperatures of 80°C or greater). The Ksp (ion activity product) values of antlerite and brochantite were determined to be 2.53 (0.01) × 10-48 and 1.01 (0.01) × 10-69, respectively, using atomic absorption spectroscopy and Visual MINTEQ after equilibration in solutions of varying ionic strength and pH for six days. These values are in good agreement with those from the literature. However, after 6 months, antlerite in contact with solution is partially converted to brochantite and hence is metastable with a relatively low conversion rate. The Ksp value for antlerite must therefore be considered approximate. The relative stabilities of the copper hydroxyl sulphates are rationalized using appropriate equations and Gibbs energy calculations. A Gibbs free energy of formation for Cu5 (SO4) 2(OH)6.4 H2O of -3442. kJ/mol was obtained from the simple salt approximation. The very restricied conditions required for the formation, of antlerite are rationalized with a stability field diagram at 80°C. © 2007 The Mineralogical Society.





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