Richard W. Schaeffer

The relative stabilities of the copper hydroxyl sulphates

Abstract 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 sodiumhydroxide and sodium carbonate, reverse titration of sodiumhydroxide 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 м CuSO4 with 0.05-1.0 м NaOH or Na2CO3 at room temperature; (2) antlerite is formed in the addition of small concentrations of base (≤0.01 м) to 1 м CuSO4 at 80ºC, but not at room temperature or with 0.01 м CuSO4 at 80ºC; (3) the formation of Cu5(SO4)2(OH)6‧4H2O 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‧4H2O; (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‧4H2O are converted to antlerite in the presence of 1 м CuSO4 (the latter requires temperatures of 80ºC 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‧4H2O of -3442 kJ/mol was obtained from the simple salt approximation. The very restricted conditions required for the formation of antlerite are rationalized with a stability field diagram at 80ºC.

A Comparative Study of the Synthesis of Calcium, Strontium, Barium, Cadmium, and Lead Apatites in Aqueous Solution

Abstract The aqueous syntheses of the hydroxy and halo apatites of calcium, strontium, barium, lead, and cadmium were explored. Because these cations represent the main group s‐ and p‐fillers and a transition metal, they present different synthetic challenges. The alkaline earth cations and lead form hydrogen phosphates at slightly acidic and slightly basic conditions, the alkaline earths form the fluorides (MF2) in the preparation of the fluoroapatites with excess fluoride, and ammonia is required for the preparation of the cadmium apatites through decomplexation. A variety of reagents were utilized with most of the derivatives, but, in general, the source of the cation was its nitrate or halide, and the phosphate is best provided as ammonium hydrogen phosphate. The requirements for pH, heating, and reaction time were also explored. A number of literature syntheses for pure phase apatites could not be reproduced: calcium iodoapatite, strontium fluoroapatite, and cadmium hydroxyapatite. Several apatites were prepared for the first time in aqueous solution: barium fluoroapatite, lead bromoapatite, and cadmium chloroapatite. The relative ease of formation of the compounds is rationalized with arguments based upon lattice and hydration energies.

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

Abstract Syntheses 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 м KNO3 solution after equilibration at 25ºC 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.

The synthesis of copper/zinc solid solutions of hydroxyl carbonates, sulphates, nitrates, chlorides and bromides

Abstract The relative stabilities of the copper/zinc solid solutions of hydroxyl sulphates, carbonates, nitrates, chlorides and bromides were studied by attempting their preparation using a variety of methods. All of the naturally occurring solid solutions except rosasite were obtained as single phases. Rosasite crystallized in a mixture with malachite and calcium carbonate during a room temperature preparation from calcium carbonate, copper nitrate and zinc nitrate. The solid solution with the antlerite stoichiometry [endmember Cu3SO4(OH)4] as well as the nitrates were not produced by the methods employed. All of the natural polymorphs of Cu2(OH)3Cl were obtained and a new method for the preparation of botallackite is reported. Botallackite was found to be stable in solution for over a year, contrary to previous reports. A bromine-bearing analogue of botallackite was prepared. Compounds were characterized by X-ray diffractometry, which was used to determine the unit-cell parameters, and by atomic absorption spectroscopy. The relative instability of solid solutions with certain stoichiometries is discussed in terms of the Jahn-Teller effect and relative solubilities.