Pure and Applied Chemistry

Titration of aqueous copper(II) sulfate solutions with aqueous NaOH at temperatures ranging from 0 to 25 °C results in a complex Ostwald step rule cascade of basic copper sulfate phases. At 25 °C, the thermodynamically stable phase is brochantite [Cu₄SO₄(OH)₆], but posnjakite [Cu₄SO₄(OH)₆·H₂O] is formed first. At lower temperatures, but above 0 °C, wroewolfeite [Cu₄SO₄(OH)₆·2H₂O] forms first. If left in contact with the reaction solution, wroewolfeite is converted to posnjakite and brochantite in turn. However, at 0 °C, synthetic wroewolfeite is stable for periods longer than a week, even in contact with the reaction solution, and a stability constant could be determined for its formation. For the reaction below, lg K = -16.3(1) at 0 °C and I = 0, as determined by solution methods.

0.25Cu₄SO₄(OH)₆·2H₂O(s,wroewolfeite) = Cu²⁺(aq) + 0.25SO₄^2-(aq) + 1.5OH^-(aq) + 0.5H₂O(l)

Stability relations between minerals of stoichiometry Cu₄SO₄(OH)₆·2H₂O (n = 0, 1, 2) are discussed. High concentrations of Mg²⁺ ions (1 M) prevent the isolation of wroewolfeite at any temperature down to 0 °C.