A calorimetric and thermodynamic investigation of the synthetic analogue of chalcomenite, CuSeO3·2H2O

Abstract

Thermophysical and thermochemical calorimetric investigations were carried out on the synthetic analogue of chalcomenite. The synthesis of CuSeO 3 ·2H 2 O was realized from copper nitrate and sodium selenite solutions heated to 80 °C for 2 to 3 h. The precipitate was characterized by X-ray powder diffraction, X-ray fluorescence, IR spectroscopy, and Raman spectroscopy. The low-temperature heat capacity of CuSeO 3 ·2H 2 O was measured using adiabatic calorimetry between 5 and 323 K, and the third-law entropy was determined. A value of S °(298 K, CuSeO 3 ·2H 2 O, cr) = 181.0 ± 1.0 J/(K mol) was obtained with an uncertainty of 0.5%. The enthalpy of formation of CuSeO 3 ·2H 2 O was determined by solution calorimetry with H 2 SO 4 solution as the solvent, giving Δ f H °(298 K, CuSeO 3 ·2H 2 O, cr) = −1035.3 ± 4.9 kJ/mol. The Gibbs energy of formation for CuSeO 3 ·2H 2 O at T = 298 K, 1 atm can be calculated on the basis on Δ f H ° (298 K) and Δ f S °(298 K): Δ f G °(298 K, CuSeO 3 ·2H 2 O, cr) = −835.3 ± 5.3 kJ/mol. Smoothed C p °( T ) values between T = 0 K and T = 320 K for CuSeO 3 ·2H 2 O (cr) are presented along with values for S ° and the functions [ H °( T ) – H °(0)]and[ G °( T ) – H °(0)]. These results motivate a re-evaluation of the natural conditions under which selenites and selenates replace selenides, and sulfides in the oxidation zones of sulfide ore deposits or upon weathering of technologic waste. The value of Δ f G ° for CuSeO 3 ·2H 2 O was used to calculate Eh–pH diagrams of the Cu–Se–CO 2 –H 2 O system. The behaviour of selenium and copper in the surface environment has been quantitatively explained by variations of the redox potential and the acidity–basicity of the mineral-forming medium. Precisely, these parameters determine the migration ability of selenium compounds and their precipitation in the form of various solid phases.