The thermal dehydration and decomposition of Ba[Cu(C₂O₄)₂(H₂O)]·5H₂O

Abstract

The thermal behaviour of Ba[Cu(C₂O₄)₂(H₂O)]·5H₂O in N₂ and in O₂ has been examined using thermogravimetry (TG) and differential scanning calorimetry (DSC). The dehydration starts at relatively low temperatures (about 80°C), but continues until the onset of the decomposition (about 280°C). The decomposition takes place in two major stages (onsets 280 and 390°C). The mass of the intermediate after the first stage corresponded to the formation of barium oxalate and copper metal and, after the second stage, to the formation of barium carbonate and copper metal. The enthalpy for the dehydration was found to be 311±30 kJ mol⁻¹ (or 52±5 kJ (mol of H₂O)⁻¹). The overall enthalpy change for the decomposition of Ba[Cu(C₂O₄)₂] in N₂ was estimated from the combined area of the peaks of the DSC curve as −347 kJ mol⁻¹. The kinetics of the thermal dehydration and decomposition were studied using isothermal TG. The dehydration was strongly deceleratory and the α-time curves could be described by the three dimensional diffusion (D3) model. The values of the activation energy and the pre-exponential factor for the dehydration were 125±4 kJ mol⁻¹ and (1.38±0.08)×10¹⁵ min⁻¹, respectively. The decomposition was complex, consisting of at least two concurrent processes. The decomposition was analysed in terms of two overlapping deceleratory processes. One process was fast and could be described by the contracting-geometry model withn=5. The other process was slow and could also be described by the contracting-geometry model, but withn=2.

The values ofE _a andA were 206±23 kJ mol⁻¹ and (2.2±0.5)×10¹⁹ min⁻¹, respectively, for the fast process, and 259±37 kJ mol⁻¹ and (6.3±1.8)×10²³ min⁻¹, respectively, for the slow process.