Enthalpy of formation of sodium bis(oxalato)cuprate(II) dihydrate

Abstract

The thermal decomposition pathways of the mineral wheatleyite (sodium bis(oxalato)cuprate(II) dihydrate, Na2Cu(C2O4)2⋅2H2O) have not been established with certainty and the enthalpy of formation of wheatleyite has not been reported. Thermogravimetric analysis (TG), attenuated total reflectance - Fourier transform infrared spectroscopy (ATR-FTIR) and powder X-ray diffraction (PXRD) were used to establish the chemical reactions that occur during the thermal decomposition of sodium bis(oxalato)cuprate(II) dihydrate under atmospheres of static air and flowing nitrogen. The decomposition occurred in three steps in either atmosphere, but the decomposition temperature and products produced depended on the atmosphere. The initial step was the loss of water to form Na2Cu(C2O4)2 in either atmosphere. The Na2Cu(C2O4)2 decomposed in the second step to form Na2C2O4 and a copper compound that depended on the carrier gas. Cu was the decomposition product formed in nitrogen while CuO was produced in air. During the final decomposition, Na2C2O4 decomposed to form Na2CO3. Once the decomposition reactions were established, differential scanning calorimetry (DSC) was used to measure the enthalpy of formation for Na2Cu(C2O4)2 and Na2Cu(C2O4)2⋅2H2O in an atmosphere of static air. The values determined for the enthalpies of formation were -2095 kJ mol−1 and -2670 kJ mol−1 for Na2Cu(C2O4)2 and Na2Cu(C2O4)2⋅2H2O respectively with a measured standard deviation of ±5 kJ mol−1 and estimated total uncertainty of ±10 kJ mol−1.