Thermodynamic properties and thermal behavior of Friedel's salt

• The decomposition of FS includes stepwise stages of dehydration, dehydroxylation, and decarbonatization.
• The temperature dependence of the heat capacity was approximated by a combination of Einstein functions and the absolute entropy was calculated.
• The corresponding value of absolute entropy decreases with a decrease of the experimental temperature range.

Crystal hydrate with the composition [Ca2Al(OH)6]Cl0.90(CO3)0.05·2H2O, close to that of Friedel's salt (FS), was synthesized by co-precipitation technique. The composition of the compound has been confirmed by X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), simultaneous thermal analysis connected with mass spectrometry (STA-MS) and chemical analysis. The heat capacity of the substance in the temperature range 150–250 K has been measured by the method of DSC. The enthalpy and entropy increments in this temperature range have been calculated. The temperature dependence of the heat capacity has been approximated using a combination of Einstein functions. The values of standard heat capacity and absolute entropy at 298.15 K have been estimated to be 333.8 ± 5.5 J mole−1 K−1 and 340 J mole−1 K−1, respectively.