An electrochemical investigation of the thermodynamic properties of the NaCl-AlCl3 system at subliquidus temperatures

The phase relations in the Nacl-AlCl3 system (0.3 less than or equal to N-A lCl 3 less than or equal to 0.65) have been determined in the temperature r ange from 373 to 623 K by isothermal equilibration, electrical conductivity , and electromotive force measurements. Only one ternary compound, NaAlCl4, was found to be stable, with a melting point of 426 K. The standard Gibbs energy of formation of NaCl and NaAlCl4 has been measured in the temperatur e range from 423 to 623 K by a novel galvanic cell technique involving in-s itu electrogenerated chlorine electrode in the Na/beta"-alumina/NaCl, NaAlC l4/Cl-2,C and Al/NaCl, NaAlCl4/Cl-2,C cells along with the Na/beta"-alumina /NaCl,NaAlCl4/Al cell. The Delta (f)G degrees (NaCl(s)) and Delta (f)G degr ees (NaAlCl 4 (l)) values have been calculated as -412.4 + 0.095 T (+/-1) k J mol(-1) and -117.5 + 0.2460 T (+/-2) kJ mol(-1), respectively. The standa rd entropy of NaAlCl4 (s) at 298 K, computed from the results of the study and the auxiliary information from the literature (184 J K-1 mol(-1)), show good agreement with the estimated JANAF value (188.28 J K-1 mol(-1)). The enthalpy of formation of NaAlCl4 (1) from NaCl (s) and AlCl3 (s) at 550 K o btained in the present study (-1850 J mol(-1)) is in agreement with that co mputed from the heat-capacity measurements (-1910 J mol(-1)). The present m easurements are unique, as a new electrochemical technique is employed in a cell with low-melting sodium chloroaluminate electrolyte to obtain the the rmodynamic properties of NaCl and NaAlCl4 at significantly low temperatures . The Gibbs energy of formation of NaCl (s) is, thus, measured at temperatu res as low as 423 K by an electrochemical technique for the first time, in this work.