Low-temperature Heat Capacities and Thermodynamic Properties of Hydrated Sodium Cupric Arsenate [NaCuAsO_4·1. 5H_2O(s) ]

摘要

Low-temperature heat capacities of the solid compound NaCuAsO_4 • 1. 5H_2O(s) were measured using a precision automated adiabatic calorimeter over a temperature range of T = 78 K to T = 390 K. A dehydration process occurred in the temperature range of T = 368—374 K. The peak temperature of the dehydration was observed to be T_D = (371. 828 ±0. 146) K by means of the heat-capacity measurement. The molar enthalpy and entropy of the dehydration were Δ_DH_m = (18. 571 ±0.142) kJ/mol and Δ_DS_m = (49. 946 ±0. 415) J/(K • mol) , respectively. The experimental values of heat capacities for the solid ( I ) and the solid-liquid mixture ( II ) were respectively fitted to two polynomial equations by the least square method. The smoothed values of the molar heat capacities and the fundamental thermodynamic functions of the sample relative to the standard reference temperature 298. 15 K were tabulated at an interval of 5 K.