Calorimetric study of the entropy relation in the NaCl–KCl system

Highlights: • The heat capacity of three (Na, K)Cl samples was measured from 5 to 300 K. • From these data, the vibrational entropy at T = 298.15 K was determined. • The calorimetrically determined solvus was compared to the directly determined solvus. -- Abstract: The heat capacity of one Na-rich and two K-rich samples of the NaCl–KCl (halite–sylvite) crystalline solution was investigated between 5 and 300 K. It deviated positively from ideal behaviour with a maximum at 40 K. The thereby produced excess entropy at 298.15 K was described by a symmetric Margules mixing model yielding WmS = 8.73 J/mol/K. Using enthalpy of mixing data from the literature and our data on the entropy, the solvus was calculated for a pressure of 105 Pa and compared with the directly determined solvus. The difference between them can be attributed to the effect of Na–K short range ordering (clustering)