The ion-pair constant and other thermodynamic properties of HCl up to 350°C

Abstract

Solubilities of silver chloride in aqueous hydrochloric acid solutions have been determined from 100 up to 350°C. From these measurements, the ionisation constant of HC1 has been evaluated up to 225°C. Evidence is presented to show that a protonated silver species, HAgCl 2 0, exists at 275°C and above. Available experimental data up to 200°C have been firted to Pitzer's equation to generate an algorithm to calculate stoichiometric activity and osmotic coefficients of HCl up to 350°C and concentrations up to at least 3.0 m. Using the present results and those of Wright et al. (1961), Pearson et al. (1963) and Lukashow et al. (1976), the dissociation constant ( K d ) of HCl as a function of temperature is described by the equation log 10 K = 2136.898 + 1.020349 T−4.5045 × 10 −4 T 2−50396.40/ T−901.770 10g 10 T ( T in ° K) which is valid in the range 25–350°C. Calculated enthalpy ( ΔH 0), entropy ( ΔS 0) and heat capacity change ( ΔC p 0) functions for HCl dissociation have been rationalized in terms of changing solute and solvent characteristics as temperature is raised.