Thermodynamics of the anion exchange of cyanide and thiocyanate on a strong base anion exchanger

The equilibrium constant for the anion exchange reaction RCN + SCN^- ⇌ RSCN + CN^- with 0.1 M K(CN + SCN) solutions and Dowex-1X8 anion exchange resin was determined at 283, 298, and 313 K. The thermodynamic equilibrium constant at 298.15 K is K°_CN,SCN = 23.2 ± 3.0. The enthalpy change for the reaction was obtained calorimetrically at 298.15 K, being ΔH°_CN,SCN = -13.23 ± 0.38 kJ mol^-1, hence the calculated ΔS°_CN,SCN = -18.3 ± 1.6 J K^-1 mol^-1 and ΔC_P° = 21 ± 13 J K^-1 mol^-1. Invasion of the thiocyanate form also releases heat 66 ± 9 kJ (mol of KSCN imbibed)^-1 and water 6.0 ± 1.5 mol of H₂O (mol of KSCN imbibed)^-1. The exchange of cyanide form resin (total water content n̄_w = 10.1 mol of H₂O equiv^-1) to thiocyanate form (n̄_w = 5.3 mol of H₂O equiv^-1) is interpreted as release of free, not of hydration, water.