The complexation of copper(I) by cyanide ions (CN − ) in aqueous solution has been studied by glass electrode potentiometry at 25°C and ionic strengths (I) of 1, 3 and 5 M in NaCl media. Overall formation constants, β n , for the equilibria: Cu + (aq) + nCN − (aq) ⇌ Cu(CN) n (n−1)− (aq) with n = 2, 3 and 4, were quantified, along with the ionization constant (K w ) of water and the acid dissociation constant (K a ) of HCN(aq). The solubility constants *K sn for the equilibria: CuCN(s) + (n − 1)HCN 0 (aq) ⇌ Cu(CN) n (n −1)− (aq) + (n − 1)H + (aq) were also determined from a re-analysis of published solubility data for CuCN(s) in acidic cyanide solutions at I = 1 M(NaCl) and 25°C. Because of the instability of uncomplexed Cu + (aq) and parameter correlations in the data, neither β 1 nor the solubility product K s0 (CuCN(s) ⇌ Cu + (aq) + CN − (aq)) could be reliably determined from the present data although estimates are presented.

The heats (enthalpy changes) associated with the ionization of water and of hydrogen cyanide have been determined by titration calorimetry at 25�C as a function of ionic strength up to 5 M in both NaCl and NaClO4 media. The enthalpy changes for both reactions exhibit a 'medium effect' with ?H being more positive in NaCl than in NaClO4 and with the difference becoming more pronounced with increasing ionic strength. This is attributed to the greater solvation of Cl- cf. CN- in aqueous solution. The present ?H values are similar to previous published results at high ionic strengths, and are in excellent agreement with the well established literature values at infinite dilution. The present ?H values were combined with literature stability constant data to calculate the corresponding entropies for the ionization of H2O and HCN as a function of ionic strength.

The enthalpy changes associated with the complexation of Cu', Ag', Zn" and Cd" by cyanide to form Cu(CN),,-, Ag(CN),-, Zn(CN),, -and Cd(CN),,-have been determined by titration calorimetry in (wherever possible) both aqueous NaCl and NaCIO, media at an ionic strength of 1 mol 1-' and 25 "C. The corresponding entropy changes were calculated by combining

1996 thermodynamic functions, thermochemistry thermodynamic functions, thermochemistry E 3000 -008Cyanide Thermodynamics. Part 4. Enthalpies and Entropies of Cyanide Complexation of CuI, AgI, ZnII, and CdII. -Enthalpies of complexation by cyanide of the title metal ions in both aqueous NaCl and NaClO4 media at an ionic strength of 1 mol/l at 25 . degree.C are determined by titration calorimetry. Entropy changes associated with the complexation are obtained by combining the present δH data with the appropriate δG (stability constant) data from the literature. The δH values show little dependence on the medium. The considerable variation of the δS values is attributed to competitive complexation of some of the metal ions by Cl-. The great stability (negative δG values) for the formation of cyanide complexes in aqueous solution is due to the highly favorable . delta.H term as δS is negative. -(SOLIS, J. S.; MAY, P. M.; HEFTER, G.; J. Chem. Soc., Faraday Trans.: Phys. Chem.