Infrared Absorption Studies of Aqueous Complex Ions: I. Cyanide Complexes of Ag(I) and Au(I) in Aqueous Solution and Adsorbed on Anion Resin

Abstract

Techniques are described for obtaining infrared absorption spectra of complex cyanide anions in aqueous solution and adsorbed on anion resin. In the system AuCN–KCN–H₂O, only the soluble complex ion Au(CN)₂^— is observed; the molar extinction coefficient (ε) is 477±25 mole^—1 liter cm^—1 at 2147±1 cm^—1. When Au(CN)₂^— is adsorbed to the extent of 47 weight percent on Dowex‐A‐1 resin, its absorption maximum appears at 2138 cm^—1. Three distinct silver cyanide complex ions were observed in aqueous solution: Ag(CN)₂^—, ε=264±12 at 2135±1 cm^—1; Ag(CN)₃^—2, ε=397±23 at 2105±1 cm^—1; Ag(CN)₄^—3, ε=556±83 at 2092±1 cm^—1. Approximate values (calculated in terms of activities) of the dissociation constants for the tri‐ and tetra‐cyanide complexes into the next lower complex are K_{3, 2}=0.20±0.05 mole liter^—1, and K_{4, 3}=13,4±4 mole liter^—1, respectively. Over ranges of silver concentrations from 0.1—1.0 formal and of free CN^— ion concentrations from 0.05—1.5 molar, the concentrations of Ag(CN)₂^—, Ag(CN)₃^—2 and Ag(CN)₄^—3 can be calculated to within 5 percent (absolute) using values of the constants adjusted to best overall fit; in terms of concentrations, K_{3, 2}^c=0.108 and K_{4, 3}^c=2.85 mole liter^—1.