Kinetics of ligand exchange reaction of Cu(OH) 2-poly(vinyl alcohol) complex with potassium cyanide

Abstract

The kinetics of the ligand exchange reaction between the green complex of Cu(OH) 2 included in poly(vinyl alcohol) (PVA) and potassium cyanide (CN −) was studied by a stopped-flow method at pH 11.6–12.60, at ionic strength μ = 0.1 (KCl) and at 25°C. The Cu(OH) 2 is believed to be of type Cu n (OH) 2+ 2 n − 2 . Therefore the polynuclear complex of the Cu n (OH) 2+ 2 n − 2 included in helical PVA chains can be expressed as PVA-Cu n (OH) 2+ 2 n − 2 . The rate law, written as: r = − d[ PVA− C n( OH) 2+ 2n−2] dt can be explained by four different reaction paths. In paths 1 and 2, the reactions are initiated by attack of H + on the polynuclear copper complex Cu n (OH) 2+ 2 n − 2 included inside the PVA helical chains. In path 1, the intermediate, Cu n (OH) 3+ 2 n − 3 is formed in the bulk water. However, in path 2, the intermediate, PVA-Cu n (OH) 3+ 2 n − 2 (H 2O), is formed inside the PVA helix. In the highly alkaline region, paths 3 and 4 involve the ligand substitution reaction. In path 3, the Cu n (OH) 2+ 2 n − 2 is believed to be released from PVA helical chain and combines with CN − in the exterior bulk water. In path 4, the rate-determining step is the bimolecular ligand exchange reaction between PVA-Cu n (OH) 2+ 2 n − 2 and CN −. A possible mechanism for the substitution reaction is discussed.