The copper(II)–hydroxide–NNN′N′-tetramethylethylenediamine system in aqueous solution: log K, ΔH, and ΔS

Abstract

The system copper(II)–NNN′N′-tetramethylethylenediamine has been investigated over a wide range of pH at 25° in 0·5M-KNO₃. Mixtures of copper salt and amine, the ratio varying from 1 : 1 to 1 : 9, containing nitric acid were titrated with KOH solution using a glass electrode calomel (0·1 M-KCl) couple. The range of total copper-ion concentration studied was 0·0008–0·0072M. Evaluation of the potentiometric data was carried out with a new program on an IBM 1130 computer. The formation constants of the species [CuL]²⁺, [Cu(OH)L]⁺, [Cu₂(OH)₂L]²⁺, [Cu₂(OH)₂L₂]²⁺, [Cu₃(OH)₄L₂]²⁺, and [Cu(OH)₂L] were determined. No evidence was found for [CuL₂]²⁺ and [Cu₂(OH)₄L₂] or for the presence of the species formed by hydrolysis of the aqueous copper(II) ion itself, that is Cu(OH)⁺, Cu₂(OH)₂²⁺, and Cu₃(OH)₄²⁺. The heats of formation of the predominant species, [CuL]²⁺, [Cu(OH)L]⁺, [Cu₂(OH)₂L₂]²⁺, [Cu₃(OH)₄L₂]²⁺, and [Cu(OH)₂L] were determined using a titration calorimeter. The mechanism of formation of the hydroxo-complexes agrees with the scheme of successive polymerisation which has been proposed for the alkaline hydrolysis of the copper ion.