Electrochemical behaviour of silver complexes of dithia-alkanedicarboxylic acids

Abstract

The composition and electrochemical behaviour of the Ag-complexes of dithia-alkane-dicarboxylic acids, viz. 2,5-dithiahexane-1,6-dicarboxylic acid (I), 3,6-dithiaoctane-1,8-dicarboxylic acid(II), 4,7-dithiadecane-1,10-dicarboxylic acid (III) and 2,2,7,7,-tetramethyl-3,6-dithiaoctane-1,8-dicarboxylic acid(IV) were investigated. The Ag+:X ratio of all these complexes was found to be 1:2 in alkaline as well as in acid solution. These complexes can ionise into H+-ions and a charged Ag complex ion. The ionisation constants were determined. The complex molecule can also dissociate into an Ag+-ion and two ionised or unionised molecules of the complex-forming acid. The complex constants were determined. The complexes are more stable in alkaline than in acid solution. The diffusion coefficients of complexes are low; in acid solution a factor of 2 higher than in alkaline solution. The activation energy of diffusion in alkaline solution is 9.2 kcal mol−1 and 7.7 kcal mol−1 in acid solution. The activation energy of Ag deposition is 15 kcal mol−1 in alkaline and 25 kcal mol−1 in acid solution. The transfer coefficient also depends on pH and is 0.50 in alkaline and 0.75 in acid solution. The rate constant in alkaline solution is a factor of 6–60 smaller than that in acid solution. For the complexes with unbranched chains ((I), (II), (III)) the rate constant, in alkaline as well as in acid solution, is proportional to the complex constant. For the branched complex (IV), a lower reaction rate is found. A cathodic Ag deposit can be obtained from these complexes as a bright, well adhering layer. The cathodic as well as the anodic current efficiency is 100%.