Pergamon Efecrrochimica Acta, Vol. 40. No. 9. pp. 1093 I 100. 1995 Copyright r; 1995 Elscwer Scicncc Ltd. Printed in &eat Bntain. All rights reserved OX3 4686195 $9.50 + 0.M ELECTROCHEMICAL STUDY OF THE Cd(II)/Cd(Hg) SYSTEM IN 2-MERCAPTOACETIC ACID SOLUTIONS IVA TURYAN and DANIEL MANDLER* Department of Inorganic and Analytical Chemistry, The Hebrew University of Jerusalem, Jerusalem 9 1904, Israel (Received 3 January 1995) Abstract-A detailed study of the Cd(lI)/Cd(Hg) system in 2-mercaptoacetic acid, H,MA, solutions has been carried out in 1 M NaClO, at pH 4.8-12.0. The electrochemical behavior of 2-mercaptoacetic acid at a dropping mercury electrode was studied as well. Since all complexes that are formed in the solution undergo a reversible two-electron reduction, mathematical analysis of the data, provided the composition of the various complexes. Thus, the stability constants of the following complexes: Cd(SCH,COJ-, Cd(SCH,CO,);-, Cd(SCH,CO,), CdH(SCH,CO,);-, CdH,(SCH,CO,):-, CdH(SCH,CO,);, CdH,(SCH,CO,), and CdH(SCH,CO,)+ were determined and their degree of formation as a function of pH were estimated. In addition, the protonation constants of the ligand were calculated. The present results indicate that H,MA forms extremely stable bidentate complexes with cadmium ions. The formation of the various complexes as a function of pH reveals that at pH 10.7-12.0 the complex Cd(SCH,CO,):- predominates, whereas at pH 4.8-6.0 the complexes CdH,(SCH,CO,), and CdH(SCH,CO,)’ are dominant. At pH 6.1-10.6 different protonated and unprotonated complexes com- posed of one to three ligands, coexist. Key words: electrochemistry, cadmium, 2-mercaptoacetic acid, stability constants. INTRODUCTION We have recently reported[l] on the application of self-assembled monolayers based on w-mercapto- carboxylic acids, HS(CH,),CO,H, for the very sensi- tive and selective determination of cadmium ions. Varying the length of the alkyl chain, ie, n = 1, 2, 5 and 10, of the monolayers adsorbed onto thin mercury films revealed that the sensitivity toward cadmium ions inversely increased with the chain length. In other words, the system consisting of 2- mercaptoacetic acid, H,MA, exhibited the highest sensitivity. Therefore, we concluded that the com- plexation between the metal ions and both function- al groups of the adsorbed w-mercaptocarboxylic acid was essential to achieving such remarkable ( < IO- ” M) sensitivity. However, the nature of the complex between Cd’+ and the monolayer was still to be explored. The complexation of H,MA with various metal ions has been reported[2-181. Usually the highest ratio between the ligand and the metal ion is 2 : 1 except Zn*+, Ni” and Cd*+ which form also 3 : 1 complexes. In addition, the formation of multi- nuclear metal complexes with H,MA has also been reported[8]. Nevertheless, only very few studies[ 14- 18] have dealt with the complexes formed between H,MA and cadmium ions. For example, the forma- * Author to whom correspondence should be addressed. tion of a polynuclear complex, ie Cd,(SCH,CO&, in an ammoniacal aqueous solution at pH 10 was studied by Cabrera and West[l6]. On the other hand, Matsui concluded[l8] that at pH 5-9 Cd(SCH,CO,) and Cd(SCH,CO& together with CdH(SCH,CO3’ were present in solution. At the same time, different values (six orders of magnitude difference) for the stability constants of Cd(SCH,CO,)[l7, 181, derived from potentiometric measurements, have been published. We report here on a detailed study of the com- plexes formed between H,MA and cadmium ions as a function of pH and at relatively low concentrations of ligand. Increasing the ligand concentration causes a polymeric complex to precipitate. zyxwvutsrqponmlkjihgfe vide-injiia. The stability constants of the complexes were determined from polarographic measurements based on DeFord and Hume method. EXPERIMENTAL Polarographic experiments were performed on a home-made dropping mercury electrode (dme) that was controlled with a BAS-1OOB electrochemical analyzer. The dme had the following characteristics: m = 3.02mgsec-’ and t = 2.34 set for E = - 0.5 V and h = 86cm (the height of the mercury column) in 1093