359 Bioelectrochemistry and Bioenergetics, 26 (1991) 359-366 A section of J. Electroanai. Chem.. and constituting Vol. 321 (1991) Elsevier Sequoia S.A., Lausanne JEC BB 01394 Short communication Voltammetric studies on the surface activity and orientation of 2-thiobarbituric acid at the mercury~solution interface Mostafa M. Kamal Chemistry Department, Faculty of Science, Assiut University, Assiut (Egypt) (Received 12 March 1991; in revised form 20 April 1991) INTRODUCTION The first hypnotic barbiturate, 5,5-diethylbarbituric acid (barbital), was intro- duced in medicine in 1905; the second was 5-ethyl-5-phenyl-barbituric acid (pheno- barbitol), used as a long acting CNS depressant. Similarly, thiobarbituric acid derivatives were used as intermediates for the synthesis of dialkyl barbituric, acid until the anaesthetic properties of some thiobarbiturie acids were discovered [1]. Thiobarbiturates, like other sulphur-containing compounds, exhibit an anodic oxidation wave at the mercury electrode surface [2-4] corresponding to the forma- tion of slightly soluble mercury compounds. Indeed, anodic waves of thiobar- biturates in 0.1 M sodium hydroxide have been used for the analysis of pharmaceutical preparations [4-7]. The polarographic and voltammetric behaviour of thiobarbituric acid derivatives has been investigated extensively [2-7], from both the analytical and the mechanis- tic view. In the present paper the adsorption properties of TBA are studied by out-of-phase sensitive ac voltammetry. A further aim of this study was to compare the surface activities of TBA (dihydroxymercaptopyrimidine) and the nucleic acid base uracil (dihydroxypyriraidine). Due to the biological significance of both com- pounds it is of great interest to investigate how the introduction of an -SH group in the hydroxypyrimidine system will influence its adsorption effects at the mercury/ solution interface. EXPERIMENTAL 2-Thiobarbituric acid (TBA, 4,6-dihydroxy-2-merce.ptopyrimidine) was purchased from Sigma and was used without further purification. Solutions containing differ- ent concentrations of TBA were prepa:ed by dissolving a known amount of the chemically pure product into a definite volume of Britton-Robinson buffer. Buffers 0302-4598/9~/$03.50 © 1991 Elsevier Sequoia S.A. All rights reserved