www.elsevier.nl/locate/jelechem Journal of Electroanalytical Chemistry 469 (1999) 182 – 188 Calculation of thermodynamic and inner layer characteristics of halide ions adsorbed on bismuth single crystal planes from solutions in methanol M. Va ¨a ¨rtno ˜ u *, E. Lust Institute of Physical Chemistry, Uniersity of Tartu, Jakobi 2, 51014 Tartu, Estonia Received 15 February 1999; received in revised form 5 May 1999; accepted 7 May 1999 Abstract The values of the surface excess and the Gibbs energy of adsorption of halide ions on the (111), (001) and (011  ) Bi single crystal planes in methanolic solutions have been calculated from the previously obtained experimental data using electrode potential as the independent electrical variable. It was found that under comparable conditions the results obtained at constant electrode potential and at constant electrode charge are coincident. The electrosorption valency and the dipole moment of the dipole formed by an adsorbed halide ion and its image charge in the Bi plane have been calculated. It was found that on the Bi(111) plane, where the chemical bonds are all saturated, the electrosorption valency does not depend on electrode charge and in the measured range of potentials a weak dependence of the dipole value formed by an adsorbed halide ion and its image charge in Bi on the electrode charge was found. In contrast, on the more active Bi planes there is noticeable dependence of both characteristics on electrode charge and at higher positive charges, covalent bonding between Bi(011  ) surface atoms and I - anions is probable. It was concluded that the dipole formed is screened significantly by the solvent molecules and the metal electron gas. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Adsorption; Ions; Methanol; Bismuth; Alcohols; Single crystal 1. Introduction In our previous work [1,2] the adsorption of Cl - , Br - and I - ions from methanolic solutions on differ- ent faces of a bismuth single crystal electrode was studied by impedance and chronocoulometric tech- niques. The charge of specifically adsorbed anions  1 was calculated using the Hurwitz – Parsons – Dutkiewicz method [3,4]. Fitting the results to the simple virial isotherm, the values of the standard Gibbs energy of adsorption -G A ° and the mutual repulsion coefficient B of the adsorbed anions were estimated. The depen- dence of the integral capacity of the inner layer on the electrode charge  was calculated according to the traditional Grahame – Parsons inner layer model [5] us- ing the electrode charge as the independent electrical variable. It was found [1,2] that the adsorption of halide ions decreases in the sequence of the Bi single crystal planes (011  ) (001) (111) in good correlation with the known crystallographic characteristics of these planes. However, it is of interest to expand the analysis of the experimental results obtained in the light of some recent work [6,7]. The main aim of the present work is, alongside the traditional mode, to obtain some addi- tional quantities such as the electrosorption valency, defined as a formal charge transfer coefficient  , and the value of the surface dipole  s , created by an ad- sorbed anion and its image charge in the metal, using the electrode potential as the independent electrical variable. 2. Calculation of the adsorption parameters using electrode potential as the independent electrical variable In the studies of the adsorption of inorganic ions, the electrode charge has been usually accepted as the inde- * Corresponding author. Fax: +372-7-375160. E-mail address: martv@chem.ut.ee (M. Va ¨a ¨rtno ˜u) 0022-0728/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII:S0022-0728(99)00225-9