Electrochimica Acta 45 (2000) 3543 – 3554 Adsorption of halide anions on bismuth single crystal plane electrodes  K. Lust, M. Va ¨a ¨rtno ˜ u, E. Lust * Institute of Physical Chemistry, Uniersity of Tartu, 2 Jakobi Street, 51014 Tartu, Estonia Received 5 October 1999; received in revised form 28 January 2000 Abstract Adsorption behaviour of Cl - , Br - and I - anions on Bi single crystal plane electrodes from aqueous, methanol and 2-propanol solutions has been studied by impedance spectroscopy and chronocoulometry methods. The values of surface excess and the Gibbs energy of adsorption of halide ions on the Bi(111), Bi(001) and Bi(011  ) single crystal planes have been calculated using the electrode potential E and the surface charge density  as the independent electrical variables. It was found that under comparable conditions the result obtained at constant E and at constant  are coincident. The formal charge transfer coefficient (so called electrosorption valency) and the dipole moment of the dipole formed by an adsorbed halide ion and its image charge in Bi plane have been calculated. It was found that on the Bi(111) plane, where the chemical bonds are all saturated, the formal charge transfer coefficient and the dipole moment of adsorbed anion are independent of electrode potential. For more active Bi planes there is noticeable dependence of both characteristics on electrode charge and at higher positive charges the covalent bonding between Bi(011  ) surface atoms and I - is probable. It was concluded that the dipole formed is significantly screened by the solvent molecules and the metal electron gas, but the influence of the solvent dielectrical and other characteristics on the effective dipole moment values is comparatively small. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Adsorption; Ions; Water; Methanol; 2-Propanol; Bismuth; Single crystal www.elsevier.nl/locate/electacta 1. Introduction Adsorption behaviour of various inorganic ions at the metal – electrolyte interface has been studied by electrochemists for a long time as the surface-active anions are involved in many technological processes. Usually the specific adsorption of different inorganic ions at metal electrode in system with constant ionic strength xcK A* +(1 -x )cK A have been studied. A* and A denote the surface-active anion and surface-inac- tive anion, respectively (F - ions for aqueous and ClO 4 - ions for nonaqueous mediums as practically surface-in- active anions have been used); x is the mole fraction of the surface-active component in the mixture; and c is the total concentration of electrolyte. The method of mixed electrolyte with constant ionic strength was orig- inally proposed more than 30 years ago [1,2] and thereafter used in many papers [1–5]. For many years the model description of these data was based only on the Grahame – Parsons (GP) theory [6,7]. Recently some new approaches simulating the reversible specific ad- sorption of ions on electrodes have been developed [8 – 15]. In the contemporary theories of specific adsorp- tion on electrodes, different electrical variables are used: the electrode surface charge  in the GP model [1,2,6,7]; potential drop across in the inner layer  M-2 in the Alekseyev – Popov – Kolotyrkin (APK) model [8]  Based on a presentation made at the 2nd Baltic Confer- ence on Electrochemistry, 1999. * Corresponding author. E-mail address: cnn@chem.ut.cc (E. Lust). 0013-4686/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII:S0013-4686(00)00432-1