Ions in solution were present due to dissociation in the salt-free water-lutidine mixture. In order to extract the critical Casimir force from the measured potentials of interactions, the distances z from the substrate for which the electrostatic contribution was estimated to be negligible have been considered. The estimation of the electrostatic interactions was based on measurements of the electrostatic potential far from the critical point. However, different solubilities of ions in water and lutidine and the enhancement of one of components of the near-critical mixture close to the surfaces might result in a change of the screening of the electrostatic interaction compared to the case of a noncritical homogeneous medium. Unfortunately, possible effects of the critical adsorption on the electrostatic contribution to the interaction potential could not be estimated due to the lack of any theoretical predictions for such effects. Motivated by these issues in the present paper we study critical adsorption in a simple generic system, i.e., for a binary mixture solvent containing ionic solutes in the presence of a single charged planar wall that preferentially adsorbs one component of the solvent. Recently, the properties of bulk near-critical binary mixtures with ions have been studied theoretically using a modified Poisson-Boltzmann theory by Onuki and Kitamura [9]. The Landau-Ginzburg functional was used to describe the binary solvent. The presence of ions was accounted for by including electrostatic contribution and the terms due to the entropy of ions. Preferential solvation effects were modeled by a linear coupling between the density of ions and the order parameter of near-critical binary mixture. Using this phenomenological functional in the mean-field approximation, the authors found that the presence of ions shifts the critical point of demixing. This result is consistent with the well known experimental observations that adding salt to the binary mixture solvent changes the position of its phase separation curve, such that the upper and lower critical points are shifted upwards [10, 11] and downwards [12], respectively. If the ion-containing mixtures are exposed to external electric fields, e.g., due to charged walls or charged macromolecules immersed in the solution, the phase separation temperature is also shifted [13]. In addition to the preferential solvation, a dielectric inhomogeneity is important for this effect; the high permittivity solvent component is attracted to the charged surface thus enhancing the phase separation. Recently, the effects of these, so called, dielectrophoretic forces and of the preferential solvation have been studied for ion- containing mixtures confined between two-charged surfaces in a single phase region away 3