A cell of isotropic liquid in the shape of a slab of thickness d and containing ionic impurities is considered. It is shown that the screening effect produced by the ionic charges on the external field is characterized by an effective surface length, lambda(S)( U), depending on the applied voltage U. The analysis indicates that lambda(S)( U)) << lambda(D) when the applied voltage is very large, and lambda(S)( U) --> lambda(D) for very small values of the applied voltage, where lambda(D) is the Debye screening length. The presence of the ions is responsible also for a counterpotential, upsilon, that for small U is such to cancel the effective electric field in the sample, whereas in the opposite limit it is inversely proportional to the applied difference of potential.