The attainment of the limiting state for ion-exchange membranes is the result of establishment of a maximum concentration gradient or a minimum concentration at the interphase boundary. Hence, among all the known methods laser interferometry alone performs the direct measurements of minimum concentration. The proposed method allows for the measuring not only of the systems where the commonly used technique can be applied but also in such systems which so far were studied only by an indirect method. In particular, these are weak electrolytes since their solutions give no inflection in the voltage-current curve.