A new formalism which describes the irreversible processes through observable quantities is reviewed. This is featured by the notion of observable electric potential (OEP) measured with reversible electrodes. Only electrolyte solutions with a common ion in isothermal conditions are studied. In this formalism the OEP depends on electrolyte concentration gradients and electric current density. The charge density is a quantity without interest. The OEP profile is not deduced from Maxwell`s equations of electrodynamics. The issues studied in this review are: (i) electrolyte solutions; (ii) gravitational and centrifugal solutions; and (iii) membrane systems.