A method of application to provide a workable solution for tracking the hardness of water utilizing an ion selective electrode by tracking the relative hardness of water at the outflow or egress of an ion exchange column. A monovalent cation exchange membrane (ion selective electrode) distinguishes hard water and softened water in a water flow stream. A voltage is applied across the membrane, facilitating the movement of sodium though the membrane (such that anions and divalent ions are excluded), and the current is measured. The change in current (delta current) is used to determine the hard water concentration or level of hardness in an influent stream. A second application estimates or detects the exhaustion of an ion exchanger, and/or determines the regeneration time/cycle of the ion exchanger through the use of an ion selective membrane. Blending of the influent hard water and effluent soft water allows a user to control hardness levels of the effluent.