A simple and robust sensor system is developed and evaluated for measurement of overall water hardness. Fabrication of the sensor including encapsulation, reference electrode and wiring are based on reliable and inexpensive thin film technology.
 
 
 

Measuring principle:

To determine the concentration of the analyte ions, the combination of the impedimetric and potentiometric measurement method is applied. With the aid of ionophores, which determine the selectivity of the ion-sensitive membrane forming the phase boundary electrode/solution, a potential difference is established. Change with the extraction of the ions by the ionophore in the sensitive membrane affects not only the electrode potential. At the same time also the conductivity of the membrane increases with the number of extracted ions. This effect is exploited in the impedimetric measurement methods.

By suitable combination of magnesium and calcium ionophores in these membranes one ionophore causes especially the extraction into the membrane and thus changes its conductivity, while the other ionophore allows only a short transport across the phase boundary of the membrane . The resulting potential difference at the phase boundary can be determined potentiometrically.

This innovative use of two physicochemical methods of measurement on a single sensor substrate, in contrast to previously used methods, allows the simultaneous quantitative determination of calcium and magnesium ions in sample solutions with a simple, portable instrument..
 
 

    Fig. 1  Sensor
 
 

Previous work:

The Working Group Electrochemistry of TU Chemnitz \performed within this project impedimetric measurements for the characterization and optimization of the ion-selective membranes are .

Focus of the investigations are currently:

•  Effects on membrane conductance by interfering ions

•  Long-term stability of the membranes

Fig. 2  Prototype (dummy) of the instrument