Monitoring of both copper and cadmium is essential in the electrolytic production of zinc because these elemetns must be removed in order to improve the efficiency of the zinc electro-refining process. A new high-volume flow-through cell with an assembly of four ion-selective electrodes (four copper, four cadmium or two copper and two cadmium electrodes), reference electrode and temperature probe coupled with microprocessor-based instrumentattion can be used for this purpose. Determinations can be done continuously for 72-h periods without maintenance in an on-line mode. when multiple electrode determinations and redundncy principles are implemented. A microcomputer system incorporating low-power CMOS technology with multichannel and multiplexing capabilities is used for data acquisition. The use of the battery-powered data acquisition system provides excellent signal-to-noise ratios, meets the special demands of the harsh industrial environment, and is preferable to conventional mains-powered monitoring systems based on ion-selective electodes.