NOVEL THICK-FILM PH SENSORS BASED ON RUTHENIUM DIOXIDE GLASS COMPOSITES

Potentiometric pH electrodes are described, which consist of 30 mu m f ilms of a ruthenium dioxide-glass composite, prepared by sintering mix tures of powdered lead borosilicate glass and ruthenium dioxide hydrat e onto Pyrex substrates at 900 degrees C. Electrical contact is made e ither at a point remote from the solution-contacting portion of the el ectrode or via a platinum wire contact immediately underlying the acti ve area. Composite electrodes with an RuO2:lead borosilicate glass rat io of 1:1 show a near-Nernstian dependence of potential upon pH in aqu eous buffer between pH 2 and 12, a maximum hysteresis of 30 mV and a r esponse time of approximately 90 s. Response rates appear to be limite d by the diffusion of protons through the porous composite and decreas e with decreasing RuO2:glass ratio. The composite electrodes are compl etely indifferent to changes in the concentration of dissolved oxygen, but show a small response to chloride ions. Changes in electrode char acteristics with aging are reported, of which the most significant is a -65 mV drift in E(o).