Direct contact tungsten bronze electrodes for calibration-free potentiometric pH measurements

Tungsten wires were oxidized in a Na2WO4/WO3 melt, resulting in pure well-ordered crystal layers of Na0.75WO3 with cubic perovskite structure. These electrodes exhibited an excellent linear behavior of the open-circuit potential Eocp depending on the pH of the electrolyte solution with an almost nernstian slope in the pH range 1 to 10 at 25 °C. Because of the direct contact between tungsten and the bronze of well-defined stoichiometry, the electrode potentials are highly reproducible allowing calibration-free pH measurements. The electrodes showed fast response times, and long life times.