The Preparation Of W/WO₃pH Electrode And Its Response Mechanism

Metal/metal oxide pH electrodes have several advantages over traditional glass pHelectrodes, including a fast response, high mechanical strength and miniaturisation etc., andcan be employed for pH measurements in high temperature or strong stirring system,clinical, environmental and food monitoring and alkaline or HF solutions, where thetraditional glass electrodes are not suitable to be used. As the primary commercializedmetal/metal oxide pH electrodes, Sb and Pb oxide electrodes have poor E-pH linearrelationship and narrow pH test range, thus they are not applied extensively. In this case,development of a metal/metal oxide pH electrode with good performance is of greatsignificance.In this paper, W/WO₃pH electrode was prepared with pure tungsten rods （99.95wt%）using cyclic voltammetry, potential or voltage control, chemical oxidation and thermaloxidation. After comparison, the electrolyzer voltage control method was selected toprepare W/WO₃pH electrodes in the present work. The sensitivity of W/WO₃electrode is50±3mV/pH in the pH range of4-12, and its response time is less than3min with theaccuracy of0.2pH. The effects of K+, Cl-, I-and F-ions on the response of the W/WO₃pHelectrode is not obvious, but NO₃-has great influence on its pH response. The W/WO₃pHelectrode saved in dry air for230days still maintains the sensitivity of46±3mV/pH, whichis better than those that were saved in pure water and pH2.00buffer solutions.In the pH range of0.5-13.5, the W/WO₃electrode shows different pH responsesensitivity in five different pH regions, i.e. region A （pH<1）, region B （pH2-4）, region C（pH4-12）, region D （pH12-13） and region E （pH>13）. Moreover, electrochemicalimpedance spectroscopy （EIS）, X-ray photoelectron spectroscopy （XPS）, X-ray diffraction（XRD） were used to investigate the structure and composition of the oxide films formed onthe W substrate in each pH region so as to make clear the pH response mechanism of theW/WO₃electrode in different pH regions.In the region A （pH<1）, hydroxylation leads to the occurrence of super-Nernstianresponse, and the sensitivity of the W/WO₃electrode reaches69±2mV/pH. In the region B （pH2-4, near to zero charge point）, the oxide film is more compact to impede the transferof H+and electron, so the sensitivity becomes as low as30±10mV/pH. In the region C （pH4-12）, the oxide film is in a relatively stable state with a loose hydration layer, and theelectrode reaction is simple, resulting in the most outstanding pH response with thesensitivity of50±3mV/pH. In the region D （pH12-13）, the oxide film is gradually dissolvedand the electrode shows a lower sensitivity （20±3mV/pH）. In the region E （pH>13）, withalkali metal ions participating in the reaction, tungsten bronzes are formed, resulting insuper-Nernst response with a much higher pH sensitivity （102±3mV/pH）.