Production and isolation of pH sensing materials by carbonate melt oxidation of iridium and platinum

Abstract

Structure and pH sensing properties are presented for solid state pH sensing materials prepared by Li⁺/H⁺ ion-exchange of Li₂MO₃ (M = Ir, Pt) as well as by carbonate melt oxidation of iridium and platinum wires. To obtain reproducible results with the latter technique, a new vertical melt oxidation technique was developed. The structures and morphologies of Li₂MO₃ before and after H⁺ ion-exchange are characterized using high resolution powder X-ray diffraction and scanning electron microscopy. Iridium wires melt oxidized 30 min and 5 h possessed near Nernstian responses with −0.060(3) and −0.057(3) V pH⁻¹, respectively, while melt oxidized platinum wires resulted in unstable pH sensing. Ion-exchange of Li₂IrO₃ results in the formation of H_1−xLi_x[Li_1/3Ir_2/3]O₂ with a HCrO₂ structure, which was determined to be the active compound in pH-sensing of melt oxidized iridium electrodes and possessed a pH response of −0.060 V pH⁻¹.