Improved electrodeposited iridium oxide pH sensor fabricated on etched titanium substrates

In the present paper, the preparation and characterization of an improved solid-state pH sensor are described. The sensor is based on anodically electrodeposited iridium oxide film, as a pH-sensing layer. Merits of the present sensor include (i) excellent adhesion of the pH sensitive layer to the substrate, (ii) excellent reproducibility of sensor fabrication, (iii) faster preparation procedure, and (iv) low cost of the titanium substrate. These advantages are realized by combining acid-etched titanium as the electrode substrate with an optimized electrodeposition solution consisting of IrCl₄ as an iridium source, hydrogen peroxide, potassium oxalate, and potassium carbonate. Heating the electrodeposition solution to 90 °C reduced the time required for solution development from ∼3 days to 10 min. The pH-sensing layer is protected with a layer of Nafion and a microporous polyester membrane. The improved sensor showed a super-Nernstian response (-73.7 ± 1.2 mV/pH unit) in the pH range of 1.5-11.5. The present pH sensor, fabricated in a tubular form, is used as a detector in a flow injection analysis (FIA) system for pH measurements. Optimization of the FIA experimental parameters resulted in a linear dependence of peak heights on the pH of the injected samples in the pH range of 2-11.