Important issues regarding the application of pH sensors are response
time and long-term stability. As stated above, stable potential
values were obtained within 1 min for all electrodes used,
which agrees with the available literature reports [26,30,43]. For
example, Yamamoto et al. [26] demonstrated that response time
of W/WO3 ultramicro sensor depended on pH with a minimum
around pH = 7, being under 1 min in the range of 2 < pH < 12. In
addition, there is a consensus in the literature regarding to the
long-term stability and reproducibility over time for W/WO3 electrode
[26,30,43].