in strong alkaline solutions. These reactions are expected to reduce
local pH around the electrode, hindering the pH response, especially
if one deals with the quiescent solutions. This is supported by the
findings of El Wakkad et al. [42] who measured pH of a standard
series of pH buffer solutions upon saturation with WO3. Namely,
these authors found that final pH values remained almost identical
to the initial pH value of the buffer, when pH approached 7. However,
when initial pH was set to a value greater than 7, the final
pH value was different by as much as 3 pH units upon saturation
of the initial solution with WO3. This agrees with the linear sweep
voltammograms presented in Fig. 3 pointing to a much lower solubility
of WO3 in acids in comparison to alkalis.