The choice of the most appropriate working pH was carried out by comparing the analytical signal of the biosensor for a 20 μM sulfite solution with that achieved for a wine sample with a similar amount of sulfite, using SOx–SPCEs. A huge cathodic current was recorded at +200 mV vs screen-printed Ag/AgCl electrode for the wine sample in relation to the sulfite solution at pH 5 and 7, which can be attributed to the presence of sulfite and different interferences. However, the reduction currents recorded at pH 6 were of analogous magnitude for both solutions (Table 2). The level of interferences due to the compounds present in the wine sample was minimized at pH 6, so this value was chosen as the working pH.