## 3 Results and Discussion
### Phenomena Related to Overlimiting Mass Transfer Mechanisms  
#### 3.2.1 Chronopotentiometric Features  
As demonstrated in the previous section, registration of distinctive features in chronopotentiometric curves can be utilized to better understand changes occurring in the membrane phase and within  
Figure 5: (**a**) Normalized current-voltage curves obtained for different solutions and (**b**) Cowan-Brown plot obtained for Cr\({}_{2}\)(SO\({}_{4}\))\({}_{3}\) solutions of different concentrations. the DBLs as a consequence of concentration polarization. These changes are demonstrated to not only necessarily cause increases in the system resistance but also reduce it when dealing with weak electrolytes. Nonetheless, application of intense polarization levels can also involve the activation of overlimiting mass transfer mechanisms, mainly water dissociation and electroconvection. Figure 6 presents the evolution of different chronopotentiometric curves as the level of current density increases until reaching the overlimiting regime. Except for the case of Fe\({}_{2}\)(SO\({}_{4}\))\({}_{3}\) solutions (Figure 6b) where the overlimiting region was suppressed due to the formation of a blocking and isolating precipitate layer, the rest of panels showcase system responses where both shifts in the chemical equilibrium or formation of precipitates coexist with overlimiting mechanisms of mass transfer. Figure 6a shows an example where a shift in the chemical equilibrium in the membrane system favoring the generation of more conductive ions takes place simultaneous to the formation of electroconvective vortices. First, at the lowest curve in the graph, only a characteristic decrease of U\({}_{\text{m}}\) with time can be seen, whereas no oscillations attributable to electroconvective vortices appear. However, in the two upper curves obtained at currents significantly higher than i\({}_{\text{lim}}\), both phenomena are present: the decrease and the oscillations of U\({}_{\text{m}}\) over time. This type of evolution proves that electroconvection is not suppressed by shifts in the chemical equilibrium. The coexistence of both phenomena in the same curves could be an indication that dissociation reactions actually predominate in the membrane phase rather than in the DBL, so that the membranes become more conductive when charged with multivalent ions. [PERSON] et al. suggested that the conductivity of the membrane phase increases when the fixed charges are compensated with multiply charged counter-ions [20]. The same effect caused by a shift in the equilibrium condition inside the membrane phase would explain the decrease of U\({}_{\text{m}}\) with time. Moreover, the strong depletion at the diluting DBL would not impede the development of electroconvection. It is also noticeable that the size of the voltage oscillations increases with the current density, which signals the formation of larger vortices. Figure 6b shows the formation of Fe(III) precipitates during the registration of chronopotentiograms. The formation of a precipitate layer is tracked by the continuous and notorious increase of U\({}_{\text{m}}\) with time at the highest currents. Here it is to be noted that the voltage drop through the membrane reaches a value higher than 3 V. The formation of precipitates at the anodic side of the membrane was visualized after finalization of the experiments (see Figure 4b). As already seen from the current-voltage curve lacking overlimiting region in Figure 5a, the increase in U\({}_{\text{m}}\) is continued, which implies that the precipitate layer blocks ion transport through the membrane, thus suppressing any possible overlimiting mass transfer mechanism. In contrast with the case of Fe\({}_{2}\)(SO\({}_{4}\))\({}_{3}\), Figure 6c,d obtained using NiSO\({}_{4}\) solutions, are representative of chronopotentiograms where the formation of precipitates and overlimiting mass transfer mechanisms are coupled.