Problem database

The total current density in electrolytes is the sum of the current density of positive ions and the current density of negative ions: $j = e_+n_+v_+ + e_- n_- v_-$, where $e \pm$, $v \pm$ and $n \pm$ are the charge, velocity of positive and negative ions and their number per unit volume. Why is the mass of the substance released at the cathode proportional to the total current, and not to the current of only positive ions?