= Electroneutrality
<Electroneutrality> means that positive and negative charges balance on the length scale under consideration. For a bulk electrolyte with ionic concentrations $c_i$ and charge numbers $z_i$, the approximation is $\sum_i z_ic_i=0$.
A solution containing sodium and sulfate ions therefore needs twice as many singly charged sodium ions as doubly charged sulfate ions if these are its only ions. Local charge separation still occurs near an electrode in an <electric double layer>. Thus bulk electroneutrality does not mean that every microscopic region is neutral, and it should not be imposed pointwise when modeling a charged interface.
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