The present invention is directed to a reversible electrochemical sensor for polyions. The sensor uses active extraction and ion stripping, which are controlled electrochemically. Spontaneous polyion extraction is suppressed by using membranes containing highly lipophilic electrolytes that possess no ion-exchange properties. Reversible extraction of polyions is induced by constant current pulse of fixed duration applied across the membrane. Subsequently, polyions are removed by applying a constant stripping potential. The sensors provide excellent stability and reversibility and allow for measurements of heparin concentration in whole blood samples via protamine titration.