A design of a rotating ring-disk electrode with an electrolytic back-contact cell for the disk has been implemented and tested. Both the disk, acting as a bipolar electrode between a conventional cell and back compartment, and the back-electrolyte are made readily interchangeable. The most significant application is to semiconductor disks where ohmic contacts may be intricate to fabricate, as for the p-InP example experimentally investigated here. However, control of the back-cell characteristics when there is a front metallic disk allows blocking current flow (anodic or cathodic) in a selected direction over some potential range. Application of this principle to amperometric end-point detection in titrimetry illustrates how selective response to mixed redox couples introduced by this diode-like effect can be put to effective use with conventional metal electrodes.