Laboratory measurements with different electrodes in parallel under conditions of varying chemistry of the flotation pulp are reported for the case of Cu--Pb separation. It is found that the redox potential as measured with a glassy-carbon electrode correlates better with the response of an oxygen probe, than the traditional platinum-foil redox electrode. The glassy-C electrode also behaves in a way which averages the responses of mineral electrodes of pyrite, galena and, to some extent, chalcopyrite