The steady-state i-V behavior of an electrochemical cell comprising two polarizable Au microelectrodes in a mixed redox solution containing reversible and soluble redox species (FcCH(2)OH and Ru(NH)(6)(3+)) is reported. Analytical solutions describing the cell i-V response in the absence of a well-poised reference electrode are presented, and these expressions are shown to agree with the experimental i-V results. Depending on the relative areas of the electrodes, the i-V response in the absence of a reference electrode is particularly sensitive to the presence of redox impurities. Specifically, we demonstrate that FcCH(2)OH(+) impurity in the solution yields diffusion limited current plateaus that are equal to the currents for the primary redox species present in nearly 100-fold excess. (C) 2012 Elsevier B.V. All rights reserved.