Total internal reflection second harmonic generation (TIR SHG) is used to investigate the interface between two immiscible electrolyte solutions (ITIES) under potential control. The observed potential dependent second harmonic response is attributed to a resonance enhanced process arising from the tetraphenylborate (TPB-) ion. As the aqueous phase is biased positive of the organic phase, a large increase in the SH response is observed and is attributed to the adsorption of the TPB- ion at the interface. The potential dependent SH data allows for the determination of the TPB- anion concentration at the interface. TIR SHG compliments measurements of the electrolyte surface excess and can function as a means of measuring the relative concentration of electrolytic species responsible for the potential drop across the electrochemical double layer.