Measurements of 𝑘𝑘 0 for facilitated ion transfer at the ITIES are further complicated by the absence of a well-defined equilibrium potential about which rate-driving force measurements should be made. This absence is because, in a typical facilitated proton-transfer experiment, only the neutral proton acceptor is added to the organic phase-a situation analogous to only adding the "reduced species" of a redox couple to the electrolyte in a voltammetry experiment. In the absence of a well-defined equilibrium potential, the formal potential for ion transfer Δ 𝑜𝑜 𝑤𝑤 𝜙𝜙 0′ must be known to accurately calculate the driving force for facilitated ion transfer. Most commonly this value is not known a priori and must be included as a free fitparameter along with 𝑘𝑘 0 and 𝛼𝛼 in a mixed diffusive-kinetic model (to fit a steady-state voltammogram), introducing additional uncertainty in the desired kinetic parameters 𝑘𝑘 0 and 𝛼𝛼. Together, the uncertainty in the interfacial pathway for facilitated proton transfer and the absence of a well-defined equilibrium potential warrants caution in the interpretation of previously reported 𝑘𝑘 0 values for facilitated proton transfer. Figure | Scheme of electrochemical cell used for measurements of facilitated PT. Laser-pulled quartz pipettes with diameters ca. 2-10 μm were filled with aqueous HCl and submerged in an immiscible nonaqueous electrolyte (TFT containing 5 mM supporting electrolyte) to form a micro liquid-liquid interface at the tip. Nanoamp-scale currents are observed when a hydrophobic organic base is initially added to the nonaqueous electrolyte, and a potential difference is applied between the two phases. Interfacial-direct PT is the fully coupled reaction limited to the interfacial region when a proton would be transferred from H3O + at the aqueous|organic interface to B(org). Interfacial-sequential transfer is a nonequilibrium process where neutral phase transfer and protonation within the aqueous phase occur prior to the electrochemical step. Highly lipophilic bases will spontaneously partition between the two phases and result in a bulk concentration of conjugate acid species at equilibrium capable of carrying current at positive overpotentials. Distinguishing between, and studying the kinetics of, the various pathways provides insight into what controls the rate of ion transfer broadly.