Rapid Determination of Diffusion Coefficients Utilizing Electrochemical Time of Diffusion (ETOD)

Abstract

The electrochemical time of diffusion (ETOD) experiment (formerly called electrochemical time of flight (ETOF)) is a generate/detect experiment that is performed on electrode arrays(1-5) . This method has been utilized to determine diffusion coefficients of electrochemically generated analytes, by measuring the time over which an analyte travels a given distance. Here we show an alternate analysis for ETOD data from what is typically found in the literature. We rearrange the equation from d=K√(D*tmc) where d is the distance, K is a geometric constant determined by the array, D is the diffusion coefficient of the analyte, and tmc is the time of maximum collection on the chronoamperometric curve(2) to √tmc=d/(K√D). This rearrangement allows for a “calibration curve” to be developed for an electrode array at a single distance with multiple analytes, each with a different and known diffusion coefficient, instead of using only a single analyte measuring tmc at electrodes of multiple distances from a generator. This decreases the number of experiments that need to be performed to determine an unknown diffusion coefficient. This method also has all the advantages of more traditional ETOD/ETOF experiments in that it greatly simplifies the determination of diffusion coefficients as the concentration of the analyte, the area of the electrode, the viscosity of the solution, and the electron transfer kinetics can remain unknown. Here we show that diffusion coefficients determined by ETOD are comparable to those found in the literature, and that diffusion coefficients of probe molecules thickened by ethylene glycol are in reasonable agreement to those estimated using the Stokes-Einstein equation. Diffusion coefficients for ferricyanide, ferrocyanide, ruthenium(III) hexamine and dopamine were found within 5% of those from the literature, while the diffusion coefficients for the same probes through thickened solutions fell within 16% of the values predicted using the Stokes-Einstein Equation. With the consensus in the community being that the Stokes-Einstein equation can provide predicted diffusion coefficients within 35% of actual observed values (6), it can be considered that our method can provide diffusion coefficients that are reasonable for the previously unknown diffusion coefficients.