Rapid determination of diffusion coefficients using electrochemical time of flight.
by Moldenhauer, Jonathan C.; Meier, Madeline; Paul, David
Electrochem. time of flight is a generate-detect expt. that can be used to rapidly det. unknown diffusion coeffs. for Red/Ox species, compared to methods commonly used. A specific advantage of ETOF is that much of the foreknowledge about the Red/Ox systems can remain unknown (i.e. electron-transfer kinetics, electroactive area of the electrode surface, analyte concn., and soln. viscosity). ETOF uses an array of micro-electrode bands; one microband is used as the generator, briefly polarized so that a portion of a Red/Ox analyte is generated and diffuses to flanking electrodes used as collectors; polarized so as to return the Red/Ox analyte to its original state. The result is that a current peak appears in the collector current as the generator products pass by. The time lapse between polarizing the generator and the peak in the collector current is the time of max. collection, tmc. In previous literature, diffusion coeffs. were detd. by graphing tmc as a function of the distance traveled, d, changing d by using collector electrodes that are farther away from the generator. The diffusion coeff. of the Red/Ox species could then be detd. using the following equation: d = Kv(Dtmc), where D is the diffusion coeff. and K is an empirical const. for the electrode array. With this method, a d vs v(tmc) curve has to be constructed to det. the diffusion coeff. for each unknown species. In this presentation we treat the data in a new way: rewriting the equation in terms of tmc as a function of the diffusion coeff., K/(dvD) + B = v(tmc). This new treatment uses a single spacing (d const.), and measures tmc for multiple Red/Ox species with known D. Plotting v(tmc) vs D-0.5 generates a straight line with slope K/d and becomes a calibration curve that can be used to det. diffusion coeffs. of Red/Ox species simply by measuring tmc. Since K and d depend only on geometry, the calibration can be performed in one solvent, and diffusion coeffs. estd. in other solvents by simply measuring tmc in that solvent. B is another empirical const. related to the array geometry defining where the diffusion coeff. is fast enough to appear as if it was tmc = 0 between the two electrodes because of the closeness of their spacing. Data will be given for previously unknown diffusion coeffs. in electrolytes contg. 20% V/V ethylene glycol.