Determination of diffusion coefficients of electroactive species in time-of-flight experiments using a microdispenser and microelectrodes

Two novel methods for the determination of diffusion coefficients of redox species combining the special properties of microdispensing devices and mic roelectrodes are presented Both are based on the local application of tiny volumes of the redox-active species by means of a dispenser nozzle at a def ined distance from the surface of a microelectrode. The microelectrode, whi ch is inserted through the bottom into an electrochemical cell, is held at a constant potential sufficient to oxidize or reduce the electro-active spe cies under diffusional control. The dispenser, which is filled with the ele ctro-active species, can be positioned by means of micrometer screws over t he microelectrode, After dispensing a defined number of droplets near the m icroelectrode surface, the current through the microelectrode is recorded, usually yielding a peak-shaped curve having a defined time delay between th e shooting of the droplets and the maximum current, The time that is necess ary to attain maximum current, together with the known distance between two dispensing points, can be used to determine the diffusion coefficient of t he electroactive species without knowledge of any system parameters, such a s concentration of the redox species, diameter of the electroactive surface or number of transferred electrons. A similar method for the determination of diffusion coefficient of redox species involves a second redox species for calibration purposes. A mixture of both species is shot close to the mi croelectrode surface. Due to the different formal potentials of the redox s pecies that are used, they can be distinguished in sequential experiments b y variation of the potentials that are applied to the microelectrode, and i t is thus possible to determine the individual transit times of the redox s pecies independently. The difference in the transit times, together with th e known diffusion coefficient of one of the redox species, can be used to c alculate the unknown diffusion coefficient of the second one.