Diffusion potential micropipettes

An example of a diffusion potential that be can described by Equation 3.11 occurs at the open end of the special micropipettes used for measuring electrical potential differences across membranes (Fig. 3-6). The fine tip of the glass micropipette provides an electrically conducting pathway into the cell or tissue. Ions diffusing through this fine tip give rise to a diffusion potential between the interior of the micropipette and the aqueous compartment into which the tip is inserted. To estimate the magnitude of this potential for KC1 as the electrolyte, we will assume that there is 3000 mol m-3 (3 m) KC1 in the micropipette (Fig. 3-6) and 100 mol m-3 KC1 in the cell. The chloride mobility, uci, is about 1.04 times uK, so the diffusion potential calculated from Equation 3.11b at 25°C is... 

The theory has been verified by voltammetric measurements using different hole diameters and by electrochemical simulations [13,15]. The plot of the half-wave potential versus log[(4d/7rr)-I-1] yielded a straight line with a slope of 60 mV (Fig. 3), but the experimental points deviated from the theory for small radii. Equations (3) to (5) show that the half-wave potential depends on the hole radius, the film thickness, the interface position within the hole, and the diffusion coefficient values. When d is rather large or the diffusion coefficient in the organic phase is very low, steady-state diffusion in the organic phase cannot be achieved because of the linear diffusion field within the microcylinder [Fig. 2(c)]. Although no analytical solution has been reported for non-steady-state IT across the microhole, the simulations reported in Ref. 13 showed that the diffusion field is asymmetrical, and concentration profiles are similar to those in micropipettes (see... 

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