Current Measurements in Flowing Solution

This section will explore the quantitative behavior of current-potential curves (voltammetry) and steady-state current measurements in flowing solution prior to considering liquid chromatographic assays. A similar examination of quiet solutions is reserved for the section on in vivo measurements. (In this and all subsequent voltammetry discussions, only oxidation reactions are treated, not because reductions are unimportant, but because there are to date few, if any, neurochemical applications.) 

As seen in Fig. 4A, in the absence of oxidizable material a small residual current flows up to potentials of about -I-1.1 V vs. Ag/AgCl. This residual current is the sum of a slight charging current (since Eapp is continuously varying) plus faradaic current from trace impurities in solution or surface oxidation of the electrode. Finally, at ca. +1.2 V, a large increase in current occurs— in this case due to oxidation of water, as 

Since there is an almost inexhaustible supply of water molecules, the 

If the current is measured by applying a fixed potential of, say, + 0.6 V, the response will be that seen in Fig. 4B. A very short pulse of charging current (z c) flows when the potential is first applied, then the current is constant. This steady current corresponds to z l for the DA oxidation. 

The value of App at i = iJ2 is termed the half-wave potential, 1/2 (voltammograms are also called waves in the polarographic literature). The quantity E1/2 is closely related to the formal and standard redox potentials for a given compound and in this case it identifies the oxidation as that of DA. More detailed quantitative treatments of forced convection 

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