VOLTAMMETRY AT MICROPIPET ELECTRODES

The use of micropipet electrodes for quantitative voltammetric measure ments of ion-transfer (IT) and electron-transfer (ET) reactions at the interface between two immiscible electrolyte solutions (ITIES) requir es knowledge of geometry of the liquid interface. The shape of the men iscus formed at the pipet:tip was studied in situ by video microscopy under controlled pressure. The shape of the interface: dan be changed from a complete sphere to: a concave spherical cap by varying the pres sure applied to the pipet, and the diffusion current to the pipet chan ges accordingly. With no external pressure applied, the water/organic interface turned out to be flat, and the voltammetric response of a pi pet must follow the well known theory for a microdisk electrode. The l arge deviations from this theory observed previously can: be attribute d to a small amount of the filling aqueous solution which escapes from the pipet and forms a thin layer on its outer wall. This effect can b e eliminated by making the outer pipet:wall hydrophobic. Procedures ha ve been developed for independent silanization of the inner and outer walls of the pipet. Pipets with a silanized inner wall can be filled w ith an organic solvent (e.g., 1,2-dichloroethane) and be used for volt ammetric measurements in aqueous solutions. Another mode of voltammetr y is based on trapping of a thin layer of organic solvent in the narro w shaft of a pipet between the filling solution and the aqueous outer phase. This arrangement is potentially useful for. electrochemical cat alysis and sensor applications.