pH-microscopy - theoretical and experimental investigations

Ultramicroelectrodes with a pH selective neutral carrier-based liquid membrane and a tip diameter of about 1 mu m are used to investigate pH profiles above microelectrodes with different shape. The maximum local resolution of a pH-microscope with these electrodes is tested. It is in the order of the tip diameter (<5 mu m). The theory of stationary diffusion fields at microdisk electrodes is adjusted and summarized. The differences between constant current and constant concentration conditions at the surface are calculated and shown to become negligible at distances larger than two times the electrode's radius. Comparison between theory and locally resolved measurements at the microelectrode's surface shows the real behavior lies between these limits. Exact calculations are presented for pH gradients in solutions of protolytics with several protolysis steps. Excellent agreement is found between theory and experimental data. Images of pH profiles above microelectrodes electrochemically generating OH--ions are presented. (C) 1997 Published by Elsevier Science Ltd.