A new method for fabrication of anodically electrodeposited iridium oxide film pH microelectrodes has been developed in this study. Novel for its tip size (3-10-microm tip diameter), the microelectrode is fabricated in a tapered glass micropipet filled with a low melting point alloy. The tapered end is recessed and platinized. Thereafter, iridium oxide is electrodeposited over the platinized end in the recessed part. The microelectrode has a very short response time (t80 < 5 s) in the pH range of 0-12 with an accuracy of 0.05 pH unit. The pH microelectrode is not affected by most ions and complexing agents of relevance in environmental and biological studies; it can be used in fluids over wide ranges of stirring speeds (0-55 rpm) and temperatures (approximately 5-40 degrees C). Redox agents such as dissolved oxygen and hydrogen peroxide have no effect on the pH response while quinhydrone, ferro- and ferricyanide, and sodium sulfide have marked effects. However, the microelectrode can still be used in any sample when calibration is done in standards having similar redox characteristics.

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Microenvironmental studies regarding plant oxygen release in a wastewater environment are important to understand the principles of constructed wetlands for wastewater treatment. pH, oxidation reduction potential (ORP), and dissolved oxygen (DO) microprofiles for the lateral and main roots of the bulrush (Scirpus validus) in a vertical flow constructed wetland fed with municipal wastewater were measured using microelectrode techniques. pH was found to be low (6.91-6.98) near the lateral root surface, indicating possible nitrification or H(+) extrusion. The ORP at the lateral root surface was between +250 and +317 mV and gradually reached the bulk solution ORP (+14 to -54 mV) at a radial distance of approximately 4,750 microm. DO values at the lateral root surface varied from 0.64-2.04 mg L(-1) as bulk biochemical oxygen demand (BOD) changed from 24 to 1,267 mg L(-1). DO at the lateral root surface and the thickness of the oxygen layer around the root marginally increased with an increase in bulk BOD, while ORP at the lateral and main root surface decreased. pH and DO values did not change near the main root and had the bulk solution values. The results of this study provide insights into root-induced microenvironments and would be helpful for the quantification of the total amount of oxygen contributed by plants in constructed wetlands.