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Calibration of Glass Microelectrodes for in Situ pH Measurements

Department of Soil and Atmospheric Sciences, 144 Mumford Hall, Univ. of Missouri, Columbia, MO 65211

*Corresponding author.

ABSTRACT

Glass pH-sensitive microelectrode fabrication, calibration, and measurement techniques have been altered to improve the applicability of the method. Insulating pipettes with inner filaments were drawn to 20-µm tips and welded to the pH-sensitive glass so the inner filament was not sealed. The filament allowed air to be displaced as the filling solution flowed to the pH-sensitive tip, eliminating the need for a vacuum filling procedure. In situ calibration allowed soil pH to be measured without moving the reference electrode and altering the junction potential. Calibration is done in the same soil sample with the electrodes placed at a distance >5 cm from the site of pH measurement. The pH-sensitive microelectrode is positioned above the soil close enough to make electrical contact. Two 0.05-mL drops of pH buffer are added so that the drops flow down the electrode shaft to the soil as the electrode potential is measured. Soil pH values obtained by calibrating the electrodes in pH buffer solutions were 0.2 pH unit lower than those obtained by in situ calibration.

The reference electrode contacted the soil through a saturated KCl salt bridge placed in the moist soil. At soil water potentials below saturation, better electrical contact is made when the soil-salt bridge contact point is wetted with water, 1 mol L^–1 KCl, or 0.01 mol L^–1 CaCl₂.

Contribution from the Missouri Agric. Exp. Stn. Journal Series no. 11870. This research was supported by USDA Prime Farmland Reclamation Grant 90-34168-5699.

Received for publication February 25, 1993.