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Calibration of Glass Microelectrodes for in Situ pH Measurements
Author(s) -
Yang J.,
Blanchar R. W.,
Hammer R. D.
Publication year - 1994
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1994.03615995005800020015x
Subject(s) - microelectrode , electrode , ph meter , analytical chemistry (journal) , glass electrode , calibration , pipette , reference electrode , buffer (optical fiber) , salt bridge , ph indicator , chemistry , glass tube , materials science , capillary action , composite material , chromatography , electrochemistry , electrical engineering , biochemistry , statistics , mathematics , mutant , gene , engineering
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 CaCl 2 .