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Glass Microelectrode Techniques for In Situ pH Measurements

Dep. of Agronomy, 135 Mumford Hall, Univ. of Missouri, Columbia, MO 65211

* Corresponding author.

ABSTRACT

Glass pH-sensitive microelectrodes with lengths < 0.1 mm and diameters < 0.05 mm have many potential uses for in situ soil pH measurements. Physically stable glass microelectrodes which responded rapidly to pH were constructed and their response to soil chemical and physical influences measured. The pH of nine inorganic salts and organic acid solutions measured with the glass microelectrode were compared to those measured with a glass macroelectrode. A slope of 0.92, an intercept of 0.35, and regression coefficient of 0.99 indicated good agreement. Soil suspension pH measured with glass micro- and macroelectrodes was compared for eight soils. A slope of 1.26, an intercept of –1.19, and regression coefficient of 0.96 indicated a slight variation. These differences were partly attributed to unavoidable electrode placement problems. In general pH measured with macro- and microelectrodes compared very well. The influence of soil water pressure gradients in the range of –10 to –1500 kPa near the pH and reference electrodes on microelectrode pH measurements was studied. No significant differences in pH values due to moisture gradients were observed when the reference electrode was placed in soil at water pressures greater than –33 kPa. Standard deviations in pH where the glass microelectrode was placed in soil with water pressures from –10 to –1500 kPa varied from 0.07 to 0.17 indicating that moisture gradients have little influence on measured pH. Results indicate that glass pH microelectrodes are suitable for use in soil systems.

Contribution from the Missouri Agric. Exp. Stn. Journal series no. 10551. This research was supported by USDA acid precipitation special grant 87-CRSR-2-3202.

Received for publication May 5, 1988.