A Comparison of Different Types of Glass Electrode for Determining the pH of Soils

During the past decade the quinhydrone electrode has been used extensively for determining the hydrogen-ion concentration of soils. It is admirably adapted to the study of reaction in soils due to its simplicity and ease of operation, and to its accuracy in most soils. As is well known, however, its use is restricted to soils having a reaction less than pH 8.0 to 8.5, and to soils containing no oxidizing or reducing substances in sufficient concentration to affect the oxidation-reduction potential of the quinhydrone. In recent years a number of investigators, notably Karraker, McGeorge, Baver, and Heintze and Crowther, have noted that erroneous results were obtained when the quinhydrone electrode was used to determine the hydrogen-ion concentration of certain soils having a neutral or even an acid reaction. The presence in soils of comparatively large amounts of manganese dioxide has been found to upset the oxidation-reduction potentials of the quinhydrone and hence to cause an error in the measurement of the hydrogen-ion concentration