NITRATE REDUCTION, AND REDOX POTENTIALS MEASURED WITH PERMANENTLY AND TEMPORARILY PLACED PLATINUM ELECTRODES IN SATURATED SOILS

The influence of nitrate-nitrogen on the redox potential measured with permanently and temporarily placed bright platinum electrodes was studied in samples of the Ap, BA and Bt horizons of a Huron silty clay loam in the laboratory. An anaerobic condition was induced by saturating the soils with distilled water. Redox potentials measured with the temporary Pt electrodes decreased with time to about −300 mv, but they were temporarily poised at +200 mv when NO3 was present. Similar results were obtained for untreated soils and for soils treated with 200 ppm NO3-N when measurements were made with the permanent Pt electrodes. With a second addition of 200 ppm NO3-N, the potential measured with the permanent electrodes increased approximately from −300 mv to −200 mv and remained poised at this potential for the duration of the experiment. The Eh values measured with the temporary electrodes increased to +200 mv and remained there temporarily until the NO3 disappeared, and then decreased to −300 mv again. The poising of the permanent electrodes at approximately −200 mv was attributed to the occurrence of chemical reactions at the platinum surface resulting in a reduction of the electrical conductivity or responsiveness of the electrodes.