Measurements of Soil Redox Potential

Abstract

Soil redox potentials (Eh) are commonly measured and operationally defined using platinum (Pt) electrodes, but there are variations in the methodology and instrumentation for measuring the associated voltages that may lead to imprecise and inaccurate estimates of oxidation–reduction intensity in the soil environment. Most field measurements of Eh are either made manually using a volt meter or automatically with a recording device such as a data logger. A mesocosm experiment was conducted to evaluate the impact of these two measurement approaches on resulting Eh values in three different wetland soils. Noticeable differences were observed between data collected using a multimeter (simple volt meter) and those collected with a data logger. Because the multimeter permitted the observed voltages to drift while the Pt and reference electrodes equilibrated following closure of the circuit, it was initially suspected that these were the more accurate data. By adding a 10‐Mohm resistor (standard component of commercially available multimeters) to a closed circuit containing the Pt and reference electrodes, data loggers could be modified to collect data similar to those obtained using a multimeter. Further testing, however, demonstrated that the voltage drift observed when using a closed 10‐Mohm circuit was an artifact of alterations in the electrochemical soil environment surrounding the Pt electrode. By using an instrument with very high (200 Gohm) input resistance, the artifactual drift could be essentially eliminated. Our comparisons and experiments suggest that the most accurate measurements of soil Eh and soil redox status are obtained using volt meters with high input resistance (e.g., 200 Gohm). Accurate Eh values also are obtained using data loggers (20 Gohm resistance) in the standard configuration that maintains an open circuit except during the actual instantaneous voltage measurement.