Making Soil Oxidation–Reduction Potential Measurements using Multimeters

Researchers studying the chemistry of soil systems may be interested in documenting the oxidation–reduction potential (Eh) for a variety of purposes, and this is often accomplished by measuring the Eh using platinum (Pt) electrodes in conjunction with a reference electrode and a voltmeter. It has been shown that Eh values obtained when using high resistance research grade instruments often are much different from values measured using standard multimeters. This difference appears to be a function of the input resistance, with the low resistance multimeters allowing a higher current to flow during measurement which alters the electrochemical environment causing the voltage to drift. The objectives of this paper are (i) to report on a device which when attached to a low resistance multimeter facilitates the accurate measurement of soil redox potential, and (ii) to better understand the nature of the drift observed when measurements are made using a standard multimeter. Redox potentials were measured with Pt and calomel electrodes in soil mesocosms using a research grade voltmeter and a standard multimeter, with and without an inexpensive device that effectively increased the input resistance of the multimeter to 1 Tohm. The device was constructed using a TL082 wide bandwidth dual JFET input operational amplifier which effectively raised the input resistance from 10 Mohm to approximately 1 Tohm. When a small correction factor was applied to account for the internal offset error from the amplifier, the Eh data recorded using the modified multimeter were essentially identical to those collected using higher end, research grade instruments (n = 162; Y = 0.9996X– 0.09; r2 = 1.0000). Depending on the pH of the system and the type of reference electrode used, and the redox couple of interest, Eh data collected using standard multimeters could lead to erroneous conclusions regarding whether a soil is oxidized or reduced.