title: Automated and continuous redox potential measurements in soil.
abstract: Redox potential (Eh) describes the electrical state of a matrix. In soils, Eh is an important parameter controlling the persistence of many organic and inorganic compounds. A popular, but also criticized, manual measuring method makes use of a small tip of Pt placed on a copper wire that is placed in the soil; a reference electrode is placed in the same soil at a fixed distance. Fluctuations in redox potential values measured in the soil can be very large and depth-dependent. This will be overlooked when making single-point measurements. We developed the datalogger Hypnos 2.0 for continuous redox potential and temperature measurements at various depths in the soil and without disturbance of the site. Hypnos is field-deployable, relatively cheap, and runs on batteries. The datalogger can use a "sleep mode" between sampling events. In sleep mode, there is no constant voltage on the Pt wire or the reference electrode, but there is only a short pulse during sampling. We did not measure an effect of this short pulse on the measured redox potential. In sandy soils in mesocosms and in a salt marsh soil we measured changes in the Eh as large as from -400 to +100 mV within 4 d, and daily cycles of 200 mV. Both absolute redox potential values and their diurnal variations were depth-dependent. Because single redox measurements are insufficient in describing redox conditions in some soil systems, Hypnos can be a powerful tool when studying the effects of fluctuating redox conditions on metal availability and pollutant degradation.
authors.affiliation: Institute of Ecological Science, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands. michel.vorenhout@ecology.falw.vu.nl
authors.forename: Michel
authors.initials: M
authors.lastname: Vorenhout
authors.forename: Harm G
authors.initials: HG
authors.lastname: van der Geest
authors.forename: Daan
authors.initials: D
authors.lastname: van Marum
authors.forename: Kees
authors.initials: K
authors.lastname: Wattel
authors.forename: Herman J P
authors.initials: HJ
authors.lastname: Eijsackers
mesh_terms: D001331:Automation; D004563:Electrochemistry; D004566:Electrodes; D004784:Environmental Monitoring / Q000379:methods*; D010084:Oxidation-Reduction; D012987:Soil
chemical_list: D012987:Soil