An evaluation of electrode insertion techniques for measurement of redox potential in estuarine sediments Elizabeth K. Hinchey * , Linda C. Schaffner Department of Biological Sciences, School of Marine Science, College of William and Mary, Virginia Institute of Marine Science, 1208 Greate Road, Gloucester Point, VA 23062, USA Received 9 December 2003; received in revised form 22 September 2004; accepted 15 October 2004 Abstract E h measurements by electrodes are commonly used to characterize redox status of sediments in freshwater, marine and estuarine studies, due to the relative ease and rapidity of data collection. In our studies of fine-grained estuarine seabeds, we observed that E h values measured in intact sediment cores were influenced by different electrode insertion techniques. Sediment E h measurements generated via lateral insertion of platinum electrodes through silicone-filled ports in acrylic cores were systematically more positive (on the order of 10–100 mV) than profiles generated via vertical insertion of platinum electrodes downward through the sediment–water interface of the same cores. A review of the literature indicated that while researchers routinely use both insertion techniques to measure E h , no discrepancy in out- put has previously been reported. We discuss the results of three experiments conducted to determine if the discrepancy in output was caused by electrode poisoning by sulfides during the stepwise vertical insertion technique, or was caused by contact of the electrode with the silicone plug during the lateral insertion technique. We conclude that contact between the platinum surface of the electrode and the silicone plug biases the E h measurements, resulting in erroneously positive E h values. Insertion of electrodes into sediment through silicone plugs produced E h values that were an average of 105.6 mV (±10.4 SE) more positive than values generated upon electrode insertion directly into sediment. Thus, we recommend against using an insertion technique where the platinum electrode remains in contact with the silicone plug, as this method results in misclassification of sediment redox state and estimated depth of the redoxcline. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: E h ; Electrode fouling; Oxidation–reduction potential; Platinum electrode 1. Introduction Oxidation–reduction potential, or redox potential, is a quantitative measure of the tendency of a biogeochem- ical system to receive or supply electrons, and can be viewed as a measure of the reducing or oxidizing capac- ity of a system (Zobell, 1946; Mitsch and Gosselink, 1993). For over half a century, measurement of sediment redox potential has been used widely as a means by which to approximate the depth of the redoxcline, or the interface between oxidized and reduced conditions in sediments. Sediment redox potential (expressed as E h when referred to a hydrogen standard, with units of millivolts) is most commonly measured by inserting an inert metal electrode (usually platinum) into the 0045-6535/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2004.10.029 * Corresponding author. Present address: US EPA Atlantic Ecology Division, Office of Research and Development, National Health and Environmental Effects Research Labora- tory, 27 Tarzwell Drive, Narragansett, RI 02882, USA. Tel.: +1 401 782 9659; fax: +1 401 782 3030. E-mail address: hinchey.elizabeth@epa.gov (E.K. Hinchey). Chemosphere 59 (2005) 703–710 www.elsevier.com/locate/chemosphere