title: Evaluation of redox indicators for determining sulfate-reducing and dechlorinating conditions.
abstract: An in situ methodology based on covalently bonded redox indicators has been developed for determining when sulfate-reducing conditions exist in environmental samples. Three immobilized redox indicators [thionine (Thi, formal potential at pH 7 (E(0')7) equals 52 mV), cresyl violet (CV, E(0')7 = -81 mV), and phenosafranine (PSaf, E(0')7 = -267 mV)] were tested for their response to sulfide in synthetic solutions and under sulfate-reducing conditions in wastewater slurries. The byproduct of the sulfate-reducing process, sulfide, was found to couple well to CV in the concentration range of 1-100 microM total sulfide ([S(-II)]) and the pH range of 6-8. Thi, the indicator with the highest formal potential, reacts rapidly with sulfide at levels well below 1 microM while PSaf, the indicator with the lowest formal potential, does not couple to sulfide at levels in excess of 100 microM [S(-II)]. The degree of reduction of the indicators (i.e., the fraction of cresyl violet oxidized) in contact with a given level of sulfide can be modeled qualitatively with an equilibrium expression for [S(-II)]-indicator based on the Nernst equation assuming that rhombic sulfur is the product of sulfide oxidation. In a groundwater sample with dechlorinating microbes, reduction of Thi and partial reduction of CV correlated with dechlorination of TCE to cis-DCE.
authors.affiliation: Department of Chemistry, Oregon State University, Corvallis, OR 97331-4001, USA.
authors.forename: Brian D
authors.initials: BD
authors.lastname: Jones
authors.forename: James D
authors.initials: JD
authors.lastname: Ingle
mesh_terms: D048588:Benzoxazines; D019149:Bioreactors; D002713:Chlorine / Q000378:metabolism*; D004784:Environmental Monitoring; D006863:Hydrogen-Ion Concentration; D007202:Indicators and Reagents; D010078:Oxazines / Q000737:chemistry; D010084:Oxidation-Reduction; D010619:Phenazines / Q000737:chemistry; D010640:Phenothiazines / Q000737:chemistry; D013431:Sulfates / Q000378:metabolism*; D013440:Sulfides / Q000378:metabolism; D017186:Titrimetry; D014241:Trichloroethylene / Q000378:metabolism; D014867:Water / Q000737:chemistry*; D014874:Water Pollutants, Chemical / Q000378:metabolism; D018508:Water Purification
chemical_list: D048588:Benzoxazines; D007202:Indicators and Reagents; D010078:Oxazines; D010619:Phenazines; D010640:Phenothiazines; D013431:Sulfates; D013440:Sulfides; D014874:Water Pollutants, Chemical; D014867:Water; D014241:Trichloroethylene; C028911:cresyl violet; D002713:Chlorine; C011977:phenosafranine; C009469:thionine