Electrochemical Techniques OCN 633 - Nov. 25, 2013 Brian Glazer glazer@hawaii.edu Redox chemistry refresher Life on Earth is comprised of e - transfer reactions Intro to electrochemistry voltaic cells, cell potential Nernst equation Electrochemical techniques reference electrodes amperometric, potentiometric, voltammetric methods Case studies coastal biogeochemistry deep-sea hydrothermal systems Oxidation state sulfide oxidation S 2- SO 3 2- sulfite SO 4 2- sulfate S SO 2- tetrathionate 3 S SO 3 S polysulfide HS - H 2 S S S 8 S 0 S x thiosulfate elemental sulfur sulfur oxidation state SO 3 2- reduction -2-1 0 1 2 3 4 5 6
Oxidation-reduction reactions Involve the transfer of electrons M " M 2+ + 2e - (oxidation loses electrons) 2H + + 2e - " H 2 (reduction gains electrons) Occur at unique potentials* Abiotic (e.g., corrosion) or biotic (e.g., respiration) CH 2 O + O 2 à CO 2 + H 2 O 2CH 2 O + SO 4 2- + 2H + à H 2 S+2CO 2 +2H 2 O Energy sources Light is used directly by phototrophs Geothermal energy is utilized mainly via heatcatalyzed production of reduced inorganics Nealson and Rye 2004
O 2 NO 3 - Redox profiling General guideline for OATZ progression SO 4 2- Fe ++ S 2- NH 4 + Mn ++ Vertical scale changes across environments has been traditionally oversimplified to SO 4 2- and H 2 S CH 4 Availability and energy yield à dominant reaction Electrochemistry the science of the interaction of electrical and chemical phenomena blurring the lines between methods & results Electrochemistry = electron transfer rxns
A simple electrochemical cell FeCl 2 at different Fe oxidation states in the two sides Salt bridge Wire with inert Pt at ends -- voltmeter between electrodes Electrons flow along wire, and Cl - diffuses through salt bridge to balance charge Voltmeter measures electron flow Charge remains neutral A simple electrochemical cell Salt bridge Container on right side is more oxidizing and draws electrons from left side Electron flow and Cl - diffusion continue until an equilibrium is established steady voltage measured on voltmeter If container on right also contains O 2, Fe 3+ will precipitate and greater voltage is measured 4Fe 3+ + 3O 2 + 12e - = 2Fe 2 O 3 (s) The voltage is characteristic for any set of chemical conditions Electrochemical potentials
Electrochemical potentials Electrochemical techniques Reference Electrodes Standard Hydrogen Electrode
Reference Electrodes Pathway of a general electrode reaction Amperometric sensors Clark-type oxygen electrode (A) Pt (B) Ag/AgCl-electrode (C) KCl electrolyte (D) Teflon membrane (E) rubber ring (F) voltage supply (G) galvanometer
Potentiometric sensors liquid junction potential --> ph ph electrodes (1) glass membrane (2) AgCl precipitate (3) internal solution (4) internal electrode (5) non-conductive body (6) reference (7) junction membrane = charge separation = potential
Voltammetric sensors 3- electrode cell, voltage applied across reference and working, current is measured by the counter ú Reference: Ag/AgCl ú Counter: Pt ú Working: Hg Detectable species Voltammetric sensors 100 micron Au wire sealed in glass or PEEK using marine epoxy 100µm Au Polished epoxy Glass O 2, H 2 O 2, Fe 2+, Mn 2+, HS -, S 2 O 3 2-, S 4 O 6 2-, S x 2-, S 8(aq), Fe 3+ (aq), FeS (aq) are all simultaneously detectable
Voltammetry 101 - construction Newly constructed Au glass electrode 100µm Au Voltammetry 101 - polishing Long glass electrode, delicately polished using a micromanipulator Polishing pad and custom polishing unit Voltammetry 101 - polishing electrode tip 220 grit, 15µm, 9µm, 1µm, 0.25µm diamond pastes
Voltammetry 101 - polishing 100µm Au Following coarse sanding Voltammetry 101 - polishing 100µm Au Following 15 micron paste polishing Voltammetry 101 - polishing 100µm Au Following 9 micron paste polishing, poorly rinsed
Voltammetry 101 - polishing 100µm Au Following 1 micron paste polishing Voltammetry 101 - polishing 100µm Au Following 0.25 micron paste polishing Voltammetry 101 - polarizing Hg film Hydrolysis 90s polarization following 240s of Hg plating
Voltammetry 101 - O 2 calibration Glazer et al. 2004 Voltammetry 101 - Fe 2+ calibration Voltammetry 101 - calibration sets Pilot Ion Method (Meites, 1965): the ratio of the slopes for the calibration curves are constant
Voltammetry 101 - calibration Luther et al. 2008 Voltammetry Applications analytical comparison Glazer et al. 2004 Voltammetry Applications open ocean water columns Konovalov et al. 2003, Glazer et al. 2006a, Glazer et al. 2006b
Voltammetry Applications coastal bays and sediments Luther et al. 2004, Taillefert et al. 2002, Rozan et al. 2002 Mn(III) desferal cyclic voltammograms (CV) in Chesapeake Bay waters (August 2003) No Mn(III) desferal Mn(III) desferal Unbound Desferal Just above suboxic zone in suboxic zone Voltammetry Applications limnology & hydroelectric Luther et al. 2003
Voltammetry Applications microbial mats Glazer et al. 2002, Glazer et al. submitted Voltammetry Applications isolates & mixed cultures inoculated control gradient tube cultures adapted from Emerson et al. 1997 Roden et al. 2004 Voltammetry Applications isolates & mixed cultures 0.5 µm D. Emerson, Bigelow Lab
Voltammetry Applications hydrothermal vents electrode Luther et al. 2001, 2008 In situ voltammetry - advantages No need to collect samples A small amount of analyte is used, allowing for multiple measurements Coupled to micromanipulators, high spatial resolution can be achieved Continuous measurement allows for high temporal resolution Electrodes can be deployed in a variety of water column, sediment, or laboratory environments Simultaneous detection of several analytes In situ voltammetry - disadvantages Labor intensive construction, preparation, maintenance, and data interpretation expensive High data acquisition rates is actually a two-edged sword, presenting new challenges in data reduction
Case studies coastal biogeochemistry deep-sea hydrothermal systems Summary In situ voltammetry is one of several effective tools for measuring many chemical redox species in many aquatic environments with high spatial & temporal resolution Moving toward unobtrusive, unattended, continuous measurements on the seafloor and in lab experiments aids in describing biogeochemical processes of interest