Electrochemical Techniques
OCN 633 - Nov. 25, 2013
Brian Glazer
[email protected]
•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
on
ati
id
ox
SO3
sulfite
S
S
sulfide
-2
-1
SO3
SO32-
thiosulfate
S
S8
S0
polysulfide
HSH2S
SO32-
tetrathionate
S
S2-
SO42sulfate
2-
elemental
sulfur
on
Sx
ti
uc
d
re
sulfur oxidation state
0
1
2
3
4
5
6
Oxidation-reduction reactions
§ Involve the transfer of electrons
 M " M2+ + 2e-­‐ (oxidation – loses electrons)
 2H+ + 2e-­‐ " H2 (reduction – gains electrons) § Occur at unique potentials*
§ Abiotic (e.g., corrosion) or biotic (e.g., respiration)
CH2O + O2 à
CO2 + H2O
2CH2O + SO42-­‐ + 2H+ àH2S+2CO2 +2H2O Energy
sources
Light is used
directly by
phototrophs
Geothermal
energy is utilized
mainly via heatcatalyzed
production of
reduced inorganics
Nealson and Rye 2004
O2
NO3Mn++
Fe++
SO42S2-
NH4+
Redox profiling
General guideline for
OATZ progression
Vertical scale changes across
environments
has been traditionally
oversimplified to SO42and H2S
CH4
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
• FeCl2 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 O2, Fe3+ will
precipitate and greater voltage
is measured
4Fe3+ + 3O2 + 12e= 2Fe2O3 (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
• O2, H2O2, Fe2+, Mn2+, HS-, S2O32-, S4O62-, Sx2-, S8(aq),
Fe3+(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 - O2 calibration
Glazer et al. 2004
Voltammetry 101 - Fe2+ 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)
Mn(III) desferal
No 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