Title: Redox Geochemistry
1Redox Geochemistry
2Oxidation Reduction Reactions
- Oxidation - a process involving loss of
electrons. - Reduction - a process involving gain of
electrons. - Reductant - a species that loses electrons.
- Oxidant - a species that gains electrons.
- Free electrons do not exist in solution. Any
electron lost from one species in solution must
be immediately gained by another. - Ox1 Red2 ? Red1 Ox2
LEO says GER
3Fundamental electromagnetic relations
- Electric charge (q) is measured in coulombs (C).
- The magnitude of the charge of a single electron
is 1.602 x 10-19 C. 1 mole of electrons has a
charge of 9.649 x 104 C which is called the
Faraday constant (F) - qnF
- The quantity of charge flowing each second
through a circuit is called the current (i). The
unit of current is the ampere (A) 1 A 1 C/sec - The difference in electric potential (E) between
two points is a measure of the work that is
needed when an electric charge moves from one
point to another. Potential difference is
measured in volts (V) 1 V 1 J/C - The greater the potential difference between two
points, the stronger will be the "push" on a
charged particle traveling between those points.
A 12 V battery will push electrons through a
circuit 8 times harder than a 1.5 V battery. - Ohms Law V I R ? potential is equal to
current resistance
4Half Reactions
- Often split redox reactions in two
- oxidation half rxn ? e- leaves left, goes right
- Fe2 ? Fe3 e-
- Reduction half rxn ? e- leaves left, goes right
- O2 4 e- ? 2 H2O
- SUM of the half reactions yields the total redox
reaction - 4 Fe2 ? 4 Fe3 4 e-
- O2 4 e- ? 2 H2O
- 4 Fe2 O2 ? 4 Fe3 2 H2O
5Examples
- Balance these and write the half reactions
- Mn(IV) H2S ? Mn2 S0 H
- CH2O O2 ? CO2 H2O
- H2S O2 ? S8 H2O
6Redox Couples
- For any half reaction, the oxidized/reduced pair
is the redox couple - Fe2 ? Fe3 e-
- Couple Fe2/Fe3
- H2S 4 H2O ? SO42- 10 H 8 e-
- Couple H2S/SO42-
7Half-reaction vocabulary part II
- Anodic Reaction an oxidation reaction
- Cathodic Reaction a reduction reaction
- Relates the direction of the half reaction
- A ? A e- anodic
- B e- ? B- cathodic
8ELECTRON ACTIVITY
- Although no free electrons exist in solution, it
is useful to define a quantity called the
electron activity - The pe indicates the tendency of a solution to
donate or accept a proton. - If pe is low, there is a strong tendency for the
solution to donate protons - the solution is
reducing. - If pe is high, there is a strong tendency for the
solution to accept protons - the solution is
oxidizing.
9THE pe OF A HALF REACTION - I
- Consider the half reaction
- MnO2(s) 4H 2e- ? Mn2 2H2O(l)
- The equilibrium constant is
- Solving for the electron activity
10WE NEED A REFERENCE POINT!
- Values of pe are meaningless without a point of
reference with which to compare. Such a point is
provided by the following reaction - ½H2(g) ? H e-
- By convention
- so K 1.
11THE STANDARD HYDROGEN ELECTRODE
- If a cell were set up in the laboratory based on
the half reaction - ½H2(g) ? H e-
- and the conditions a H 1 (pH 0) and p H2
1, it would be called the standard hydrogen
electrode (SHE). - If conditions are constant in the SHE, no
reaction occurs, but if we connect it to another
cell containing a different solution, electrons
may flow and a reaction may occur.
12STANDARD HYDROGEN ELECTRODE
½H2(g) ? H e-
13ELECTROCHEMICAL CELL
Fe3 e- ? Fe2
½H2(g) ? H e-
14ELECTROCHEMICAL CELL
- We can calculate the pe of the cell on the right
with respect to SHE using - If the activities of both iron species are equal,
pe 12.8. If a Fe2/a Fe3 0.05, then - The electrochemical cell shown gives us a method
of measuring the redox potential of an unknown
solution vs. SHE.
15DEFINITION OF Eh
- Eh - the potential of a solution relative to the
SHE. - Both pe and Eh measure essentially the same
thing. They may be converted via the
relationship - Where ? 96.42 kJ volt-1 eq-1 (Faradays
constant). - At 25C, this becomes
- or
16Free Energy and Electropotential
- Talked about electropotential (aka emf, Eh) ?
driving force for e- transfer - How does this relate to driving force for any
reaction defined by DGr ?? - DGr - n?E
- Where n is the of e-s in the rxn, ? is
Faradays constant (23.06 cal V-1), and E is
electropotential (V) - pe for an electron transfer between a redox
couple analagous to pK between conjugate
acid-base pair
17Nernst Equation
- Consider the half reaction
- NO3- 10H 8e- ? NH4 3H2O(l)
- We can calculate the Eh if the activities of H,
NO3-, and NH4 are known. The general Nernst
equation is - The Nernst equation for this reaction at 25C is
18Eh Measurement and meaning
- Eh is the driving force for a redox reaction
- No exposed live wires in natural systems
(usually) ? where does Eh come from? - From Nernst ? redox couples exist at some Eh
(Fe2/Fe31, Eh 0.77V) - When two redox species (like Fe2 and O2) come
together, they should react towards equilibrium - Total Eh of a solution is measure of that
equilibrium
19FIELD APPARATUS FOR Eh MEASUREMENTS
20CALIBRATION OF ELECTRODES
- The indicator electrode is usually platinum.
- In practice, the SHE is not a convenient field
reference electrode. - More convenient reference electrodes include
saturated calomel (SCE - mercury in mercurous
chloride solution) or silver-silver chloride
electrodes. - A standard solution is employed to calibrate the
electrode. - Zobells solution - solution of potassium
ferric-ferro cyanide of known Eh.
21CONVERTING ELECTRODE READING TO Eh
- Once a stable potential has been obtained, the
reading can be converted to Eh using the equation - Ehsys Eobs EhZobell - EhZobell-observed
- Ehsys the Eh of the water sample.
- Eobs the measured potential of the water sample
relative to the reference electrode. - EhZobell the theoretical Eh of the Zobell
solution - EhZobell 0.428 - 0.0022 (t - 25)
- EhZobell-observed the measured potential of the
Zobell solution relative to the reference
electrode.
22PROBLEMS WITH Eh MEASUREMENTS
- Natural waters contain many redox couples NOT at
equilibrium it is not always clear to which
couple (if any) the Eh electrode is responding. - Eh values calculated from redox couples often do
not correlate with each other or directly
measured Eh values. - Eh can change during sampling and measurement if
caution is not exercised. - Electrode material (Pt usually used, others also
used) - Many species are not electroactive (do NOT react
electrode) - Many species of O, N, C, As, Se, and S are not
electroactive at Pt - electrode can become poisoned by sulfide, etc.
23Figure 5-6 from Kehew (2001). Plot of Eh values
computed from the Nernst equation vs.
field-measured Eh values.
24Other methods of determining the redox state of
natural systems
- For some, we can directly measure the redox
couple (such as Fe2 and Fe3) - Techniques to directly measure redox SPECIES
- Amperometry (ion specific electrodes)
- Voltammetry
- Chromatography
- Spectrophotometry/ colorimetry
- EPR, NMR
- Synchrotron based XANES, EXAFS, etc.
25REDOX CLASSIFICATION OF NATURAL WATERS
- Oxic waters - waters that contain measurable
dissolved oxygen. - Suboxic waters - waters that lack measurable
oxygen or sulfide, but do contain significant
dissolved iron (gt 0.1 mg L-1). - Reducing waters (anoxic) - waters that contain
both dissolved iron and sulfide.
26Redox titrations
- Imagine an oxic water being reduced to become an
anoxic water - We can change the Eh of a solution by adding
reductant or oxidant just like we can change pH
by adding an acid or base - Just as pK determined which conjugate acid-base
pair would buffer pH, pe determines what redox
pair will buffer Eh (and thus be reduced/oxidized
themselves)
27Redox titration II
- Lets modify a bjerrum plot to reflect pe changes
28The Redox ladder
O2
Oxic
H2O
NO3-
N2
MnO2
Post - oxic
Mn2
Fe(OH)3
Fe2
SO42-
Sulfidic
H2S
CO2
CH4
H2O
Methanic
H2
The redox-couples are shown on each stair-step,
where the most energy is gained at the top step
and the least at the bottom step. (Gibbs free
energy becomes more positive going down the
steps)