Download - REDOX CLASSIFICATION OF NATURAL WATERS
REDOX 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 (> ~0.1 mg L-1).
Anoxic waters - waters that contain both dissolved iron and sulfide.
DEFINITION OF EhEh - 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 (Faraday’s constant).At 25°C, this becomes
or
EhRT
pe303.2
Ehpe 9.16
peEh 059.0
Eh – 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+/Fe3+=1, 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
FIELD APPARATUS FOR Eh MEASUREMENTS
PROBLEMS 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 at electrode)
• Many species of O, N, C, As, Se, and S are not electroactive at Pt
– electrode can become poisoned by sulfide, etc.
Figure 5-6 from Kehew (2001). Plot of Eh values computed from the Nernst equation vs. field-measured Eh values.
Other 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.
Free 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 Gr ??
Gr = nE or G0r = nE0
– Where n is the # of e-’s in the rxn, is Faraday’s 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
Electromotive Series• When we put two redox species together, they will
react towards equilibrium, i.e., e- will move which ones move electrons from others better is the electromotive series
• Measurement of this is through the electropotential for half-reactions of any redox couple (like Fe2+ and Fe3+)– Because Gr = nE, combining two half reactions in a
certain way will yield either a + or – electropotential (additive, remember to switch sign when reversing a rxn)-E - Gr, therefore spontaneous
• In order of decreasing strength as a reducing agent strong reducing agents are better e- donors
Reaction directions for 2 different redox couples brought together?? More negative potential reductant // More positive potential oxidant Example – O2/H2O vs. Fe3+/Fe2+ O2 oxidizes Fe2+ is spontaneous!
Biology’s view upside down?
Nernst 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 25°C is
QnRTEEh log303.20
100
3
4log8
0592.0
HNO
NH
aa
aEEh
Let’s assume that the concentrations of NO3- and
NH4+ have been measured to be 10-5 M and
310-7 M, respectively, and pH = 5. What are the Eh and pe of this water?
First, we must make use of the relationship
For the reaction of interest rG° = 3(-237.1) + (-79.4) - (-110.8)
= -679.9 kJ mol-1
nGEor0
volts88.0)42.96)(8(
9.6790 E
The Nernst equation now becomes
substituting the known concentrations (neglecting activity coefficients)
and
10
3
4log8
0592.088.0HNO
NH
aa
aEh
volts521.01010103log
80592.088.0 1055
7
Eh
81.8)521.0(9.169.16 Ehpe
Stability Limits of Water• H2O 2 H+ + ½ O2(g) + 2e-
Using the Nernst Equation:
• Must assign 1 value to plot in x-y space (PO2)• Then define a line in pH – Eh space
20
21
2
1log0592.0
HO apn
EEh
UPPER STABILITY LIMIT OF WATER (Eh-pH)
To determine the upper limit on an Eh-pH diagram, we start with the same reaction
1/2O2(g) + 2e- + 2H+ H2O
but now we employ the Nernst eq.
20
21
2
1log0592.0
HO apn
EEh
20
21
2
1log2
0592.0
HO ap
EEh
As for the pe-pH diagram, we assume that pO2
= 1 atm. This results in
This yields a line with slope of -0.0592.
221
2log0296.023.1
HO apEh
pHpEh O 0592.0log0148.023.12
volts23.1)42.96)(2()1.237(0
0
nGE r
pHEh 0592.023.1
LOWER STABILITY LIMIT OF WATER (Eh-pH)
Starting withH+ + e- 1/2H2(g)
we write the Nernst equation
We set pH2 = 1 atm. Also, Gr° = 0, so E0 =
0. Thus, we have
pHEh 0592.0
H
H
ap
EEh2
1
2log1
0592.00
O2/H2O
C2HO
Redox 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)
Making stability diagrams
• For any reaction we wish to consider, we can write a mass action equation for that reaction
• We make 2-axis diagrams to represent how several reactions change with respect to 2 variables (the axes)
• Common examples: Eh-pH, PO2-pH, T-[x], [x]-[y], [x]/[y]-[z], etc
Construction of these diagrams
• For selected reactions:Fe2+ + 2 H2O FeOOH + e- + 3 H+
How would we describe this reaction on a 2-D diagram? What would we need to define or assume?
2
30 log
10592.0
Fe
H
aa
EEh
• How about:• Fe3+ + 2 H2O FeOOH(ferrihydrite) + 3 H+
Ksp=[H+]3/[Fe3+]
log K=3 pH – log[Fe3+]
How would one put this on an Eh-pH diagram, could it go into any other type of diagram (what other factors affect this equilibrium description???)
Redox 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)
Redox titration II
• Let’s modify a bjerrum plot to reflect pe changes
Greg Mon Oct 25 2004
-4 -2 0 2 4 6 8 10 1250
60
70
80
90
100
pe
Som
e sp
ecie
s w
/ S
O4-- (
umol
al) H2S(aq) SO4
--
Redox titrations in complex solutions
• For redox couples not directly related, there is a ladder of changing activity
• Couple with highest + potential reduced first, oxidized last
• Thermodynamics drives this!!
The Redox ladder
H2O
H2
O2
H2ONO3
-
N2 MnO2
Mn2+
Fe(OH)3
Fe2+SO4
2-
H2S CO2
CH4
Oxic
Post - oxic
Sulfidic
Methanic
Aerobes
Dinitrofiers
Maganese reducers
Sulfate reducers
Methanogens
Iron reducers
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. (Gibb’s free energy becomes more positive going down the steps)