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REDOX CLASSIFICATION OF NATURAL WATERS

Oxicwaters - 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 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 (Faraday’s constant).

At 25°C, this becomes

or

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

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 DGr ??

DGr = nDE or DG0r = nDE0

- 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 DGr = nDE, combining two half reactions in a certain way will yield either a + or – electropotential (additive, remember to switch sign when reversing a rxn)

-E - DGr, 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!

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

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

The Nernst equation now becomes

substituting the known concentrations (neglecting activity coefficients)

and

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

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.

As for the pe-pH diagram, we assume that pO2 = 1 atm. This results in

This yields a line with slope of -0.0592.

LOWER STABILITY LIMIT OF WATER (Eh-pH)

Starting with

H+ + e- 1/2H2(g)

we write the Nernst equation

We set pH2 = 1 atm. Also, Gr° = 0, so E0 = 0. Thus, we have

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?

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

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!!

Aerobes

Oxic

H2O

Dinitrofiers

NO3-

N2

Maganese reducers

Post - oxic

MnO2

Mn2+

Iron reducers

Fe(OH)3

Fe2+

SO42-

Sulfate reducers

Sulfidic

H2S

CO2

Methanogens

CH4

Methanic

H2O

H2

The Redox ladderThe 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)

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