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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 ).

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redox classification of natural waters
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
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 – 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
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.
slide6

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
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
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 = nDE or DG0r = nDE0

    • 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
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 = nDE, 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
slide10

Biology’s view  upside down?

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
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

slide12
Let’s assume that the concentrations of NO3- and NH4+ have been measured to be 10-5 M and 310-7 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

slide13
The Nernst equation now becomes

substituting the known concentrations (neglecting activity coefficients)

and

stability limits of water
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
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.

slide16
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
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

slide19

O2/H2O

C2HO

redox titrations
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
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
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?

slide23
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 titrations1
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
Redox titration II
  • Let’s modify a bjerrum plot to reflect pe changes
redox titrations in complex solutions
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

O2

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 ladder

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)

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