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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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.
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
Figure 5-6 from Kehew (2001). Plot of Eh values computed from the Nernst equation vs. field-measured Eh values.
DGr = nDE or DG0r = nDE0
-E - DGr, therefore spontaneous
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!
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
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
substituting the known concentrations (neglecting activity coefficients)
Using the Nernst Equation:
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.
This yields a line with slope of -0.0592.
H+ + e- 1/2H2(g)
we write the Nernst equation
We set pH2 = 1 atm. Also, Gr° = 0, so E0 = 0. Thus, we have
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?
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???)
Post - oxic
H2The 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)