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

Qn

RTEEh log

303.20

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

n

GE

or0

volts88.0)42.96)(8(

9.6790 E

The Nernst equation now becomes

substituting the known concentrations (neglecting activity coefficients)

and

10

3

4log8

0592.088.0

HNO

NH

aa

aEh

volts521.01010

103log

8

0592.088.0 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

1log

0592.0

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

1log

0592.0

HO apn

EEh

20

21

2

1log

2

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(00

n

GE r

pHEh 0592.023.1

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

pHEh 0592.0

H

H

a

pEEh

21

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

1

0592.0

Fe

H

a

aEEh

• 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

So

me

sp

eci

es

w/

SO

4-- (

um

ola

l) 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

H2O

NO3-

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)