Limnology: Lecture 10
Chemical Limnology: Redox
Limnology: Lecture 10
I. Oxidation and Reduction
A. Originally "oxidation" referred to reactions that involved the reaction of oxygen with another
compound
1. "Reduction" was the removal of oxygen
2. Now the definition of oxidation and reduction encompasses all elements, not just oxygen,
and involves electron loss and gain.
B. Oxidation is the process of the loss of electrons from an atom or ion
1. Oxidation results in the increase of the oxidation number of an atom or ion
2. Oxidizing agents are substances which increase the oxidation number of another atom or ion
(electron acceptors)
C. Reduction is the process of the gain of an electron by an atom or ion
1. Reduction results in the decrease of the oxidation number of an atom or ion
2. Reducing agents are substances which decrease the oxidation number of another atom or
ion (electron donors)
D. Example:
Fe++
Fe+++ + eFe++ is the reduced form (ferrous iron)
Fe+++ is the oxidized form (ferric iron)
Limnology: Lecture 10
II. Redox Potential
A.
In any given reduction reaction there will be a flow of
electrons (See Figure 10.1)
1. If an electrical current (a flow of electrons) is applied in
the opposite direction, the strength of the electron
activity or the flow of electrons in the reduction reaction
can be measured. The potential is measured at the
point where there is no net flow of electrons.
2. Redox potential is proportional to the equivalent free
energy change per mole of electrons associated with a
given reduction
a. Just as we measure the activity of the hydrogen
ion by pH, pH = ‑log(H+), we describe the activity
of the electrons as pE (redox potential) pE =
‑log(e-)
b. pE is large and positive in strongly oxidizing
solutions (low electron activity)
c. Redox reactions are standardized against the
redox reaction of hydrogen: H2
2H+ + 2ei. A hydrogen electrode is created by
bubbling H2 gas at one atmosphere of
pressure over a platinum plate or wire.
(See Figure 10.2)
ii. The redox potential standardized against
the hydrogen electrode is termed the Eh
Limnology: Lecture 10
II. Redox Potential
3.
4.
5.
The Eh of natural waters ranges from about 400 ‑ 600 mv (millivolts) depending upon the pH of
the water
a. The Eh of distilled water at pH 7 should be ca. 800 mv
b. Eh is relatively insensitive to changes in oxygen concentration except under anoxic
conditions: Upon total depletion of dissolved oxygen, Eh decreases greatly (0 ‑ 200 mv)
c. Eh is sensitive to changes in pH
i. A decrease of one pH unit is accompanied by an increase in Eh of 58 mv
ii. Eh is frequently adjusted to reflect the redox potential at neutrality
a) add 58 mv for every pH unit on the basic side of neutrality
b) subtract 58 mv for every pH unit on the acid side of neutrality
Vertical profiles of Eh in lakes and reservoirs
a. In well‑oxygenated waters, there is little change in Eh with depth
b. In anoxic waters of the hypolimnia of eutrophic lakes, Eh drops precipitously (to
approximately 0 ‑ 200 mv; Eh may even be negative in the sediments)
Eh = Eo + (RT / nF) [(oxidized state) / (reduced state)] where:
a. R = gas constant (.001987 Kcal/deg)
b. F = Faraday constant (23.06 Kcal/volt-gram eq)
c. T = absolute temperature (oK)
d. n = number of electrons involved
e. Eo = voltage of reaction when all substances involved are at unit activity
Limnology: Lecture 10