Review of Oxidation/Reduction, Oxidation/Reduction in the Natural Environment

We want to enter an examination of the effect of oxidizing media on species in water.

Oxidation state is important

Hg, HgCH₃⁺, and Hg(CH₃)₂

Methyl Mercury (both forms) is much more toxic than mercury metal;

Chromium VI is more toxic than other forms of chromium (i.e. Cr³⁺).

Philosophy: Thermodynamics vs. Kinetics

Remember: Oxidation states

Hydrogen is always +1

Oxygen is always -2

Alkali +1, Alkane earths +2

Element in its natural state is defined to equal zero

Look at Nitrogen, Sulfur, Carbon

N₂, NH₃, NO₃^-, NO₂, NO, NO₂^-, H₂S, S,

SO₂, SO₃^2-, SO₄^2-, CH_4,, CH₂O, CO, CO₂

Reduction is defined as the gain of electrons

Oxidation is defined as the loss of electrons

The hydrogen is defined to be zero and everything is relative to this standard.

The more positive the potential, the greater the tendency for the reaction to proceed as written.

G = -nFE

G = -2.303 nRT pE

Because we do not know "all" of the species present in any particular sample of natural water, we can not write the balanced redox reaction that should occur.

We define a new quantity called:

pE = -log a_e-

Where A_e- is the activity of an electron

Remember the Nerst Equation

Low pE is reducing

Figure 12.1. Stable Forms of Nitrogen in the Natural Environment

pE diagrams tell us the most thermodynamic stable form in certain water environments.

pE of water in equilibrium with the air = + 13.58

Many times the form of a species in water is a function of both pH and pE

These ideas can be incorporated into one concise diagram -pE - pH

Oxidizing limit for water pE = 20.75 - pH

Reducing limit for water pE = 0.0 - pH

The common forms of nitrogen in the aqueous environment are:

The transformations between these nitrogen oxidation states are goverened by the following equations:

Use this data to construct a pE/pH Diagram for Nitrogen in water:

Figure 12.2. pE/pH Diagram for Nitrogen (N-O-H System)

What about Carbon?

Figure 12.3. Diagram for Nitrogen, Sulfur and Carbon

ENV 440 - Course Topics