Introduction to Soil Chemistry

potentiometry (electrodes in soil measurements) 103
oxidizing, electron accepting, agents. Likewise, there are a number of reducing,
electron donating agents; chief among these is hydrogen. Generally speaking,
in soil reduced species are more soluble and more easily removed from
the soil. For this reason the oxidation–reduction condition of a soil sample can
be important in any extraction or analytical procedure.
In accessing the redox potential it is also essential to know the pH of the
medium being analyzed. There is a direct relationship between pH and the
potential measured:
E =-59.16mV ¥ pH
For each unit of increase in pH (decrease in H + ), there is a decrease in millivoltage.
In soil, this simple equation does not hold because it does not take
into account the complex electrical characteristics of soil and thus cannot be
used in soil analysis. However, it does illustrate the fact that any measurement
or consideration of the oxidation–reduction situation in soil must also take
into account the soil’s pH.
In soil stable redox reactions occur between the limits of the oxidation and
reduction of water as shown in Figure 5.3. The y axis (ordinate) shows millivolts
(mV); the x axis (ordinate), pH. Chemists and physical chemists will use
these two terms in describing what happens in a redox reaction. Environmental
and soil chemists will refer to the measured voltage [expressed in millivolts
(mV)] as Eh; thus these types of graphs are called Eh–pH diagrams. Eh
is defined by the Nernst equation [equation (5.3a)], which can be simplified to
equation (5.3b) when the activities of (Red) and (Ox) are equal. Many redox
couples have the relationship shown in equation (5.3c), and thus their reactions
as electron acceptors or donors is straightforward (however, in soil significant
variations from this simple relationship exist where m π n):
0
( d)
Eh = Eh -
+ ( Ox) ( H )
RT Re
ln
0 m
Eh = Eh - 0 059pH
n .
m
n
+ H
= = 1 -
e
nF n m
(5.3a)
(5.3b)
(5.3c)
where Eh 0 = standard electrode potential
R = gas constant
T = absolute temperature
n = number of electrons
F = Faraday constant
(Red),(Ox),(H + ) = concentrations of reduced, oxidized species, and hydrogen
ion, respectively