Direct Energy, 2018a

208 9.2 Measures of the Ability of Charges to Flow
bond per unit charge. It is more often used by experimentalists than theorists,
and it is often used to describe solids instead of liquids. Redox
potential is a macroscopic property, describing a larger piece of material as
opposed to describing just an individual atom. It is also sometimes called
oxidation reduction potential or the standard electrode potential [137]. It
is a relative measure of the ability of a substance to lose an electron. A
list of redox potentials can be found in references [60, p. 158] and [137].
There are dierent ways of dening redox potential in the literature. The
denitions vary in their choice of a ground reference voltage, and they vary
in their sign conventions. American and European researchers tend to use
dierent denitions.
Redox potential is measured on a relative scale. To measure redox
potential [138], electrodes are put in the system being studied. A potential
is applied to balance the internal voltage. By measuring this externally
applied voltage, the potential of an electrode is determined with respect
to a reference electrode. Often, the potential of a platinum electrode is
used as a reference and said to have zero volts at standard conditions of
T =25 0 C and P =1atm. The reaction at the platinum electrode is
H 2 → 2H + +2e − . (9.7)
9.2.6 pH
pH is a unitless measure of the likelihood that a water molecule is bonded
or has been ionized in a liquid solution. It is used to classify liquids as
acidic or basic. When discussing pH, we assume the material under test is
a liquid solution at a temperature of 25 ◦ C and a pressure of 1 atm [12]
[81]. A liquid solution is a mixture of water and another material called a
solute. More specically, pH is dened as
( )
1
pH =log 10
[ ] H
+
. (9.8)
The quantity [ H +] is the amount concentration of hydrogen ions in the
units of mol
L
[68, p. 39].
[ ] H
+
= concentration H+ ions, mol
(9.9)
volume of solution in L
This quantity was formerly called molarity or molar concentration, but
these terms are no longer recommended for use [68, p. 39]. pH is a measure
often used by experimentalists.