Page 72 - Fundamentals of Geomorphology

GEOMORPHIC MATERIALS AND PROCESSES 55

Box 3.2

pH AND Eh

pH is a measure of the acidity or alkalinity of aque-
ous solutions. The term stands for the concentration
of hydrogen ions in a solution, with the p stand-
ing for Potenz (the German word for ‘power’). It is
expressed as a logarithmic scale of numbers rang-
ing from about 0 to 14 (Figure 3.2). Formulaically,
pH =− log[H ], where [H ] is the hydrogen ion
+
+
concentration (in gram-equivalents per litre) in an
aqueous solution. A pH of 14 corresponds to a hydro-
gen ion concentration of 10 −14 gram-equivalents per
litre. A pH of 7, which is neutral (neither acid nor alka-
line), corresponds to a hydrogen ion concentration of
10 −7 gram-equivalents per litre. A pH of 0 corresponds
to a hydrogen ion concentration of 10 −0 (= 1) gram-
equivalents per litre. A solution with a pH greater than
7 is said to be alkaline, whereas a solution with a pH
less than 7 is said to be acidic (Figure 3.2). In weather-
ing, any precipitation with a pH below 5.6 is deemed
to be acidic and referred to as ‘acid rain’.
The solubility of minerals also depends upon the Eh
or redox (reduction–oxidation) potential of a solu-
tion. The redox potential measures the oxidizing or
reducing characteristics of a solution. More specifically,
it measures the ability of a solution to supply electrons
to an oxidizing agent, or to take up electrons from a
reducing agent. So redox potentials are electrical poten-
tialsorvoltages.Solutionsmayhavepositiveornegative
redox potentials, with values ranging from about −0.6
volts to +1.4 volts. High Eh values correspond to oxi-
dizing conditions, while low Eh values correspond to
reducing conditions.
Combined, pH and Eh determine the solubility
of clay minerals and other weathering products. For
example, goethite, a hydrous iron oxide, forms where
Eh is relatively high and pH is medium. Under high
oxidizing conditions (Eh > +100 millivolts) and a Figure 3.2 The pH scale, with the pH of assorted
moderate pH, it slowly changes to hematite. substances shown.