Page 30 - Soil and water contamination, 2nd edition

Basic environmental chemistry 17

Table 2.2 Additional parameters related to concentrations in soil and water (after Appelo and Postma, 2005).
Parameter Unit Description
+
+
pH pH units [-] Logarithm of the H activity; -log[H ]
TDS mg l -1 Total dissolved solids;
EC µS cm -1 Electrical conductivity of water; as the EC is temperature dependent, it
is commonly reported as the EC at 25°C
Eh Volt Redox potential (see Section 2.10.3)
-
-
pe - Redox potential expressed as – log[e ], where [e ] = the activity of
electrons; pe = Eh/0.059 at 25 °C (see Section 2.10.4)
The pH is defined as the reciprocal of the logarithm of the ‘effective concentration’
+
(activity ; see Section 2.3) of hydrogen ions (H ). Water with a pH < 7 is considered acidic
and with a pH > 7 is considered basic. The pH of pure water (H 0) is 7 at 25 °C, but when
2
exposed to the carbon dioxide in the atmosphere, the pH decreases to approximately 5.2.
The normal range for pH in surface water is 6.5 to 8.5; for groundwater it is 6 to 8.5.
Section 2.9 goes into more detail about the pH, acids, and bases.
The total dissolved solids (TDS) represents the total concentration of dissolved minerals
in the water and is determined by evaporating a known volume of a filtered sample in an
oven heated to 105 °C. The electrical conductivity (EC ) is a measure of the water’s ability
to conduct electricity, and therefore a measure of the water’s ionic concentration. The
major dissolved phase constituents (see Chapter 3) contribute most to the EC; the larger
their concentration, the higher the conductivity. Therefore, the EC is generally found to be
a good measure of the concentration of total dissolved solids (TDS) and salinity. The EC
-1
-1
expressed in μS cm ranges from 1.0 to 2.0 times the TDS concentration in mg l and can
be estimated more accurately using:
EC ≈ 100 × meq (cations or anions) l -1 (2.1)

The EC increases substantially with temperature , which can have a confounding effect on
attempts to compare the EC across different waters or different seasons. To eliminate this
complication and allow comparisons to be made, the EC is normalised to a temperature of
25 °C.
The redox potential is defined as the ability of a system or species to consume or donate
electrons; it is measured in volts or millivolts. The more negative the redox potential is
relative to the standard potential, the greater a system’s tendency to lose electrons and the
greater that system’s reducing potential. Conversely, a more positive redox potential indicates
an oxidising environment, which implies a greater affinity for electrons and less tendency to
donate electrons. The concept of redox potential is further explained in Section 2.10.

2.3 ACTIVITY
Because the dissolved ions in an aqueous solution influence each other through the
electrostatic forces between them, the concentration of the dissolved ions is not the correct
measure of their reactivity . Therefore, instead of the total concentrations, an ‘apparent’
concentration should be used for calculations of chemical reactions in aqueous solutions.
This apparent concentration, called the activity , is corrected for the non-ideal effects in
aqueous solutions arising from the electrostatic forces between all dissolved ions in the water.
It is related to the molar concentration:

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