Civil Engineering Reference
In-Depth Information
Mineral acidity.
The initial solution pH is below the H
2
CO
3
equivalence point.
The equivalents of base required to raise the pH to the H
2
CO
3
equivalence point
(pH
4.5) are a measure of the mineral acidity.
•
CO
2
acidity.
The initial solution pH lies below the HCO
equivalence point. The
3
•
equivalents of base required to raise the pH to the HCO
equivalence (pH
8.3)
3
are a measure of the CO
2
acidity.
Total acidity.
The initial solution pH lies below the CO
equivalence point.
2
•
3
The equivalents of base required to raise the pH to the CO
equivalence point
2
3
(pH
10) are a measure of the total acidity.
SOLUBILITY
Gases, liquids, and solids dissolve in water. The amount of solute present may vary
continuously below a certain limit, called the
solubility limit.
Solubility is the concen-
tration of solute present when the solution is in a state of equilibrium with an excess
of the pure solute, and is a fixed number for a given temperature, pressure, and solvent.
This equilibrium and its application are important in water engineering.
Liquids and Solids
In general, the solubility of solids and liquids is highly dependent on temperature but
only slightly on pressure. In most water engineering situations, solubility may be con-
sidered as a function of temperature alone. Exceptions are situations where extreme
pressures exist, such as deep underground aquifers, great ocean depths, or certain
industrial applications. The solubilities of most inorganic salts increase with temper-
ature, but a number of calcium compounds, such as CaCO
3
, CaSO
4
, and Ca(OH)
2
,
decrease in solubility with increasing temperature. The activity of a solid in regard to
solubility is constant at a given temperature, and this activity is stated mathematically
by the
solubility product
(
K
sp
), which is expressed as
K
sp
[A
][B
], where [A
][B
]
represents the product of the molar concentrations of the ions making up the substance.
The solute concentrations (i.e., [A
]) are usually expressed in moles per liter:
milligrams per liter
Molarity
3
gram-molecular weight
10
Tables 5-5 and 5-6 list typical
K
sp
constants of interest in water engineering. In an
unsaturated solution, the ion product ([A
][B
]) is less than
K
sp
. If the ion product is
greater than
K
sp
, the solution is supersaturated and will tend to form a precipitate.
Other ions present can affect the solubility of a substance through either the
common-ion effect
or the
secondary salt effect
. For example, in a solution containing
100 ppm of carbonate alkalinity, CaCO
3
has a solubility of only 0.5 ppm, although its
solubility in pure water is about 13 ppm. This repression of solubility in the presence
of an excess of one of the ions concerned in the solubility expression is known as the
common-ion effect.
The solubility of slightly soluble salts is increased when other salts
that do not have an ion in common with the lightly soluble substance are present. The
increased ionic strength of the solution resulting from the foreign salt causes a decrease
in the activity coefficients of the slightly soluble substance. For example, calcium
