Inorganic Nutrients
95
the American Public Health Association et al. (1989)]. The most reliable methods are
thought to be turbidimetric, in which the sulfate ion is precipitated in acidic solution
with barium chloride [e.g., TabaUibai (1974)]. The barium sulfate crystals then are
stabilized in suspension. The absorbance of the suspension then is measured
spectrophotometrically, and the S04 concentration is determined by comparison with
known concentrations. At concentrations less than about 2 mg SO 4/1, the turbidimetric
method is highly unreliable, and other procedures such as the Thorin method should be
used [e.g., Rainwater and Thatcher (1960, p. 279)].
Chloride
Concentrations of chloride ions in natural waters are commonly determined by the
titration of chloride with mercuric nitrate in acidic solution to an end point (violet
color) where HgCl l forms a complex with diphenylcarbazone-bromphenol blue mixed
indicator [cr., Golterman and Clymo (1969) and the American Public Health
Association et al. (1989)].
Note: See the "Automated Analyses" section below.
SPECIFIC CONDUCTANCE
In some chemistry or physics class you undoubtedly observed that a light bulb would
not light when an electrical current was passed through a circuit which had two
separate electrodes immersed in pure water; no current passed through the water.
When salt was added to the water, the electrical current passed and the bulb glowed.
The resistance of the water to electrical current was reduced by the addition of ions
of salt and the electron flow was increased. Specific conductance (conductivity) of
fresh water is based on the same principle. The purer the water (i.e., the fewer the
dissolved electrolytes in the water), the greater will be the resistance to electrical
current.
By definition, specific conductance of an electrolyte is the reciprocal of the specific
resistance of a solution and is expressed in mhos* (reciprocal of ohms). The amount of
current conducted is proportional to the concentration of ions in solution and, hence,
to both the concentration and extent of dissociation of the dissolved salts. The standard
conductance cell consists of two platinum-black electrodes 1 cm l in area and 1 cm
apart.
The temperature of the electrolyte affects ionic velocities and must be controlled
carefully. Conductance increases about 2 to 3 %rc. The international standard of25°C
is taken as the standard temperature of conductance measurements. A custom-made
water bath is very convenient for routine analyses of conductance (Fig. 7.3). When
samples are not maintained at 25°C during measurements, an appropriate factor for
temperature compensation must be applied (Table 7.1).
It has been found that the specific conductance of water in bicarbonate-dominated
lakes and streams is closely proportional to the concentrations of major cations (Ca 1 + ,
Mgl +, Na +, K +) (Rodhe, 1949; Likens and Johnson, 1968). Once the concentrations of
these cations are known for an individual body of water, changes in the specific
* Also referred to as Siemens and expressed as ~S/cm.
95
the American Public Health Association et al. (1989)]. The most reliable methods are
thought to be turbidimetric, in which the sulfate ion is precipitated in acidic solution
with barium chloride [e.g., TabaUibai (1974)]. The barium sulfate crystals then are
stabilized in suspension. The absorbance of the suspension then is measured
spectrophotometrically, and the S04 concentration is determined by comparison with
known concentrations. At concentrations less than about 2 mg SO 4/1, the turbidimetric
method is highly unreliable, and other procedures such as the Thorin method should be
used [e.g., Rainwater and Thatcher (1960, p. 279)].
Chloride
Concentrations of chloride ions in natural waters are commonly determined by the
titration of chloride with mercuric nitrate in acidic solution to an end point (violet
color) where HgCl l forms a complex with diphenylcarbazone-bromphenol blue mixed
indicator [cr., Golterman and Clymo (1969) and the American Public Health
Association et al. (1989)].
Note: See the "Automated Analyses" section below.
SPECIFIC CONDUCTANCE
In some chemistry or physics class you undoubtedly observed that a light bulb would
not light when an electrical current was passed through a circuit which had two
separate electrodes immersed in pure water; no current passed through the water.
When salt was added to the water, the electrical current passed and the bulb glowed.
The resistance of the water to electrical current was reduced by the addition of ions
of salt and the electron flow was increased. Specific conductance (conductivity) of
fresh water is based on the same principle. The purer the water (i.e., the fewer the
dissolved electrolytes in the water), the greater will be the resistance to electrical
current.
By definition, specific conductance of an electrolyte is the reciprocal of the specific
resistance of a solution and is expressed in mhos* (reciprocal of ohms). The amount of
current conducted is proportional to the concentration of ions in solution and, hence,
to both the concentration and extent of dissociation of the dissolved salts. The standard
conductance cell consists of two platinum-black electrodes 1 cm l in area and 1 cm
apart.
The temperature of the electrolyte affects ionic velocities and must be controlled
carefully. Conductance increases about 2 to 3 %rc. The international standard of25°C
is taken as the standard temperature of conductance measurements. A custom-made
water bath is very convenient for routine analyses of conductance (Fig. 7.3). When
samples are not maintained at 25°C during measurements, an appropriate factor for
temperature compensation must be applied (Table 7.1).
It has been found that the specific conductance of water in bicarbonate-dominated
lakes and streams is closely proportional to the concentrations of major cations (Ca 1 + ,
Mgl +, Na +, K +) (Rodhe, 1949; Likens and Johnson, 1968). Once the concentrations of
these cations are known for an individual body of water, changes in the specific
* Also referred to as Siemens and expressed as ~S/cm.