The Inorganic Carbon Complex
115
where bicarbonate buffering is exhausted. Nonlinearity in this region of the plot
indicates either a malfunctioning electrode or the presence of noncarbonate buffers
(e.g., organic acids).
Typically, the volume of acid required to reach the equivalence point is determined
by extrapolation of the linear portion of the plot (between pH values of 4 and 3.5 or
less). Alkalinity is computed as the microequivalents of acid added at the equivalence
point divided by the sample volume in liters and reported as Ileqjl:
Alk, Ileq/l = (0.1N)(0.OO01251)(106Ileqjl/N)/0.050 I
= 250 Ileq/l
Alternately, a plot of the pH values at each step in the titration versus the titrant
volume can be used to estimate the equivalence point from the point where the change
in the pH curve is most rapid. For noncritical analyses the pH of the equivalence point
can be assumed, and then the sample is titrated to that point with the aid of a pH meter.
Historically, aquatic chemists used a somewhat different method of measuring and
reporting alkalinity. With this method a pH indicator was used to indicate the
equivalence end point pH. Typically, the "faintest pink" color of the methyl orange
indicator was used [pH of approximately 4.25; Asbury et al. (1989)]. This method has
now been replaced with a mixed bromcresol green methyl red indicator that has a
sharper and less ambiguous end point. Terms such as total alkalinity have been used to
describe alkalinity measured with color indicators, and the units are expressed
frequently in terms of mg CaC0 3 /l, because this was the common chemical referred to
in the treatment of water and waste waters. The following equation may be used to
convert between the two sets of alkalinity units:
(8)
Note that this calculation does not compensate for the overtitration bias caused by the
lower end point of the methyl orange indicator as compared to the Gran procedure.
For many soft water lakes, subtracting 551leqjl from the methyl orange alkalinity value
will make them approximately comparable to Gran alkalinities (Asbury et al., 1989).
The term phenolphthalein alkalinity is used to express the portion of alkalinity
contributed by the hydroxide and carbonate ions. The pH indicator phenolphthalein
turns from pink to clear as the pH is lowered to 8.3, where essentially all of the
carbonate is converted to bicarbonate (Figs. 8.2 and 8.3).
ACIDITY AND CARBON DIOXIDE
Acidity is, in a sense, the opposite of alkalinity. It is the total amount of acids that can be
titrated with a strong base. Acidity commonly is not measured in limnological
investigations, although acidity titrations sometimes are used to estimate the free CO2
in the solution, as CO 2 is used more readily by phytoplankton than either HC0 3 - or
C0 3 2 -, and it can become toxic to aquatic organisms if present in high concentrations.
The amount of carbon dioxide (actually, CO 2 acidity) can be determined by adding
phenolphthalein to a sample of water and titrating with a standard basic solution (free
of CO 2 ) until the pink color just appears (pH approximately 8.3). There are numerous
difficulties with this assay, and, in general, the results are not very accurate. The best
methods employ sparging and analysis by gas chromatography (Schindler et al., 1972),
1973; Stainton 1973) or infrared CO 2 analysis (see Chapter 9). Older titrimetric
methods are accurate when done carefully, but are tedious [e.g., Nygaard (1965)].
115
where bicarbonate buffering is exhausted. Nonlinearity in this region of the plot
indicates either a malfunctioning electrode or the presence of noncarbonate buffers
(e.g., organic acids).
Typically, the volume of acid required to reach the equivalence point is determined
by extrapolation of the linear portion of the plot (between pH values of 4 and 3.5 or
less). Alkalinity is computed as the microequivalents of acid added at the equivalence
point divided by the sample volume in liters and reported as Ileqjl:
Alk, Ileq/l = (0.1N)(0.OO01251)(106Ileqjl/N)/0.050 I
= 250 Ileq/l
Alternately, a plot of the pH values at each step in the titration versus the titrant
volume can be used to estimate the equivalence point from the point where the change
in the pH curve is most rapid. For noncritical analyses the pH of the equivalence point
can be assumed, and then the sample is titrated to that point with the aid of a pH meter.
Historically, aquatic chemists used a somewhat different method of measuring and
reporting alkalinity. With this method a pH indicator was used to indicate the
equivalence end point pH. Typically, the "faintest pink" color of the methyl orange
indicator was used [pH of approximately 4.25; Asbury et al. (1989)]. This method has
now been replaced with a mixed bromcresol green methyl red indicator that has a
sharper and less ambiguous end point. Terms such as total alkalinity have been used to
describe alkalinity measured with color indicators, and the units are expressed
frequently in terms of mg CaC0 3 /l, because this was the common chemical referred to
in the treatment of water and waste waters. The following equation may be used to
convert between the two sets of alkalinity units:
(8)
Note that this calculation does not compensate for the overtitration bias caused by the
lower end point of the methyl orange indicator as compared to the Gran procedure.
For many soft water lakes, subtracting 551leqjl from the methyl orange alkalinity value
will make them approximately comparable to Gran alkalinities (Asbury et al., 1989).
The term phenolphthalein alkalinity is used to express the portion of alkalinity
contributed by the hydroxide and carbonate ions. The pH indicator phenolphthalein
turns from pink to clear as the pH is lowered to 8.3, where essentially all of the
carbonate is converted to bicarbonate (Figs. 8.2 and 8.3).
ACIDITY AND CARBON DIOXIDE
Acidity is, in a sense, the opposite of alkalinity. It is the total amount of acids that can be
titrated with a strong base. Acidity commonly is not measured in limnological
investigations, although acidity titrations sometimes are used to estimate the free CO2
in the solution, as CO 2 is used more readily by phytoplankton than either HC0 3 - or
C0 3 2 -, and it can become toxic to aquatic organisms if present in high concentrations.
The amount of carbon dioxide (actually, CO 2 acidity) can be determined by adding
phenolphthalein to a sample of water and titrating with a standard basic solution (free
of CO 2 ) until the pink color just appears (pH approximately 8.3). There are numerous
difficulties with this assay, and, in general, the results are not very accurate. The best
methods employ sparging and analysis by gas chromatography (Schindler et al., 1972),
1973; Stainton 1973) or infrared CO 2 analysis (see Chapter 9). Older titrimetric
methods are accurate when done carefully, but are tedious [e.g., Nygaard (1965)].