The Inorganic Carbon Complex
113
Since alkalinity includes HC0 3 - and C0 3 2 -, alkalinity can be predicted from DIC
and pH:
Alk = DIC[O(l + 20(2] + [OH-] - [H+]
When any compound dissolves in water it produces equal numbers of positive
(cationic) charges and negative (anionic) charges. Therefore, electroneutrality is
maintained, and this charge balance can be expressed mathematically as:
[H+] + {basic cations} = [HC0 3 -] + 2[C0 3 2 -] + [OH-] + {strong acid anions}
( 5)
where
and
{acid anions} = 2[SO/ -] + [Cl-] + [N0 3 -] + ...
Alkalinity may be thought of as an "imbalance" between acid [H+] and the weak
anions, mainly bicarbonate with some carbonate and hydroxide. Alkalinity represents
the number of protons that can be neutralized by all of the weak anions after
subtracting the protons already present and can be written as:
Alkalinity = [HC0 3 -] + 2[C0 3 2 -] + [OH-] - [H+]
(6)
If electroneutrality were to be maintained, then the following also would be true:
Alkalinity = {base cations} - {strong acid anions}
(7)
These two equations are statements about equivalents. In practice, alkalinity can be
measured by adding known amounts of strong acid until all of the weak anions have
been used, at which point the pH will decline precipitously and all acid added
subsequently will persist as protons. This point is often called "the equivalence point"
and in fresh water usually occurs at a pH between about 4.2 and 5.1, depending on the
amount of dissolved inorganic carbon present. It also is possible to calculate the
alkalinity by mea suing all of the base cations and strong acid anions and subtracting
the moles of charges. Note that in acidic waters it is possible to have negative
alkalinities.
The terms alkalinity, carbonate alkalinity, alkalinity reserve, titratable base, and acid
neutralizing capacity (ANC) have been used commonly to express the total quantity of
bases that can be titrated with a standard solution of a strong acid (e.g., 0.1 N H 2 S0 4 ).
Currently, several methods of measuring alkalinity are used, depending on conditions
and on the degree of accuracy required. Since alkalinity is not affected by the exchange
of CO 2 with the atmosphere, sampling procedures for most surface waters are
straightforward, only requiring the use of clean bottles. If the water sample were anoxic
and reduced substances were present, such as Fe 2 + or H 2 S, then care would need to be
taken to insure that these substances do not become oxidized by contact with
atmospheric oxygen. Because of the uptake and release of other ions during
photosynthesis or decomposition, samples should be analyzed promptly, preferably
within 3 h for high precision. Although exchange of CO 2 with the atmosphere does not
affect alkalinity itself, it may affect the pH and the equivalence point if substantial
changes in the total amount of dissolved inorganic carbon were to occur during
titration. Thus, it is best to titrate quickly and without undue surface agitation of the
sample.
113
Since alkalinity includes HC0 3 - and C0 3 2 -, alkalinity can be predicted from DIC
and pH:
Alk = DIC[O(l + 20(2] + [OH-] - [H+]
When any compound dissolves in water it produces equal numbers of positive
(cationic) charges and negative (anionic) charges. Therefore, electroneutrality is
maintained, and this charge balance can be expressed mathematically as:
[H+] + {basic cations} = [HC0 3 -] + 2[C0 3 2 -] + [OH-] + {strong acid anions}
( 5)
where
and
{acid anions} = 2[SO/ -] + [Cl-] + [N0 3 -] + ...
Alkalinity may be thought of as an "imbalance" between acid [H+] and the weak
anions, mainly bicarbonate with some carbonate and hydroxide. Alkalinity represents
the number of protons that can be neutralized by all of the weak anions after
subtracting the protons already present and can be written as:
Alkalinity = [HC0 3 -] + 2[C0 3 2 -] + [OH-] - [H+]
(6)
If electroneutrality were to be maintained, then the following also would be true:
Alkalinity = {base cations} - {strong acid anions}
(7)
These two equations are statements about equivalents. In practice, alkalinity can be
measured by adding known amounts of strong acid until all of the weak anions have
been used, at which point the pH will decline precipitously and all acid added
subsequently will persist as protons. This point is often called "the equivalence point"
and in fresh water usually occurs at a pH between about 4.2 and 5.1, depending on the
amount of dissolved inorganic carbon present. It also is possible to calculate the
alkalinity by mea suing all of the base cations and strong acid anions and subtracting
the moles of charges. Note that in acidic waters it is possible to have negative
alkalinities.
The terms alkalinity, carbonate alkalinity, alkalinity reserve, titratable base, and acid
neutralizing capacity (ANC) have been used commonly to express the total quantity of
bases that can be titrated with a standard solution of a strong acid (e.g., 0.1 N H 2 S0 4 ).
Currently, several methods of measuring alkalinity are used, depending on conditions
and on the degree of accuracy required. Since alkalinity is not affected by the exchange
of CO 2 with the atmosphere, sampling procedures for most surface waters are
straightforward, only requiring the use of clean bottles. If the water sample were anoxic
and reduced substances were present, such as Fe 2 + or H 2 S, then care would need to be
taken to insure that these substances do not become oxidized by contact with
atmospheric oxygen. Because of the uptake and release of other ions during
photosynthesis or decomposition, samples should be analyzed promptly, preferably
within 3 h for high precision. Although exchange of CO 2 with the atmosphere does not
affect alkalinity itself, it may affect the pH and the equivalence point if substantial
changes in the total amount of dissolved inorganic carbon were to occur during
titration. Thus, it is best to titrate quickly and without undue surface agitation of the
sample.