Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 114

injected into a carrier stream which is merged with a stream of sulphuric acid. Gaseous
carbon dioxide is then formed which diffuses through a gas permeable membrane into an
indicator stream. The indicator stream comprises a mixture of acid-base indicators and
the reaction can be written.
By this reaction the anion concentration will decrease and the colour shift is measured
photometrically. Coloured samples and samples containing small particles can be
analysed as long as clogging can be prevented.
In a further flow injection analysis method for the determination of 20-180mg L −1
[147] or 100-1 000mg L −1 [148] total carbonate, the aqueous sample is injected into a
carrier stream and mixed with 0.1M sulphuric acid (Fig. 2.7). The joint stream passes
along a PTFE membrane in a gas diffusion cell. The carbon dioxide formed diffuses
through the membrane into an indicator stream. The resulting colour change of the
indicator is measured at 585-590nm.
The application of this technique to the determination of carbonate is also discussed in
section 14.4.1.4.
2.14.1.2 Ion chromatography
This technique has been applied to the codetermination of carbonate and bicarbonate in
natural and potable waters [149].
The application of this technique is also discussed under multianion analysis in section
12.2.5.
2.14.2 Total alkalinity
2.14.2.1 Titration methods
Alkalinity of water is its acid-neutralising capacity. It is the sum of all the titratable bases,
and it is determined by titration with a standard solution of a strong mineral acid. In non
saline waters, alkalinity is primarily a function of carbonate, bicarbonate and hydroxide
content though borates, phosphates, silicates and other bases may contribute. If the only
anions present in a solution are hydroxide and carbonate there will be two endpoints in
the titration with acid; the first endpoint occurs at about pH 8.3 and corresponds to the
hydroxide and half the carbonate; the second endpoint occurs at about pH 4.5 and
corresponds to the rest of the carbonate [150].
In the classical determination of alkalinity by titration with an acid, phenolphthalein is
used to indicate the pH 8.3 endpoint and methyl orange to indicate the pH 4.5 endpoint.
Alkalinity determined with reference to these two endpoints is often called 'P and M
alkalinity'. The terminology is still in use, even though it is more usual today to use a
calibrated pH electrode as an endpoint indicator. The 'total alkalinity' is based on the
amount of acid required to neutralise the sample to the pH 4.5 endpoint; in many