Anions in seawater - Determination of Anions in Natural and Treated Waters - page 413

15200 12600-18000

Source: Reproduced with permission from Elsevier Science [225]

A commonly used procedure for the determination of phosphate in sea water and
estuarine waters involves the formation of the molybdenum blue complex at 35-40°C in
an Autoanalyser and spectrophotometric evaluation of the colour produced.
Unfortunately when applied to sea water samples, depending on the chloride content of
the sample, peak distortion or even negative peaks occur which make it impossible to
obtain reliable phosphate values. This effect can be overcome by the replacement of the
distilled water used in such methods by a solution of sodium chloride of an appropriate
concentration related to the chloride concentration of the sample. The chloride content of
the wash solution need not be exactly equal to that of the sample. For chloride contents in
the sample up to 18,000mg L −1 (ie seawater) the chloride concentration in the wash
should be within ±15% of that in the sample (Table 3.17). The use of saline standards is
optional but the use of saline control solutions is mandatory. Using good equipment,
down to 0.02mg L −1 phosphate can be determined by such procedures. For chloride
contents above 18,000mg L −1 the chloride content of the wash should be within ±5% of
that in the sample.
Airey et al. [226] have described a method for the removal of sulphide prior to the
determination of phosphate in anoxic estuarine waters. Mercury(II) chloride was used to
precipitate free sulphide from samples of anoxic water. The sulphide-free supernatant
liquid was used to estimate sulphide by measuring the concentration of unreacted
mercury(II), as well as to determine phosphate by a spectrophotometric method in which
sulphide interferes. The detection limit for phosphate was 1µg L −1 .

3.32.2 Electrostatic ion chromatography, bromide, nitrate and iodide
In a series [227-233] employing stationary phases coated with zwitterionic surfactants (ie
those containing both positively and negatively charged functional groups but carrying no
formal net charge), it has been demonstrated that inorganic anions can be separated using
pure water as eluent, with unique separation selectivity. This method has been termed
electrostatic ion chromatography the separation has been attributed to a simultaneous
electrostatic attraction and repulsion mechanism occurring at both the positive and the
negative charges on the stationary phase. A drawback of electrostatic ion chromatography
is that when the sample contains multiple anions and cations, each analyte anion may be
eluted as more than one peak, with each peak being a specific combination of the anion
with one of the cations of the sample. Recently, Hu et al. [234] showed that addition of a
small quantity of a suitable electrolyte to the eluent causes analyte anions to be eluted
only as a single peak, irrespective of the number and type of cations in the sample. In
further studies, Hu et al. [235] investigated the separation mechanism in more detail and
have applied the method to the determination of nitrate, bromide, and iodide in seawater.
The determination of nitrate, bromide, and iodide in seawater is of importance to
oceanographic research [236-238]. Ion exchange chromatography is generally
inapplicable to this analysis for several reasons. First, the large amount of matrix ions