Anions in estuary and coastal waters - Determination of Anions in Natural and Treated Waters - page 423

4.3 Phosphate
4.3.1 Spectrophotometric method
A commonly used procedure for the determination of phosphate in seawater 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 [3].
Unfortunately when applied to seawater 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. 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.
The determination of phosphate is also discussed under multianion analysis in section
4.6.1.
4.4 Selenate and selenite
4.4.1 Spectrofluorometric method
Itoh et al. [4] determined selenium(IV) in estuarine waters by an anion exchange resin
modified with bismuthiol(II) and diaminonaphthalene. An Amberlite IRA-400 anion
exchange resin was modified by mixing with an aqueous solution of bismuthiol(II) to
give 0.2mmol bismuthiol per resin. Sample solution was eluted through a column packed
with the modified resin. Selenium(IV) adsorbed as selenotrisulphate was then eluted from
the column with either 0.1mol L −1 penicillamine or 0.1mol L −1 cysteine. The eluate was
then subjected to an acid digestion procedure to reduce selenium to the tetravalent state
with diaminonaphthalene for fluorometric determination. Approximate agreement with
the tellurium coprecipitation method was obtained. The application of both methods to