Anions in seawater - Determination of Anions in Natural and Treated Waters - page 394

3.22.5 Reverse phase ion interaction liquid chromatography
Ito et al. [162] used this technique employing octadecyl silane reverse phase columns
coated with cetyltrimethyl ammonium chloride for the determination of nitrite and nitrate
in seawater.
3.23 Perrhenate
3.23.1 Determination and preconcentration
This is discussed in section 15.3.2.
3.24 Phosphate
3.24.1 Spectrophotometric method
Spencer and Brewer [115] have reviewed methods for the determination of phosphate in
seawater. Earlier methods for the determination of phosphate in seawater are subject to
interferences, particularly by nitrate. In one early method [163] the filtered seawater
sample is acidified with nitric acid and perchloric acid in the presence of
hydroxylammonium chloride and ammonium chloride and the phosphomolybdate
complex extracted with methylisobutyl ketone. This extract is then reduced with acidic
stannous chloride and ascorbic acid and the extinction measured at 725nm. Hosokawa
and Ohshima [164] heated the sample (50ml) for 20min on a boiling water bath with 2ml
Mo(VI)-Mo(V) reagent (hydrochloric acid added (10ml) to 2M-Mo(VI) (10ml) then
added zinc (0.3g), and after the zinc dissolved added hydrochloric acid to 100ml). The
mixture was cooled and the extinction at 830nm measured. The resulting blue complex
remains stable for at least 2 months and has an ε value of 26000. Nitrate interferes if
present in concentrations more than 1µg L−1and should first be reduced to nitrite with
zinc in acidic medium, AsO 4 3 − interfere at 10mg L −1 . The salt error was about 5% at a
chloride concentration of 1.9%.
Isaeva [165] described a phosphopolybdate method for the determination of phosphate
in turbid seawater. Molybdenum titration methods are subject to extensive interferences
and are not considered to be reliable when compared with more recently developed
methods based on solvent extraction [166-171] such as the solvent extraction-
spectrophotometric determination of phosphate using molybdate and malachite green
[172]. In this method the ion-pair formed between malachite green and
molybdophosphate is extracted from the seawater sample with an organic solvent. This
extraction achieves a useful 20-fold increase in the concentration of the phosphate in the
extract. The detection limit is about 0.1µg L −1 standard deviation 0.05ng−1 (4.3µg L−1 in
tap water) and relative standard deviation 1.1%. Most cations and anions found in non
saline waters do not interfere, but arsenic(V) causes large positive errors.
In earlier work Motomizu et al. [171] used ethyl violet as counter-ion for