Anions in seawater - Determination of Anions in Natural and Treated Waters - page 408

water from San Francisco Bay. It was accompanied by a decrease in salinity from 33‰ to
20‰.
3.28.3 Ion exclusion chromatography
Hioki et al. [206] have described an on-line determination of dissolved silica in seawater
by ion-exclusion chromatography in combination with inductively coupled plasma
atomic emission spectrometry.
This method was developed as a second, independent method to complement the usual
colorimetric procedure in the determination of a certified concentration of dissolved silica
in a planned seawater reference material. Ion exclusion affords a separation of the
dissolved silica not only from the major seawater cations but also from potentially
interfering anions. The detection limit, conservatively estimated at 2.3ng g − 1 as Si
(0.08µM), is superior to that achievable by direct analysis by inductively coupled plasma
atomic emission spectrometry.
Other studies on the application of this technique have been carried out [207-210].
3.28.4 Miscellaneous
Determinations of the so-called 'micronutrients' (eg nitrate, phosphate, and dissolved
silica) in seawater are among the most commonly performed analyses in oceanographic
research and survey work. Surprisingly, there exists no certified reference material that
can be used to check the accuracy of such analyses, although intercomparison exercises
were conducted on a regular basis during the 1980s [211]. The lack of a seawater
certified reference material for micronutrients can be attributed both to the difficulty of
preparing a suitable material with an adequate shelf life [212,213] and to the dearth of
independent methods available for the determinations.
The National Research Council of Canada has undertaken a project, in collaboration
with the Bedford Institute of Oceanography of the Canadian Department of Fisheries and
Oceans, to address the need for a seawater certified reference material for micronutrients,
with the initial objective being the preparation of a material with certified values for
nitrate, phosphate and dissolved silica.
3.29 Sulphate
3.29.1 Titration method
Sulphate has been determined in seawater by photometric titration with hydrochloric acid
in dimethyl sulphoxide [214]. The sample (5ml) is added slowly to dimethyl sulphoxide
(230ml) and titrated with 0.02M hydrochloric acid (standardised against sulphate) with
bromocresol green as indicator. Since borate, carbonate and bicarbonate interfere, a
separate determination of alkalinity is necessary.