Anions in seawater - Determination of Anions in Natural and Treated Waters - page 403

varied by changing the tube length
Source: Reproduced with permission from the American Chemical
Society [179]
3.27 Selenate/selenite
3.27.1 Fluorometric method
Itoh et al. [181] determined selenium(IV) in sea and estuarine waters by an anion-
exchange resin modified with bismuthiol(II) and diaminonaphthalene fluorophotometry.
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 0.1M penicillamine or 0.1M 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 the analysis of
estuarine waters permitted the separate determination of both selenium(IV) and selenium
(VI) since the tellurium coprecipitation method did not differentiate between the two
species.
3.28 Silicate
3.28.1 Spectrophotometric methods
Silicon is an essential and, in some cases, growth-limiting micronutrient for marine
organisms that form siliceous tests [182]. Concentrations of available silicon are often
low (<1µM) in surface waters of high productivity. Although most of the silicon is
recycled near the surface, a fraction reaches the sediment surface where it may dissolve.
As a result, deep waters may have dissolved silicon concentrations as high as 200µM.
Upwelling returns the dissolved silicon to the surface and the cycle is repeated. Dissolved
silicon is an important species to monitor because of its influence on plankton growth
[182] and because of its use as a tracer of water mass movement [183].
Various approaches to the analysis of dissolved silicon have been tried. Most of them
are based on the formation of β-molybdosilic acid [184- 188]. Dissolved silicon exists in
seawater almost entirely as undissociated orthosilic acid. This form and its dimer, termed
'reactive silicate', combine with molybdate to form α- and β-molybdosilic acid [189].
The molybdosilic acid can be reduced to molybdenum blue, which is determined
photometrically [190]. The photometric determination of silicate as molybdenum blue is
sufficiently sensitive for most seawater samples. It is amenable to automated analysis by
segmented continuous flow analysers [191,192]. Most recent analyses of silicate in
seawater have, therefore, used this chemistry. Furthermore, reactive silicate is probably
the only silicon species in seawater that can be used by siliceous organisms [189],