Anions in seawater - Determination of Anions in Natural and Treated Waters - page 371

performance liquid chromatography with fluorometric detection. An important feature of
this method is that the enzymatic reaction occurs directly in aqueous media, even
seawater, and does not require sample pre-treatment other than simple filtration. The
reaction proceeds at room temperature at a slightly alkaline pH (7.5-8.5) and is specific
for formate with a detection limit of 0.5µM ( S/N =4) for a 200µL injection. The precision
of the method was 4.6% relative standard deviation ( n =6) for a 0.6µM standard addition
of formate to Sargasso seawater. Average recoveries of 2µM additions of formate to
seawater were 103%. Intercalibration with a Dionex ion chromatographic system showed
an excellent agreement of 98%. Concentrations of formate present in natural samples
ranged from 0.2 to 0.8µM for Biscayne Bay seawater.
3.16 Hypochlorite
3.16.1 Spectrophotometric method
Williams and Robertson [77] have described a simple inexpensive method for
determining reactive chlorine in non saline waters. It involves addition of bromine, which
is oxidised by the reactive chlorine in the sample, and which in turn brominates
fluorescein to give a pink derivative; this can be measured visually or
spectrophotometrically, or the decrease in fluorescein can be measured fluorometrically.
Potential applications of the method are indicated.
The basis of this method is that when normal seawater is chlorinated at the usual levels
of 1 to 10mg/L to chloride, the bromide in seawater (8.1× 10 −4 M, 65mg L −1 at
salinity=35 ‰) is rapidly and quantitatively oxidised to BrO − and HBrO. If 50mg L −1 of
bromide is added to distilled or fresh waters containing HClO plus ClO − , then HBrO plus
BrO − are both formed. The HBrO plus BrO − , in turn, will rapidly brominate fluorescein
(9-[ o- carboxyphenyl]-6-hydroxy-3-isoxanthenone) to give the pink tetrabromo derivative
eosin yellow (2,4,5,7-tetrabromo-9-[ o -carboxyphenyl]-6-hydroxy-3-isoxanthenone),
provided the molar ratio of bromide to fluorescein is 4:1. The resultant increase in eosin
can be measured visually or spectrophotometrically, and the decrease in fluorescein
measured fluorometrically. If the molar ratio of bromide to fluorescein is <4:1, then the
mono-, di- and tri-bromo derivatives are formed reproducibly. These derivatives have
extinction coefficients close to eosin and are accounted for in the standardisation.
3.17 Iodate
3.17.1 Spectrophotometric method
Truesdale [78] has described autoanalyser procedures for the determination of iodate and
total-iodine in seawater. The total iodine content of seawater (approximately 50-60µg
L−1) is believed to be composed of iodate- (30-60µg L −1. I) and iodine-iodine (0-20µg
L −1 ) with perhaps a few µg L −1 of organically bound iodine. In both of the methods, the
iodine species of interest are first converted to iodate. Then the iodate is reacted with acid