Anions in seawater - Determination of Anions in Natural and Treated Waters - page 366

chromium were generally 10.0%. 5.0% and 5.0% respectively. From these results, the rsd
for the calculated concentration of the bound species was 20%.
Ahern et al. [51] have discussed the speciation of chromium in seawater. The method
involved co-precipitation of trivalent and hexavalent chromium, separately, from samples
of surface seawater and determination of the chromium in the precipitates and particulate
matter by thin-film X-ray fluorescence spectrometry. An ultraviolet irradiation procedure
was used to release bound metal. The ratios of labile trivalent chromium to total
chromium were in the range 0.4-0.6 and the totals of labile tri- and hexavalent chromium
were in the range 0.3-0.5µg L −1 . Bound chromium ranged from 0 to 3µg per litre, and
represented 0-90% of total dissolved chromium. Acidification of the samples in the usual
manner for the determination of trace metals altered the proportion of trivalent to
hexavalent chromium.

3.14 Fluoride

3.14.1 Spectrophotometric method
Fletsch and Richards [52] determined fluoride in seawater spectrophotometrically as the
cerium alizarin complex. The cerium alizarin complexan chelate was formed in 20%
aqueous acetone at pH 4.35 (sodium acetate buffer) and, after 20-60min, the extinction
measured at 625nm (2.5cm cell) against water. The calibration graph was rectilinear for
8-200µg L −1 fluoride; the mean standard deviation was ±10µg L −1

at a concentration of

1100µg L −1 fluoride.
Spectrophotometric procedures based on the lanthanum-alizarin complex are also
described [53].

3.14.2 Photoactivation analysis
Photoactivation analysis has also been used to determine fluoride in seawater [54]. In this
method a sample and simulated seawater standards containing known amounts of fluoride
are freeze-dried, and then irradiated simultaneously and identically, for 20min, with high-
energy photons. The half-life of 18 F (110min) allows sufficient time for radio-chemical
separation from the seawater matrix before counting. The specific activities of sample
and standards being the same, the amount of fluoride in the unknown may be calculated.
The limit of detection is 7ng fluoride, and the precision is sufficient to permit detection of
variations in the fluoride content of oceans. The method can be adapted for the
simultaneous determination of fluorine, bromine and iodine.

3.14.3 Ion selective electrodes
Ion selective electrodes are emerging as the method of preference for the determination
of fluoride in seawater [55-60].
Anfalt and Jagner [59] measured total fluoride ion concentration by means of a single-
crystal fluoride selective electrode (Orion, model 94-09). Samples of seawater were