Anions in seawater - Determination of Anions in Natural and Treated Waters - page 367

adjusted to pH 6.6 with hydrochloric acid and were titrated with 0.01M sodium fluoride
with use of the semi-automatic titrator described by Jaguer [61]. Equations for the
graphical or computer treatment of the results are given. Calibration of the electrode for
single-point potentiometric measurements at different seawater salinities is discussed.
Rix et al. [60] have commented that earlier published potentiometric methods for
determining fluoride in seawater appear to be unnecessarily complicated in several
respects. Reagents, such as a total ionic strength adjustment buffer, TISAB, are added to
the sample at high concentrations to set the pH and ionic strength and to liberate the
majority of fluoride bound in metal complexes. The possibility of introducing solution
contamination when using high concentrations of TISAB is always a potential hazard
and, if this step can be avoided, this will obviously be advantageous. Furthermore, a
significant drawback with most earlier methods was the difficulty of establishing an
acceptable or representative matrix in which to prepare standards, as the actual matrix is
often unknown in non saline waters. This aspect of fluoride determination in seawater has
received considerable attention, and extensive and elaborate procedures have been used
to match the sample and standard matrices [55].
The method of standard additions is frequently employed in environmental analysis for
samples of variable matrix [62]. The method of data treatment used by Rix et al. [60] is
similar to that originally described by Gran [1] for the exact determination of
potentiometric end-points. Brand and Rechnitz [63], Baumann [64] and Craggs et al. [65]
have described procedures employing the method of standard additions to selective ion
electrode potentiometry, but no data are available for non saline waters using this direct
method except for the work of Liberti and Mascini [66] who determined fluoride in
mineral waters. The procedure used by Rix et al. [60] is exceedingly simple, and the
method of data treatment provides a linear plot, the slope and intercept of which give
independent measures of the original concentration of fluoride in the sample. Total
fluoride concentration is determined despite the fact that the activity of the fluoride ion is
the parameter monitored by the fluoride selective ion electrode.
The concentration of fluoride in seawater is approximately 1.4ppm or 7 ×10 −5 mol L −1
and the fluoride electrode has been shown to give a Nernstian response at this level and
indeed three orders of magnitude lower [67].
It has been well established that the fluoride electrode is highly selective in its response
to the activity of fluoride with the only common interferant being hydroxide [68].
Nevertheless, although seawater has a uniform pH of 8, possible hydroxide interference
was shown not to be a problem as indicated by the results presented in Table 3.6.
Complexation of fluoride by metal ions in seawater has previously been overcome by
the addition of TISAB solution. This reagent is presumed to release the bound fluoride by
preferential complexation of the metal ions with EDTA type ligands present in the
TISAB. Examination of the metal ions present in seawater [69,70] suggests that
magnesium is the major species forming fluoride complexes. Theoretical calculations for
the simple case when only MgF»fa, Mg 2 »fa and F − are considered demonstrate that even
this species is unlikely to interfere.
Results for seawater of 35.21 ‰ salinity are presented in Table 3.6, and the average
value of 1.35±0.05mg L −1 is in excellent agreement with previously published data

[55,56,69,71,72].