Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 170

2.31.6 Ion selective electrodes
Shiraishi et al. [326] showed that a fluoride ion selective electrode could be used to
determine fluoride in well water in the presence of aluminium. These workers showed
that the de-masking of fluorine from its aluminium complexes is enhanced by dilution of
the sample. The permissible limits of aluminium concentration increase markedly with
decreasing fluoride concentration, hence fluoride could be determined by a successive
dilution method.
Zernalova and Senyavin [327] examined conditions for the potentiometric
determination of fluoride in non saline water using an ion selective electrode. Some
micro constituents of water did interfere when determining fluorides.
Ion selective electrodes have been extensively applied to the determination of fluoride
at the mg L −1 and µg L −1 levels in water [ 328-332]. Warner [328], Sekerka and Lechner
[329] and Erdmann [330] have applied the technique to non saline waters. Sekerka and
Lechner [329] achieved a detection limit of 0.2µg L −1 fluoride. Their methods employ a
total ionic strength adjustment buffer usually comprising sodium hydroxide, 1,2-
diaminocyclohexane- N,N,N,N -tetraacetic acid and acetic acid in fixed proportions.
Aluminium, iron, calcium and magnesium do not interfere in these procedures within
prescribed concentration limits. Pakalns and Farrar [331] have discussed the effect of
surfactants on the determination of fluoride. Erdmann [330] has described an automated
ion selective electrode method for determining fluoride in non saline waters. This method
is capable of analysing 30 samples per hour and utilises an autoanalyser system.
Trojanowicz and Lewardowski [192] have described a multiple flow-through
potentiometric system for the continuous determination of fluoride in admixture with
chloride and nitrate in non saline waters. Classic ion selection electrode methods for
fluoride suffer from the disadvantages of excessive electrode drift, long response time
and inadequate precision.
Kissa [333] has described modifications to the determination of fluoride using the
fluoride ion selective electrode to enable the determination of concentrations in the range
1-10mg L −1 fluoride with relative errors from 0.2 to 0.6%.
Kissa [333] overcame the aforementioned problems by
(a) limiting the fluoride concentration to which the electrode is exposed to a 0.01-0.1mg
L −1 fluoride or 0.05-1mg L −1 fluoride concentration range;
(b) measuring the electrode potential in the analyte by approaching equilibrium in the
same direction from a higher potential to a lower potential; and
(c) keeping the temperature of the solutions constant within ±0.2°C. The fluoride
concentration in the analyte is adjusted to the concentration range of the ion selective
electrode by dilution or fluoride addition.
It has been observed by previous workers [334] that the electrode response drifted while
the fluoride solution was stirred, but when stirring was stopped, the electrode assumed a
stable potential within 5-30min, depending on the fluoride concentration. Kissa [333]
examined this procedure and concluded that the apparently stable potential resulted for a
reduced transport of fluoride ions to the electrode and did not always represent an