Anions in potable waters - Determination of Anions in Natural and Treated Waters - page 488

Kissa [34] has described modifications to the determination of fluoride using the
fluoride ion selective electrode, to enable the determination of concentration in the range
1-10mg L −1 fluoride with relative errors from 0.2 to 0.6%.
Kissa [34] 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.
7.13.4.1 Determination of fluoride by the analyte addition method
Experimental details are as follows.
The fluoride ion selective electrode, Orion Model 94-09A, was used in combination
with a double junction electrode, Orion Model 90-01. The cell potentials were measured
with the Orion Ionalyzer, model 901. The Orion electrode holder (Catalog No. 13-641-
814) was provided with a stop to keep the immersion depth of the electrodes constant.
Two sets of electrodes were used, one set being exposed only to solutions containing 10-
100µg L −1 fluoride and the other to solutions containing 50-100µg L −1 fluoride. All
volumetric flasks and beakers used were made of 'nalgene'. Agitation was provided by a
thermally insulated magnetic stirrer operating at a constant speed.
The standardising solutions were prepared by successive dilutions of a stock solution
containing 11.10g L −1 of reagent-grade sodium fluoride (dried to a constant weight at
125°C). The standardising solutions and analytes contained an acetic acid-sodium acetate
buffer (2 vol%) prepared by adding 2800ml of water, 480ml of acetic acid, reagent grade,
and 500ml of 30% sodium hydroxide solution, made of ACS certified, electrolytic
sodium hydroxide pellets, diluting to 3800ml with water and adjusting to pH 5.0-5.2 with
sodium hydroxide.
TISAB II (without CDTA) or TISAB III were used for electrolyte containing analytes.
The equilibration time needed to attain a stable electrode potential decreases with
increasing fluoride concentration and stirring rate and increases with the increasing
concentration change resulting from successive immersions. If the concentration
difference is small, the equilibrium time can be reasonably short even at low fluoride
concentrations. Kissa [34] restricted therefore, the exposure of the fluoride ion selective
electrode to solutions differing less than 10 or 20 times in fluoride concentration. By
limiting the fluoride concentration range and using a programmed immersion sequence,
the equilibration time in the analyte was reduced to 3-7min even at fluoride
concentrations as low as 20µg L −1 . This is the lower practical limit for determining
fluoride conveniently, because the electrode response is no longer linear below this
concentration.
If the millivolt reading indicates that the fluoride concentration in the analyte exceeds
the concentration range of the fluoride ion selective electrode (20-100 or 100-1000µg
L −1 fl uoride) the electrode is withdrawn immediately from the analyte and the fluoride