Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 167

potassium hydroxide solution before analysis, to ensure quantitative ion exchange.
Ion exchange/spectrophotometry was used by Capitan-Vallvey et al. for determining
low levels of fluoride in non saline waters [302]. Fluoride reacts with slightly
polymerised zirconium and xylenol orange, and the reaction product is sorbed on Dowex
1-X8. The absorbance of the resin phase at 580 and 750nm is measured directly. The
calibration graph is linear in the concentration range of 2-20µg L −1 , and the relative
standard deviation is 1.8%. The detection limit is 1.1µg L −1 for a 500mL sample.
Long and Huang [303] observed that fluoride causes a decrease in the absorbance due
to a zirconium complex of salicylfluorone and cetyltrimethylammonium bromide. The
calibration curve is linear for 5-50µg of fluoride per 50ml. Relative standard deviations
were 5.43% at 0.050% fluoride and 2.78% in 0.223% fluoride levels in non saline water.
2.31.2 Spectrofluorometric methods
Various spectrofluorometric systems have been proposed for the determination of
fluorides in non saline waters.
Substitution reactions
Aluminium quinolin-8-ol(oxine) system
A method for determining fluoride has been proposed with the aluminium quinolin-8-ol
(oxine) system [304]. This system was studied by Gentry and Sherrington [305] in
chloroform solution. The presence of fluoride in the aqueous phase before extraction
removes some of the aluminium as a complex ion, reducing the amount present as
quinolin-8-olate and extractable with chloroform. The necessity for extraction adversely
affects the precision of this method.
Morin system
The quantitative determination of fluorides with the aluminium-morin system has been
widely studied [304,306,307]. Willard and Horton [304] used this system, among others.
They obtained a curved calibration graph of fluorescence versus fluoride concentration
because more than one fluoride complex is formed by aluminium ions. As the fluoride
concentrations increase, AlF 2+ , AlF 2 + , AlF 3 , AlF 4 − and perhaps AlF 5 2− and AlF 6 3−
complexes are formed consecutively. In this series, as more and more fluoride is required
to complex a certain amount of aluminium, the rapidity of the decrease in fluorescence
decreases, with a consequent loss in sensitivity and precision for large amounts of
fluoride. Fortunately, the sensitivity is highest at the lowest levels of fluoride where other
methods have lower accuracy. To avoid the numerous interferences, the authors advise
the separation of the fluoride by Willard and Winter's method [304] for liquid samples or
as hydrogen fluoride by pyrohydrolysis [309] for heavy metal fluorides. Bouman [307]
used Willard and Horton's method to determine fluoride but he changed the conditions to
increase the sensitivity. He determined fluorides in 0.25ml of water with an accuracy of
20mg L −1 .
Eriochrome Red 8 and Superchrome Garnet Y
Powell and Saylor [310] developed two methods for the determination of trace amounts