Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 168

of fluorides based on the fact that the intensity of fluorescence in the compounds formed
in the reaction of aluminium chloride with two dihydroxyazo dyes, Eriochrome Red B
(sodium salt of 4- ρ -sulphonaphth-2-ol-α-azo-1-phenyl-3-methyl-5-hydroxypyrazole) (I)
and Superchrome Garnet Y (sodium salt of 5-sulpho-2-hydroxybenzene-azoresorcinol)
(II) is decreased on the addition of fluoride. These methods were tested by analysing
solutions of pure sodium fluoride both before and after a Willard-Winter distillation. The
ions that are often present after distillation, such as sulphate and phosphate do not
interfere seriously.
Quenching reactions—Morin system
Fluorescent aluminium chelates have been widely used to determine several anions
indirectly, especially fluoride. The aluminium-morin complex is the most commonly
utilised.
Bishop [236] studied the influence of anions on the morin test for aluminium, gallium,
beryllium and zinc and proposed several tests for the detection of fluoride (oxalate,
citrate, phosphate, tartrate and vanadate).
Calcein Blue zirconium system
Hems et al. [311] and others [312] studied the determination of zirconium with Calcein
Blue (XV) and verified the positive interference that fluoride and sulphate ions produced
on the fluorescence intensity of the complex. Based on this fact, Har and West [313]
proposed two methods for the determination of both anions.
1-(2-pyridylazo)naptho-2-ol system
Another fluorescent aluminium chelate used to determine fluoride is the aluminium 1-(2-
pyridylazo)naphth-2-ol (PAN) system [314]. Two procedures have been described, one
for high sensitivity and the other for a wider range but with poorer sensitivity.
Pontachrome Blue Black R
Bourstyn [315] suggested that the fluorescence quenching produced on the aluminium
Pontachrome Blue Black R (2,2 ′ -dihydroxy-1,1 ′ -azonaphthalene-4-sulphonic acid)
system could be used to characterise this anion.
Zirconium flavanol system
This system [313,316,317] is particularly sensitive and can be used to determine fluoride
at the 2-100µg L −1 level.