Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 293

snow and river waters.
Nasu et al. [742] have also studied the determination of sulphate with the thorium-
flavonol system. They have used it to analyse lake, river and mine waters in snow.
Thorium forms a light red non-fluorescent complex with salicyl-fluorone [9-( o -
hydroxyphenyl)trihydroxyfluorone) in a weakly acidic medium, whereas free
salicylfluorone in ultraviolet light gives a clear greenish yellow illumination [743]. If
thorium is added to a solution of sulphate, followed by the reagent, part of the thorium
becomes bound in the sulphate complex and part is liberated. The sensitivity of the
reaction depends on the ratio of thorium to salicylfluorone and it increases when the
concentration of thorium exceeds that of salicylfluorone.
A method has been proposed for determining sulphate in distilled water [735] based on
the reaction with salicylfluorone [a-( o -hydroxy-phenyl)trihydroxyfluorone].
Hems et al. [311] studied the interference of zirconium with Calcein Blue(XV) and
verified the positive interference that sulphate and fluoride ions produce on the
fluorescence intensity of the complex. Based on this fact, Har and West [313] proposed
two methods for the determination of both anions.
Nasu et al. [744] used the hydroxyflavone-thorium complex. The extinction at the
absorption maximum at 390nm of the 1:1 complex decreases in the presence of sulphate
owing to the formation of the sulphate complex of thorium. The blue fluorescence of the
hydroxyflavone thorium complex at 470nm also decreases on addition of sulphate. The
calibration graphs are almost rectilinear, and reproducible for 0.2-4.0 and 0.02-0.8mg
L −1 of sulphate in 30% aqueous ethanol at pH 2.4-2.7 and pH 2.5-3.0 by the
spectrophotometric and fluorometric method, respectively. Interference is caused by iron
(III), aluminium, zirconium, barium, fluoride and phosphate, organic anions, and other
oxy-anions of sulphur.
A further fluorescence method [745] is based on the quenching by sulphate of the
fluorescence of the thorin-thorin complex. Interfering ions are first removed by ion
exchange. Down to 25µg sulphate can be determined.
2.93.4 Turbidimetric methods
Coleman et al. [746] compared barium chloride and 4-amino-4 ′ -chlorobiphenyl
hydrochloride as precipitants in the turbidmetric determination of sulphate at 430nm with
barium chloride. Good results (coefficient of variation 1-4%) were obtained 30s after
mixing. The performance with 4-amino-4 ′ -chlorobiphenyl hydrochloride for measure-
ments 20s after mixing was better (coefficient of variation 0.33 and 1.0% for 18 and 4µg
mL −1 of sulphate respectively); internal calibration for each run is recommended. In this
method the reagent is stable for at least 1 month and there is no interference from up to
500mg L −1 of phosphate.
Bonda et al. [747] described a column turbidimetric method for determining sulphate.
The method uses solid barium chloride in crystals of 0.3-0.6mm diameter. The method is
precise and reproducibility is good. The advantage of the method is the stability of the
reagents and the speed, economy and simplicity of performance. Photometric
determination is carried out over the range 460-540nm and the method may be used to