Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 292

lead sulphate at pH 2.0-3.5 in the presence of ethanol. Excess lead(II) ions were
determined with tetraphenylporphine trisulphonic acid in alkaline solution by measuring
the absorbance at 464nm.
A method for determining sulphate in non saline waters based on measuring the excess
barium ions following precipitation of barium sulphate was reported by Yin and Zhang
[727]. In this case the excess barium ions was determined by titration with sodium
tetraphenylborate. The method had a relative standard deviation of <1.4% based on 7 to
11 measurements of sulphate at 17.6-28.5ppm levels.
An indirect spectrophotometric method for determining sulphate in non saline waters
has been developed by Kamaya and Murakami [728]. The sulphate reacts with barium
nitranilate to form barium sulphate and displace the coloured anion which has a constant
absorptivity at 300nm over the pH range of 2-9. The ions phosphate, thiosulphate,
aresenite, arsenate, chromate, sulphite, oxalate, calcium and strontium all interfere.
Honjo et al. [726] discuss a spectrophotometric method for determining trace (6-
10ppm) concentrations of sulphate in fresh waters based on precipitating sulphate as lead
sulphate at pH 2-3.5 in the presence of 50-70% ethanol. The excess lead ions are reacted
with tetraphenylpor-phinetrisulphonic acid at pH 10.9 and the absorbance is measured at
464nm.
Cheam and Chau [729] discuss the correction of historical colorimetric data for
determining sulphate in coloured waters, and Bosch Serrat [730] discusses methods for
minimising errors in the titrimetric determination of sulphate.
Greenberg et al. [731] and Colovas [732] have described a method for determining 0-
60mg L −1 sulphate in which cations are removed on an ion exchange resin and sulphate
determined by the methyl thymal blue method at a wavelength of 460nm.
In a standard UK method [733] capable of determining down to 4mg L −1 sulphate the
sulphate is precipitated at pH 3.5 to 4.5 by 2-aminoperimidine and the precipitate isolated
by centrifugation. The excess precipitant in the supernatant phase is determined by either
direct uv absorbance measurements at 305nm or treatment with nitrous acid and alkali to
give a red colour which is evaluated spectrophotometrically at 525nm.
2.93.3 Spectrofluorometric methods
Fluorometric methods for determining sulphate are mainly based on the ability of this
anion to react with thorium ion to liberate an organic reagent that initially forms a
complex with thorium. These methods are subject to interference from ions such is
phosphate and fluoride. The method using thorium-salicyfluorone [734,735] is the most
sensitive (0-2µg).
This thorium-morin system has been amply utilised to determine sulphate. Fletcher and
Milkey [736,737] studied the thorium-morin system in a dilute acid medium and noted
the serious interference of sulphate in the formation of the complex caused by the
formation of sulphate complexes with thorium. Based on these interactions, a
determination of sulphate has been developed by Guy on and Lorah [738]. This method is
more sensitive than most existing methods, is rapid, manipulatively simple and adaptable
to the determination of large numbers of samples. Vlasov et al. [739,740] have used this
method to determine sulphate in weakly mineralised waters, and Nasu [741] to analyse