Multianion analysis, miscellaneous methods - Determination of Anions in Natural and Treated Waters - page 718

14.2.3.3 Sulphide, sulphite and sulphate
Bhat et al. [12] reported methods for the determination of low levels of sulphides,
sulphites and sulphates in waste waters. The method for sulphite and sulphide involves
the reduction of the bis-2, 9-dimethyl-1-10-phenanthroline copper(II) ion by sulphide in
the presence of formaldehyde and by sulphite in its absence. In the method for sulphate,
stannous chloride in hydrochloric acid is used to reduce sulphate ions to hydrogen
sulphide which is reacted with bis-2, 9-dimethyl-1-10-phenanthroline copper(II) ion.
All analyses are completed by measuring the absorbances due to the coloured copper(I)
complex formed by chemical reduction. While the methods are less than ideal from the
point of view of sensitivity, simplicity and freedom from interferences, the colours
formed are stable and the sulphate reduction system can be used repeatedly.
14.3 Titration methods
14.3.1 Potable water
14.3.1.1 Chlorite, hypochlorite and chlorate
Oxidising agents such as chlorine and hypochlorite ion are widely used in drinking water
disinfection [13,14] in the United States. Recently, chlorate ion and possibly chlorite ion
have been suggested to exist in drinking water at measurable levels when hypochlorite
ion is used as the sole chlorinating agent. Chlorite and chlorate ions may be a potential
health hazard, and their low-level toxicity is controversial. The US EPA is considering
regulating [15,16] the amount of chlorite and chlorate permitted in drinking water. The
most likely source of [17,18] these ions is the decomposition of concentrated aqueous
sodium hypochlorite stock solutions or solid calcium hypochlorite during storage before
being applied as the disinfectant. Thus, an analytical methodology is needed for the direct
and selective determination of hypochlorite, chlorite and chlorate in concentrated sodium
hypochlorite or Ca(OCl) 2 solutions.
In solution chloride and hypochlorite have similar reactivities, and both ions react with
the reagents typically used for their determination, such as iodide and thiosulphate. The
various difficulties of other methods used to analyse solutions containing chloride and
hypochlorate have been summarised [19]. Freshly produced concentrated solutions of
commercial grade sodium hypochlorite (ie 5-15wt% measured as chlorine) contain levels
of chlorite and chlorate ions which range from 200 to 2000 times less than the
concentration of hypochlorite. Due to this large concentration difference, the analysis of
each ion is not straightforward. The primary objective of the work of Adam and Gordon
[20] was to demonstrate a selective technique for the determination of hypochlorate at
high concentration and chlorite and chlorite at low concentrations when these species are
present in solution.
These workers [20] developed a selective potentiometric technique for the
determination of hypochlorite at high concentration (eg 1-3M) and chlorite and chlorate