Discussion of the influence of pH, temperature and molybdate concentration on the performance of the triiodide method for the trace-level determination of bromate (EPA 326)
The disinfection of drinking water destroys pathogenic microorganisms and removes compounds causing bad taste and/or odour.
Most public water suppliers still disinfect their drinking water with chlorine.
However, apart from its unpleasant taste, chlorine reacts with the ubiquitous organic compounds and forms harmful disinfection by-products (DBP) such as the potentially carcinogenic trihalomethanes.
In order to control the formation of these DBPs several strong oxidants, including permanganate and ozone, are used.
While ozone is one of the most efficient oxidants, it also oxidises the naturally occurring bromide to bromate.
Since the International Agency of Research on Cancer has classified bromate as a potential carcinogen, bromate levels in drinking and mineral water have to be controlled.
The United States Environmental Protection Agency (US EPA) and the European Union currently set a maximum bromate concentration of 10ppb in drinking water.
For mineral waters the pertinent regulations stipulate a limit of 3ppb.
To satisfy the regulatory requirements, the determination of bromate requires very sensitive analytical methods.
The most widely used methods for the quantitation of bromate are based on ion chromatography.
MS detection and UV/vis detection after post-column reaction (PCR) are the most sensitive detection modes.
The post- column derivatisation of bromate with o-dianisidine (ODA) according to EPA method 317 achieves a detection limit of approximately 0.2ppb, but the potential carcinogen ODA is major drawback.
The alternative EPA method 326 stipulates post-column reaction (PCR) of bromate with the less harmful iodide, achieving a detection limit of bromate of less that 50ng/L (=50ppt).
The triiodide method.
The analysis is based on the Environmental Protection Agency EPA method 326.
In this post-column derivatisation method, bromate - aided by the catalytic effect of ammonium molybdate - oxidises iodide to triiodide in an acidic medium.
The produced triiodide molecule can be detected at high sensitivity in the UV spectrum at a wavelength of 352nm.
The method described here is exclusively suited for the determination of bromate, iodate and chlorite.
Any additional water constituents have to be determined separately.
The peristaltic pump transfers the KI solution to the PCR where it is mixed with the eluent.
The generated triiodide is subsequently conveyed to the UV/vis detector.
Summary.
Ozonolysis of bromide-containing drinking water results in the formation of bromate, a suspected carcinogen.
Bromate is effectively detected at trace-level by ion chromatography followed by post column derivatisation with iodide, according to the US EPA method 326.
In this work the derivatisation reaction in the post-column reactor was optimised for sensitivity.
The bromate response was neither dependent on the investigated reaction temperature (30-80C) nor on the examined iodide concentrations (0.26-0.75moI/l KI).
In contrast, varying molybdate and sulphuric acid concentrations had a significant influence on method sensitivity.
Increasing sulphuric acid concentrations (pH<1.5) improved sensitivity and shifted the bromate peak to shorter retention times.
Ammonium molybdate concentrations of 45 and 90uM in the eluent provided he best results.
Applying the optimum conditions for the triiodide method, a detection limit for bromate of less than 50ng/l (=50 ppt) is attained.
For a full poster on this application please contact Gemma Clocherty at Metrohm.
