Influence of reagent purity on the ion chromatographic determination of bromate in water using 3,3 '-dimethoxybenzidine as a prochromophore for photometric detection

Variable availability of the purified dihydrochloride salt of 3,3'-dimethox ybenzidine (DMB; ortho-dianisidine) led us to investigate the effects of re agent purity on the analytical results obtained when this reagent is used i n the photometric determination of the disinfection byproduct bromate. Afte r analyte ions are separated by ion chromatography, a solution of DMB (post -column reagent) is added to the eluate and the DMB is oxidized, thereby pr oducing a chromophore detected by its absorbance. Although some commercial products of undefined grade performed well, others did not. Variability was also observed between lots of purified material. Sensitivity at low concen trations (<5 <mu>g L-1 BrO3-) varied by a factor of up to 10. In some cases , the lower limit of detection for photometric detection was greater than t hat obtained using conductivity detection, as high as 5-7 mug L-1 BrO3-. An impurity or several impurities are suspected to be responsible for deviati ons from linearity at low analyte concentrations. This investigation unders cores the need for ensuring reagent purity in environmental analyses. Ideal ly, chemical manufacturers will meet the needs of analytical chemists who t est potable water and begin producing a high grade material in sufficient q uantities to meet monitoring requirements. The establishment of third-party standards for a spectrophotometric grade of DMB . 2HCl would be helpful in ensuring that a variety of manufacturers could supply products of uniforml y high quality that would be suitable for the measurement of bromate in pub lic drinking water supplies.