Influence of pH, temperature and molybdate concentration on the performance of the triiodide method for the trace-level determination of bromate (EPA 326)

Importance of the topic

Bromate is a regulated drinking water by-product formed when strong oxidants like ozone react with bromide in water. Recognized as a potential carcinogen, bromate must be monitored at trace levels to comply with environmental standards. Sensitive, reliable analytical methods are essential for safeguarding public health and meeting regulatory limits set by the US EPA and EU.

Aims and study overview

This study aims to optimize the post-column triiodide derivatization method (EPA 326) for trace-level determination of bromate by ion chromatography with UV/VIS detection. The investigation focuses on the effects of reaction temperature, molybdate concentration, iodide concentration, and pH (via sulfuric acid) on method performance.

Methodology and instrumentation

The approach employs ion chromatography on a Phenomenex Star Ion A300 HC column, eluting with 100 mmol/L H2SO4 containing varying concentrations of ammonium molybdate. The post-column reactor mixes the effluent with KI via a peristaltic pump at defined flow rates and temperatures (25–80 °C). The resulting triiodide is detected at 352 nm. Key parameters examined:

• Reaction temperature (30–80 °C)
• Ammonium molybdate (0–90 μM)
• KI reagent concentration (0.26–0.75 M)
• Eluent acidity (pH calculated from sulfuric acid concentration)

Main results and discussion

Temperature and iodide concentration had negligible impact on sensitivity, allowing room-temperature operation and moderate KI levels. Molybdate concentration significantly influenced response, with optimal sensitivity achieved at 45–90 μM. Sulfuric acid pH below 1.5 enhanced signal and reduced retention times; pH above 1.5 caused rapid sensitivity loss and longer analyses.

Benefits and practical applications

The optimized triiodide method delivers a detection limit below 50 ng/L, avoiding toxic derivatizing agents like o-dianisidine. Its straightforward setup and compatibility with standard ion chromatography systems make it ideal for routine monitoring of bromate, iodate, and chlorite in drinking and mineral water under QA/QC and regulatory compliance.

Future trends and possibilities

Potential developments include integration with mass spectrometric detection for confirmatory analysis, microfluidic reactor designs for reduced reagent consumption, automation of reactor conditions, and exploration of alternative greener catalysts to further lower detection limits and environmental impact.

Conclusion

The optimized post-column triiodide derivatization protocol enhances sensitivity and robustness for trace bromate analysis. By fine-tuning molybdate concentration and acidity, the method meets stringent regulatory requirements with a detection limit below 50 ng/L, providing a reliable tool for water quality monitoring.

Reference

• Kitamaki Y.; Takeuchi T. Cyclodextrin-aided determination of iodate and bromate in drinking water by microcolumn ion chromatography with precolumn enrichment. Analytical Sciences. 2004;20:1399–1402.