Potentiometric determination of trace bromide and iodide in chlorides

Potentiometric Determination of Trace Bromide and Iodide in Chlorides

Significance of the Topic

Potentiometric determination of bromide and iodide in the presence of a large excess of chloride addresses critical analytical challenges in environmental water analysis, food testing, and industrial quality control where accurate trace halide measurement is essential.

Objectives and Study Overview

This bulletin presents two complementary methods. The first quantifies trace bromide via cyanogen bromide distillation, absorption, acid decomposition, and potentiometric titration with silver nitrate. The second measures iodide by selective oxidation to iodate, conversion to iodine, and titration with sodium thiosulfate. Both approaches aim to deliver high selectivity and sensitivity in chloride-rich matrices.

Methodology and Used Instrumentation

• Bromide Procedure: Sample mixed with chromic and sulfuric acids undergoes distillation with KCN under nitrogen. Evolved BrCN is trapped in NaOH, decomposed by concentrated H2SO4, and titrated with AgNO3 using an AgBr-coated electrode.
• Iodide Procedure: Sample dissolved in water is treated with bromine solution and NaOH to oxidize iodide to iodate. Excess reagent is quenched with formaldehyde and acetic acid. After adding KI, liberated iodine is titrated with Na2S2O3 using a platinum electrode.

Instrumental Setup

• Metrohm Titrino Series (models 702 SET/MET, 716 DMS, 736 GP, 751 GPD or 785 DMP) or Titroprocessor (726/796) with Dosino/Dosimat.
• Magnetic stirrer for sample homogenization.
• Exchange units specific to bromide (6.3014.153) and iodide (6.3014.213).
• Ag Titrode with AgBr coating for bromide and Pt Titrode for iodide.
• Standard electrode cables.

Results and Discussion

The methods reliably determine bromide and iodide at sub-mg/kg levels in brine, seawater, and sodium chloride without significant interference from the prevailing chloride background. Bromide titrations exhibit sharp endpoints and reproducible results. Iodide analysis remains unaffected by high bromide concentrations, ensuring consistent quantification.

Benefits and Practical Applications

• Excellent selectivity in high-chloride samples.
• Low detection limits suitable for trace analysis.
• Applicable to diverse matrices including water, food, and industrial salts.
• Automated titration workflow enhances throughput and reproducibility.

Future Trends and Opportunities

Advancements in electrode materials and endpoint detection algorithms are expected to further reduce detection limits and improve robustness. Integration with autosamplers and coupling to chromatographic or mass spectrometric techniques could expand the analytical scope. Research into micro-distillation systems may streamline sample preparation and increase laboratory efficiency.

Conclusion

The described potentiometric methods provide robust, selective, and sensitive tools for routine determination of trace bromide and iodide in chloride-rich matrices, meeting the stringent requirements of environmental and industrial analysis.

Reference

• J D Winefordner M Tim Separation of trace quantities of bromide from large amounts of chloride by a distillation method and measurement of the bromide by precision null point potentiometry Anal Chem 35 1963 382–386
• W S Wooster P S Farrington E H Swift Coulometric titration of iodide by electrolytically generated bromine and an amperometric endpoint Anal Chem 21 1949 1457–1460