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SOIL CHEMISTRY

A Colorimetric Microwell Method for Determining Bromide Concentrations

^a Biological Systems Engineering, Univ. of Wisconsin–Madison, 460 Henry Mall, Madison, WI 53706
^b Dep. of Soil Science, Univ. of Wisconsin–Madison, 1525 Observatory Dr., Madison, WI 53706

^* Corresponding author (bjlepore{at}wisc.edu).

Bromide is commonly used as a tracer in studies of water and chemical transport in soil and rock because it is relatively nonreactive with soil and rock constituents and because of its low environmental background concentrations. Based on a largely ignored modification of an American Public Health Association standard colorimetric method for determining Br^– using phenol red and chloramine-T(CT), we have corrected an internal error and recast the technique for use with 96-well microplates. Furthermore, the addition of thiosulfate to quench the undesirable chlorination reaction as previously published is shown to be unnecessary and even detrimental following the use of NH₄⁺ to produce chloramine from excess Cl species. The method detection limit is 0.11 mg L^–1 Br^–; negative interference with dissolved organic C can be addressed by either standard additions or solid-phase extraction with C18 sorbent. By reducing the sample size from 300 to 20 µL, the concentration range can be expanded from 12 mg L^–1 Br^– to as much as 300 mg L^–1 Br^–.

Abbreviations: CFS, crushed foundry slag • CT, chloramine-T • DOC, dissolved organic carbon • IC, ion chromatography • ISE, ion specific electrode • MDL, method detection limit • PR, phenol red • SPE, solid-phase extraction • TS, thiosulfate