A bromide-selective electrode-redox electrode cell for the potentiometric determination of bromine and free residual chlorine.
A potentiometric cell consisting of a bromide-selective electrode and a platinum redox electrode is sensitive to free residual chlorine (FRC) when the sample solution is treated before measurement with reagents to give pH 5 and a bromide content of 0.1M. The calibration graph of e.m.f. vs. log [FRC] is linear over the [FRC] range from about 0.05 to 50 mg l . (the highest level tested). Not all forms of bromide-selective electrode prove suitable, but one with a silver bromide single-crystal membrane has been found to give a theoretical calibration slope, whereas one with an Ag(3)SBr compressed pellet membrane gave a sub-Nernstian response. The response to combined residual chlorine (CRC) is negligible but if both FRC and CRC are present, the sample treatment is likely to cause a reduction in FRC concentration. Tests have shown that a similar bias can be found with the commonly used DPD colorimetric method.
Midgley
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Central Electricity Research Laboratories, Kelvin Avenue, Leatherhead, Surrey, England.