The cationic surfactant hexadecylpyridinium chloride (cetylpyridinium chloride, CPC) enhances the sensitivity and selectivity of the spectrophotometric determination of chromium(VI) based on its reaction with iodide ion in an acid medium, which is accelerated by 2,2#x2032;-dipyridyl. The triiodide ion produced in the reaction associates with CPC micelles, which results in maximum absorption at 500 nm, with a concomitant substantially increased absorptivity and stability constant of the triiodide complex. The micellar medium allowed the determination of chromiuni(VI) at concentrations between 15 and 100 ng ml#x2212;1, with a detection limit of 9 ng ml#x2212;1(i.e. about 5 times lower than those of methods implemented in aqueous media). The molar absorptivity for the triiodide-CPC system was found to be (5.19#xB1;0.08)x1041 mol#x2212;1cm#x2212;1. The selectivity towards various ions such as Pb(II), Fe(III), Cd(II). Cu(II) and Ni(II) was also greately enhanced. The proposed method was satisfactorily applied to the determination of chromium(VI) in natural waters after extraction with trioctylamine, with an average recovery of 96.
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