DETERMINATION OF CHROMIUM(VI) IN NATURAL-WATERS BY THE INTERACTION BETWEEN TRIIODIDE ION AND HEXADECYLPYRIDINIUM CHLORIDE MICELLES

The cationic surfactant hexadecylpyridinium chloride (cetylpyridinium chloride, CPC) enhances the sensitivity and selectivity of the spectro photometric determination of chromium(VI) based on its reaction with i odide ion in an acid medium, which is accelerated by 2,2'-dipyridyl. T he triiodide ion produced in the reaction associates with CPC micelles , which results in maximum absorption at 500 nm, with a concomitant su bstantially increased absorptivity and stability constant of the triio dide complex. The micellar medium allowed the determination of chromiu m(VI) at concentrations between 15 and 100 ng ml(-1), with a detection limit of 9 ng ml(-1) (i.e. about 5 times lower than those of methods implemented in aqueous media). The molar absorptivity for the triiodid e-CPC system was found to be (5.19+/-0.08)x10(4) 1 mol(-1) cm(-1). The selectivity towards various ions such as Pb(lI), Fe(III), Cd(II), Cu( II) and Ni(II) was also greately enhanced. The proposed method was sat isfactorily applied to the determination of chromium(VI) in natural wa ters after extraction with trioctylamine, with an average recovery of 96%.