2,2′-Dihydroxyazobenzene and five substituted analogues have been examined as pre-column chelating agents for the determination of trace metal ions by reversed-phase high-performance liquid chromatography (RP-HPLC) with spectrophotometric detection. Seven metal ions, viz., AlIII, CoIII, CuIII, FeIII, MoVI, VV and ZnII, were detected as their azo chelates in the ion-pair RP mode. The optimum conditions, including pH for colour formation, column used, mobile phase parameters and detection wavelength, were investigated for each individual reagent. The method was applied to the determination of AI and Fe in coal fly ash samples. The kinetic nature and the forms of the chelates responsible for the selectivity of RP-HPLC towards metal ions are discussed in terms of "kinetic differentiation" together with ion-pair RP partition phenomena.
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