Online column preconcentration for speciation and selective determination of Cr(III) in natural water samples using flow injection with chemiluminescence detection.

Abstract

A simple, rapid, sensitive and inexpensive approach is described in this work based on a combination of solid-phase extraction of 8-hydroxyquinoline (8HQ), for speciation and preconcentration of Cr(III) and Cr(VI) in river water, and the direct determination of these species using a flow injection system with chemiluminescence detection (FI-CL) and a 4-diethylamino phenyl hydrazine (DEAPH)-hydrogen peroxide system. At different pH, the two forms of chromium [Cr(III) and Cr(VI)] have different exchange capacities for 8HQ, therefore two columns were constructed; the pH of column 1 was adjusted to pH3 for retaining Cr(III) and column 2 was adjusted to pH1 for retaining of Cr(VI). The sorbed Cr(III) and Cr(VI) species were eluted from columns using 3.0ml of 0.1N of HCl and 3.0ml of 0.1N of NaOH, respectively. The flow injection-chemiluminescence (FI-CL) method is based on light emitted due to the oxidation of DEAPH by the H2 O2 in the presence of Cr(III), which catalyzes the reaction. The flow cell is a transparent coiled tube made from glass (2.0×4.0, inner and outer diameter) and located close to the photodetector. The flow parameters: flow rate, sample volume, flow cell length, and distance to the CL detector were studied and optimized. Under optimum flow conditions, the Cr(III) concentration can be determined over the range 5-350 μg L-1 with a limit of detection of 1.2μg L-1 , as the Cr(III) concentration is proportional to the intensity of the CL signal. The relative standard deviations (%) for 10 and 50 μg L-1 Cr(III) were 1.2% and 3.2%, respectively. The effects of Al(III), Cd(II), Zn(II), Hg(II), Pb(II), Co(II), Cu(II), Ni(II), Mn(II), Ca(II), and Fe(III) were investigated. The proposed method is highly selective and sensitive, enabling a rapid determination of the Cr(III) amount in the presence of other interfering metals. Finally, the FI-CL method was examined in five river water samples with excellent recoveries.