LIMITATIONS OF ION CHROMATOGRAPHY WITH POSTCOLUMN REACTION FOR DETERMINATION OF HEAVY-METALS IN WATERS CONTAINING STRONG CHELATING-AGENTS

A published method for the determination of heavy metals was applied t o the determination of Cu(II), Ni(II), Zn(II) and Mn(II) in synthetic solutions that contained one of the following ligands (L): citrate, ni trilotriacetate (NTA), ethylenediaminetetraacetate acid (EDTA) and cyc lohexylenediaminetetraacetate (CDTA). The method involves on-column de rivatization with 2,6-pyridinedicarboxylic acid (PDCA) and ion-exchang e separation, followed by post-column reaction with 4-(2-pyridylazo)re sorcinol (PAR) to form metal-PAR chelates, which can be sensitively mo nitored by spectrophotometric detection at 520 nm. Solutions with 12.6 mu mol/l of Cu(II), Ni(II), Zn(II) and Mn(II) and 12.6 or 25.2 mu mol /l of L were analysed. A 100% recovery was obtained for all metals wit h citrate or NTA, for Cu(II), Zn(II) and Mn(II) with EDTA and only for Mn(II) with CDTA. The recoveries in further cases were Ni(II)-EDTA le ss than or equal to 68%, NI-CDTA less than or equal to 80%, Cu(II)-CDT A less than or equal to 20% and Zn(II)-CDTA less than or equal to 87%. To interpret these results, simpler solutions with 12.6 or 6.3 mu mol /l Cu(II) and Cu:L ratios (R) in the range 4.2-0.26 were analysed. For both citrate and NTA an almost 100% recovery of Cu(II) was found for all values of R. For EDTA, 100% recovery was only observed for R great er than or equal to 1.0. For CDTA the recoveries were between 76% (R = 4.2) and 0% (R less than or equal to 0.52). Speciation calculations s howed that only kinetic factors were responsible for the inefficiency of both the on-column and the post-column derivatizations. Analytical implications of the results are discussed.