KINETICS AND EQUILIBRIA OF THE ZN-EDTA-PAR POSTCOLUMN REACTION DETECTION SYSTEM FOR THE DETERMINATION OF ALKALINE-EARTH METALS

Zn-EDTA-PAR is the archetypical ''displacement'' reagent for the postcolumn detection of metals in ion chromatography. However, only a superficial understanding of the mechanism of this system exists. The displacement reaction of the Zn-EDTA-PAR reagent was investigated using stopped and continuous flow methods. The kinetics and sensitivity of the Zn-EDTA-PAR reagent are strongly dependent upon the nature and the concentration of the auxiliary complexing agent. Ethylenediamine was more effective than ammonia for the determination of alkaline earth metals, both in terms of reaction kinetics (t(1/2) of 0.7 s for ethylenediamine vs 3.6 s for NH3 for Mg2+) and sensitivity (2-9-fold improvement). Furthermore, auxiliary agents suppress the direct reaction of PAR with metal ions such as Zn2+ and so should be used sparingly in postcolumn reagents.