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 postc olumn detection of metals in ion chromatography. However, only a super ficial understanding of the mechanism of this system exists. The displ acement reaction of the Zn-EDTA-PAR reagent was investigated using sto pped and continuous flow methods. The kinetics and sensitivity of the Zn-EDTA-PAR reagent are strongly dependent upon the nature and the con centration 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 ethylenediam ine vs 3.6 s for NH3 for Mg2+) and sensitivity (2-9-fold improvement). Furthermore, auxiliary agents suppress the direct reaction of PAR wit h metal ions such as Zn2+ and so should be used sparingly in postcolum n reagents.