Determination of metal ions concentration and speciation in seawater by titration with an iminodiacetic resin.

Achelating resin containing iminodiacetic groups, Chelex 100 which is often employed for sorbing metal ions from complex matrices, was used for separating and preconcentrating heavy metal ions from seawater samples. The resin titration method was applied for determining the total concentration. The experimental procedure consists of measuring the concentration of the metal sorbed on Chelex 100 from solutions with different ratios volume of aqueous phase/amount of resin. The total metal ion concentration is obtained from the experimental data by a model for the sorption on the resin, the parameters of which are obtained by a non-linear regression procedure. In this way not only the total metal ion concentration can be determined, but also the side reaction coefficient M.I/, if the sorption equilibria of the considered metal ion on the resin are exactly known, as is the case of Chelex 100 considered here. The resin titration with Chelex 100 was utilized for determining the total metal ion concentration in certified seawater samples and real samples from the Ligurian Sea. It allowed the simultaneous determination of the total concentration of manganese(II), cadmium(II), nickel(II) and copper(II), in acceptable agreement with the certified values. Moreover, the side reaction coefficients of some complex species could be evaluated. They were very high, and strongly dependent on the acidity of the sample solution. For instance, values varying from 109 to 1011 were obtained for copper (II) in the different samples considered, at pH around 7. Chelex 100 is such a strong sorbing substance that it is able to compete with the stable complexes for the combination with the heavy metal ions considered here.