Free concentrations of Zn2+, Fe3+, Ca2+ and Mg2+ in a red wine (Raimat, Catalonia, Spain) have been determined, with the Donnan Membrane Technique (DMT) for the first time. The required equilibration time benefits from the acceptor solution including major cations. K+ and Na+, mainly unbound to any ligand in the sample, have been identified as suitable reference ions. A free Zn concentration of 1.76μmolL-1 determined with DMT was in excellent agreement with the free Zn concentration independently provided by the electroanalytical technique Absence of Gradients and Nernstian Equilibrium Stripping (AGNES), 1.7μmolL-1, amounting to 14.4% of the total Zn. The free concentrations found in this wine were 1.79μmolL-1 Fe3+, 1.11mmolL-1 Ca2+ and 3.4mmolL-1Mg2+ (8.82%, 40% and 57% of their total concentrations). Prior to the application of the techniques to the red wine, they had been cross-validated in Zn-tartrate solutions.
Keywords: Absence of Gradients and Nernstian Equilibrium Stripping; Donnan Membrane Technique; Free metal; Iron; Wine; Zinc.
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