Speciation of Zn, Fe, Ca and Mg in wine with the Donnan Membrane Technique

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.