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Modelling of proton and metal exchange in the alginate biopolymer

De Stefano, Concetta, Gianguzza, Antonio, Piazzese, Daniela, Sammartano, Silvio
Analytical and bioanalytical chemistry 2005 v.383 no.4 pp. 587-596
Macrocystis pyrifera, anions, biopolymers, calorimetry, cations, dissociation, electrodes, electrolytes, enthalpy, entropy, equations, ionic strength, lithium chloride, magnesium, metal ions, models, potassium chloride, sodium, sodium alginate, sodium chloride, titration
Acid–base behaviour of a commercial sodium alginate extracted from brown seaweed (Macrocystis pyrifera) has been investigated at different ionic strengths (0.1≤I/mol l⁻¹≤1.0) and in different supporting electrolytes (Et₄NI, NaCl, KCl, LiCl, NaCl+MgCl₂), with the aim of examining the influence of ionic medium on the proton-binding capacity and of quantifying the strength of interaction with light metal ions in the perspective of speciation studies in natural aqueous systems. Potentiometric ([H⁺]-glass electrode) and titration calorimetric data were expressed as a function of the dissociation degree (α) using different models (Henderson–Hasselbalch modified, Högfeldt three parameters and linear equations). The dependence on ionic strength of the protonation constants was taken into account by a modified specific interaction theory model. Differences among different media were explained in terms of the interaction between polyanion and metal cations of the supporting electrolytes. Quantitative information on the proton-binding capacity, together with the stabilities of different species formed, is reported. Protonation thermodynamic parameters, at α=0.5, are log K ᴴ=3.686±0.005, ΔG ⁰=−21.04±0.03 kJ mol⁻¹, ΔH ⁰=4.8±0.2 kJ mol⁻¹ and TΔS ⁰=35.7±0.3 kJ mol⁻¹, at infinite dilution. Protonation enthalpies indicate that the main contribution to proton binding arises from the entropy term. A strict correlation between ΔG and TΔS was found, TΔS=−9.5–1.73 ΔG. Results are reported in light of building up a chemical complexation model of general validity to explain the binding ability of naturally occurring polycarboxylate polymers and biopolymers. Speciation profiles of alginate in the presence of sodium and magnesium ions, naturally occurring cations in natural waters, are also reported.