Potentiometric determination of threonine in acid solutions by used MF-4SC membranes with sulfo-containing dopants
Amino acids are components of pharmaceuticals, products for children and sports nutrition, and also they use as feed additives. The electrochemical methods, in particular dialysis and electrodialysis, are being developed for the separation and purification of amino acids. The necessity to control of processes efficiency and the quality of products in the production of amino acids necessitates the development of methods for their rapid determination, taking into account the variable medium pH. The aim of this work was to reduce the influence of hydroxonium ions on the determination of threonine cations and zwitterions in aqueous solutions at pH<7. For this, cross-sensitive DP-sensors (analytical signal is Donnan potential) based on MF-4SC membranes with oxides surface-modified with sulfo-groups were used. The characteristics of DP-sensors were determined in the equimolar solutions of threonine (Thr) and hydrochloric acid with component concentrations from 1.0 10<sup>-4</sup> to 1.0 10<sup>-1</sup> М. In the pH range of the investigated solutions (1.73-4.25), threonine is present in the form of singly charged cations (Thr+) and zwitterions (Thr±). The perfluorosulfonic membranes (MF-4SC) and the hybrid materials based on them with nanoparticles of hydrated zirconia and silica, surface modified by the -(CH<sub>2</sub>)<sub>3</sub>-SO<sub>3</sub>H and -SO<sub>3</sub>H sulfo containing fragments, were used as materials for DP-sensors. The samples of membranes were provided by the Laboratory of Ionics of Functional Materials of the Kurnakov Institute of General and Inorganic Chemistry RAS (the head of laboratory is Yaroslavtsev A.B., Dr.Sci. (Chemistry), prof., corresponding member of the RAS;a synthesis of materials is carried out by Safronova E.Yu., senior scientific researcher, Ph.D. (Chemistry)). The influence of the concentration and proton-donor properties of dopants introduced into the membranes on the characteristics of cross-sensitive DP-sensors in threonine solutions at pH<7 are investigated.It is shown, that the sensitivity of DP-sensors increases to cations and zwitterions of threonine and decreases to hydronium ions with an increase in the volume fraction of dopants in membranes due to a simultaneous decrease in the volume of the intraporous space and an increase in the number of binding sites for analyte ions. Under such conditions, a transition of the bulk Thr+ and Thr± ions is apparently possible, and their interaction with the sulfo-groups on the membrane pore walls and on the oxide surface excludes the part of protons from the ion exchange. The highest sensitivity of DP-sensors to analyte ions and the smallest sensitivity to interfering ion was obtained by the using samples containing 3-5 wt.% of silica modified by -(CH<sub>2</sub>)<sub>3</sub>-SO<sub>3</sub>H groups. Using these membranes provided a reduction of the relative error determination of threonine cations and zwitterions in 2.5-4 times compared with the initial sample in the concentration range of analyte from 1.010<sup>-4</sup> to 1.010"<sup>1</sup> M (pH 1.73-4.25).