LITHIUM ION-SELECTIVE MEMBRANE

In a lithium (Li) ion-selective membrane (A) comprising a polymeric support, a plasticizer, a conductive compound and a lithium ion selective ionophore (I), (I) consists of dibenzyl-14-crown-4 (Ia) or its derivatives and forms 0.5-3 wt. % of (A). Independent claims are included for: (1) the production of (A), by completely dissolving the polymeric support, plasticizer, conductive compound and (I) in tetrahydrofuran (THF) with magnetic stirring at room temperature; decanting the obtained clear solution to cause crystallization by evaporation of the THF at room temperature and recovering the obtained membrane; (2) a Li ion-selective electrode (B), comprising an internal reference element, an internal solution of Li chloride and a (I)-containing Li ion-selective membrane as defined above for (A); (3) a device for determining the Li ion concentration of a fluid, comprising a first, comparative measuring electrode permanently immersed in a reference solution of Li ions having constant potential; a second Li ion-selective electrode as defined for (B), immersed in test solution; equipment for measuring the potential difference between the electrodes; and analytical and calculating systems for determining the Li ion concentration based on the potential difference; and (4) a corresponding Li ion concentration determination method.