Lithium Ion-selective Electrodes Employing Tetrahydrofuran-based 16-Crown-4 Derivatives as Neutral Carriers

Abstract

The potentiometric response characteristics of lithium ion-selective membrane electrodes employing three different THF-type 16-crown-4 derivatives as ionophores were investigated. Of the THF- based crown compounds tested, a cis-type isomer of the derivative with only methyl groups (H/CH₃) at four bridged positions (cis-THF-I) was found to exhibit the best selectivity for Li⁺ over other cations. The composition of the cis-THF-I-based PVC-matrix membrane was optimized and its performance was compared with those based on lithium ionophores reported previously. The new cis-THF-I-based membrane exhibits remarkably enhanced selectivities for lithium over other cations: logK^pot_Li,Na = –2.8, logK^pot_Li,K = –4.6, logK^pot_Li,NH4 = –5.4, logK^pot_Li,Ca = –5.7 and logK^pot_Li,Mg = –5.4. Furthermore, it was found that serum components do not interfere significantly with the lithium measurements when the cis-THF-I-based electrode is used. The response of the electrode was found to be linear in the clinical range of interest.