Macrocyclic compounds, due to their selective receptor properties and ease of structural modification, have been employed as ionophores in construction of ion selective electrodes for the determination of alkali, alkaline earth and many transition metal ions [1–4]. In the present paper, we introduce a new ion selective electrode based on the incorporation of 4\',4\'\'(5\'\')-Di-tert-butyldibenzo-18-crown-6 (DtBDB18C6) that is selective to lantanium (III) cations was evaluated electrochemically, and a Nerenstian slope (20.2±2 mV decade−1) over a concentration range of 1.0×10−2–0×10−6 M and a detection limit of 5.6×10−6 M were observed. The electrode reached equilibrium in a short period of time(10 s) due to the fast exchange kinetics of the complexation– decomplexation of La(III) cations with the ion carrier at the test solution–membrane interface and most significantly, very good selectivity with a number of ions the transition and heavy metal ions as well as some mono, di and trivalent cations such as K+, Na+, Ca2+, Pb2+, Ni2+, Ba2+, Zn2+, Sr2+, Cu2+, Fe3+, Al3+and Cr3+ ions. The potentiometric selectivity coefficient values as determined by Mixed solution methode (MSM) indicate excellent selectivity for La3+ ions over interfering cations. In addition, it can be used at least for a period of 9 weeks without any significant divergences in its potential response. In the present study, the effect of the nature and amount of plasticizer on the potential response of the proposed La(III) potentiometric sensor was investigated, the potential remained constant over a pH range of 7.0–11.5. Therefore, the working pH range of the proposed electrode is 7.0–11.5. In our preliminary studies, to examine the interactions between La (III) cations and the ligand, complex formation between the ligand and lantanium(III) cations was investigated. For this purpose, a conductometric titration was performed, and the change in molar conductivity (Λm) versus the ligand to cation molar ratio([L]t/[M]t) was studied to determine the stability constant of the complexation of 4\',4\'\'(5\'\')-Di-tert-butyldibenzo-18-crown-6 (DtBDB18C6) with lantanium(III) cations in methylacetate (MeOAc). The stoichiometry of the complex formed between lantanium(III) cation and DtBDB18C6 was 1:1. Additionally, the proposed sensor was used as an indicator in the potentiometric titration of EDTA and titration of sodium hydroxid.

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