Nitrogen-rich heterocyclic systems, particularly 1,2,4-triazole derivatives, are attractive sources of functionally diverse compounds and promising modifiers for electrochemical sensors. Ionometry, as a branch of potentiometric analysis, focuses on developing highly selective ion-selective electrodes (ISEs) capable of providing direct assessment of ion activity or ionogenic forms of analytes in solution. β-Estradiol is a biologically important steroid hormone; its quantification is relevant for pharmaceutical analysis. Schiff bases as membrane electroactive components represent a viable strategy to enhance sensor selectivity.

The aim of the study was to create and study the electrochemical characteristics of an ISE with a plasticized membrane based on 5-(3-fluorophenyl)-4-amino-1,2,4-triazole-3-thiol with lipophilic ionogenic additives of various nature – tetraoctylammonium bromide (TOABr) and sodium tetraphenylborate (NaB(C6H5)4, NaBPh).

Materials and methods. The electroactive Schiff base was synthesized by condensation of 2-hydroxy-5-(phenyldiazenyl)benzaldehyde with 5-(3-fluorophenyl)-4-amino-1,2,4-triazole-3-thiol in n-butanol (2 h reflux, 12 h standing), followed by purification (DMF recrystallization) and structural confirmation by elemental analysis and 1H NMR (DMSO-d6, 400 MHz). A PVC membrane (0.2 g) was prepared containing 2 wt.% electroactive compound, 0.5 wt.% ionogenic additives (NaBPh or TOABr), polyvinyl chloride (PVC; one-third of the mass), and nitrophenyloctyl ether (two-thirds) as plasticizer, using tetrahydrofuran as the casting solvent. Potentiometric measurements were performed at 20–25 °C (Ezodo PL-700PV) versus an Ag/AgCl reference electrode. Calibration was carried out in the 10-2–10-4 M β-estradiol range (1000–10 ppm) with reproducibility control every 2 h.

Results. The use of plasticized PVC membranes and a solid-contact configuration was justified to improve potential stability and minimize drift. The presence of azo and azomethine fragments in the modifier implies a dual-pathway electroanalytical process. A mathematical model incorporating diffusion supply and electrode surface coverage by reduction products was proposed; linear stability analysis using the Routh–Hurwitz criterion indicated a broad parameter region where a stable steady state is attainable, enabling formation of an interpretable analytical signal. The detection limit is associated with monotonic instability, whereas potential oscillatory behavior is expected beyond the analytical working range.

Conclusions. A plasticized PVC-based ISE incorporating a triazole-derived Schiff base and ionogenic additives (NaBPh / TOABr) was proposed for potentiometric β-estradiol determination; theoretical analysis supports its electroanalytical feasibility and controllable steady-state operation.