This study involves the preparation and investigation of a novel and highly selective poly(vinyl chloride)-based membrane of 2-((5-(2-hydroxy-3-methoxybenzylideneamino)-2H-1,2,4-triazol-3-ylimino)methyl)-6-methoxyphenol Schiff base ligand (HMBT), which is a neutral ionophore with sodium tetraphenyl borate (STB) in the form of an excluder and o-nitrophenyloctyl ether (o-NPOE) in the form of solvent mediators (plasticizing) as a Ni(II)-selective electrode. The observation of optimal performance was done wherein the membrane was shown to have the HMBT–PVC–NPOE-STB composition of 4:32:63:1.It worked effectively across a broad range of concentration (1.0 × 10−8 to 1.0 × 10−2 mol L−1). Meanwhile, the Nernstian slope was recorded as 29.3 mV per decade of activity between pH 3.0 and 8.0. The response time of this electrode was fast at 11 s which was used for a span of 100 days with sound reproducibility. According to the selectivity coefficients for trivalent, divalent, and monovalent cations, excellent selectivity was indicated for Ni(II) ions across a large number of citations, whereas no interference was caused by anions like PO43-, SO42− and Cl−. The proposed method in this study was applied successfully to determine Ni(II) content in different samples of water, obtaining suitable recoveries. Additionally, the probed sensor is utilized as indicator electrode when considering Ni2+ ion potentiometric titration against EDTA. In addition, the chelate’s geometry and structure of the complex formed between Ni2+ ions and HMBT, abbreviated as HMBT-Ni2, was evaluated by separating the solid product. Complex structure was confirmed based on alternative analytical and spectral methods to be structured in the bimetallic form with the formula [Ni2(HMBT)(H2O)2 Cl2]. The diamagnetic nature of the complex, which was concluded from the room temperature magnetic moment measurement combined with the UV-Vis measurement, suggested the square planar geometry around the Ni centers.