Spectrophotometric determination of Mercury(II) ions in laboratory and Zamzam water using bis Schiff base ligand based on 1,2,4-Triazole-3,5-diamine and o-Vaniline

•Highly sensitive spectrometric method for mercury determination.•Promising limit of detection i.e 0.016 µg/L was obtained.•mercury reaction chemistry with HMBT is discussed.•Analytical applications, validation and successful literature comparison was performed.•Future trends: Clear superiority over literature methods and as an alternative routine analysis method.