A simple methods have been developed for determination of Cu(II) ions in aqueous solutions. The spectrophotometric method relied mainly on the reaction between Cu(II) ions and the azo dye ligand named N-diaminomethylene-4-(2,4-dihydroxy-phenylazo)-benzenesulfonamide (H2L) at pH 10.0. The influence of parameters such as concentration, pH and reaction time were inspected. A linear relationship (R2 = 0.9992) between absorbance and the concentration of Cu(II) was obtained at the maximum absorption peak of 474 nm within 1.6–9.6 × 10−6 mol L−1 concentration range. The limit of detection for Cu(II) ion and limit of quantitation were 1.1 × 10−7 mol L−1 and 3.7 × 10−7 mol L−1, respectively. The potentiometric method is based on a novel poly(vinyl chloride) membrane, containing the synthesized azo dye as an ionophore, was used to developed a Cu(II)- selective sensor. This newly developed sensor revealed a Nernstian response over Cu2+ ion in a concentration range 1.0 × 10−6–1.0 × 10−2 mol L−1 with cationic slopes of 29.5 ± 0.2 mV decade−1 and detection limits of 3.0 × 10−6 mol L−1 copper(II) for o-nitrophenyl-octyl ether (o-NPOE) based membrane sensor. The electrode showed good discrimination toward Cu2+ ions with respect to most common cations. The advantages of the proposed methods are their simplicity, selectivity, and high sensitivity. In addition, the sensor has been used as indicator electrode in the potentiometric titration of Cu2+ ion against EDTA. The structure and geometry of the complex formed between Cu(II) and H2L ligand was identified via isolation of the solid complex; Co(II) an Ni(II) complexes were synthesized as well. The geometrical structure around the metal centers were proved to be square planar for Cu(II) complex and tetrahedral for Co(II) an Ni(II) complexes.