Naked-eye detection of heavy metal ions after forming complexes with xylenol orange by column method

Abstract

Preliminary naked-eye detection of heavy metal ions (Cd(II), Co(II), Cu(II), Ni(II), Pb(II), Zn(II), Fe(III) and Cr(III)) complexed with xylenol orange adsorbed onto silica gel by batch and column methods was investigated. The color change of all metal complexes in comparison with control was observed at pH 5 in acetate buffer changing from orange to red for Cd(II), Cu(II), Ni(II), Pb(II) and Zn(II), to purple for Co(II) and Cr(III) and to black for Fe(III). The colors of only Ni(II), Co(II) and Fe(III) complexes at low concentration were obviously observed. The maximum f(R) signals measured by DR-UV-Vis spectrophotometer of metal complexes adsorbed onto silica gel were found at 580 nm for Co(II), Ni(II) and 595 nm for Fe(III). The formation ratio of the complex of Co(II), Ni(II) and Fe(III) and xylenol orange in aqueous solution was found to be 1:1 by Job’s plot method. The conditions of Co(II), Ni(II) and Fe(III) naked-eye detection by the column method were optimized. The concentration of xylenol orange was 0.01 mM. The flow rate of 1 mL/min passed through a mini-column (4 mm o.d., 2 cm length) packed with 25 mg of silica gel was found. The maximum sample volumes were 50 mL for Co(II) and 100 mL for Ni(II) and Fe(III). The lowest concentration of Co(II), Ni(II) and Fe(III) detected by naked-eye were 0.6, 0.6 and 0.06 mg/L, respectively. The working concentration ranges of 0.1-60 mg/L for Co(II) and Ni(II) 0.1-120 mg/L for Fe(III) were used to prepare the naked-eye color calibrations. Interfering ions as Na⁺, K⁺, Ca²⁺ and Mg²⁺ were also studied. The results showed that high concentration of Na⁺, K⁺, Ca²⁺ and Mg²⁺ did not interfere the naked-eye detection of Co(II), Ni(II) and Fe(III). The proposed naked-eye method can be applied for determination of Co(II), Ni(II) and Fe(III) in various water sources at trace level without special instruments.