Analytical chemistry study of some hydroxyazo-metallochromic reagents

Abstract

In this research work, two hydroxyazo compounds namely 1, 3-bis(1,8-dihydroxy-3, 6-disulfo-2-naphthylazo)-benzene (m-X dye) and 1, 4-bis(1, 8-dihydroxy-3, 6-disulfo-2-naphthylazo) benzene (p-X dye) were synthesized by the author in the laboratory. Both m-X dye and p-X dye were then isolated and purified. Paper chromatographic studies of these compounds using nineteen solvent systems were carried out. Among those, the systems of water : ammonia : n-butanol with the ratio 25 : 2 : 2 by volume and of isobutanol : ethanol : water with the ratio of 5 : 3 : 2 by volume were found to be mostly suitable and practical for the characterization of m-X dye and p-X dye respectively. Additionally, some structural elucidation of these two synthesized dyes were also studied by infrared absorption spectrometry and proton and carbon-13 nuclear magnetic resonance spectroscopy. Metallochromic properties of the two synthesised dyes were primarily investigated at various pH and the effect of ethylenediaminetetra-acetic acid a mass masking agent was also observed. Among the metal ions investigated, thorium(IV) in acidic media showed quite intensive colour reactions with the two synthesised dyes whereas in neutral and moderate alkaline solutions, copper (II), chromium (III), cobalt, nickel, lead gave slightly visual colour reactions. By the use of conventional approaches, visible spectrophotometric methods for the determination of thorium using the two synthesised dyes as metallochromic reagents were systematically developed during the course of this research work. Experimentally, it was ironically found that the performance of the m-X dye as being a spectrophotometric reagent was far better than of the p-X dye. Considering the results obtained from the developed spectrophotometric method for the determination of thorium using m-X dye, it was found that, at pH 4, the blue colour 1:1 mole ratio thorium-mX dye complex was developed simultaneously and stable for at least forty eight hours. The apparent stability constant of the complex determined by Job’s plot was found to be 4.1 x 10⁷ (equivalent to pK = 7.61). The conformity to Beer’s law was in the range of 46-2320 ppm of thorium. At 610 nm, the molar absorptivity was 1.31 x 10⁴ 1 mol⁻¹ cm⁻¹, the specific absorptivity was 0.06 ml g⁻¹ cm⁻¹ and Sandell sensitivity was 0.0177 µg cm⁻². The effect of twenty-one cations and nine anions on the thorium determination using m-X dye were also reported. It may be concluded that the m-X dye which was synthesised it this work can be used as a potential spectrophotometric reagent for thorium.