Simultaneous Spectrophotometric Determination of Transition-Metal Ions Using Hydroxytriazene as a Reagent

Abstract

A simple, rapid, sensitive and economical spectrophotometric method has been developed for the simultaneous determination of copper(II), nickel(II) and cobalt(II) using a hydroxytriazene as the chromogenic reagent. Hydroxytriazenes possess the characteristic –N=N–N(OH)– functional grouping, which behaves as a bidentate (O,N) donor and forms intensely coloured, stable chelates with a large number of metal ions in the visible region. The reagent was synthesised by a diazotisation–coupling route, purified to constant melting point and characterised before use. The colour reaction proceeds optimally at pH 6.5 in acetate buffer with a molar excess of reagent, develops fully within fifteen minutes at room temperature and remains stable for at least one hour. The copper, nickel and cobalt chelates absorb maximally at 545, 470 and 590 nm, with molar absorptivities of 1.86 × 10⁴, 1.42 × 10⁴ and 1.65 × 10⁴ L mol⁻¹ cm⁻¹ and Sandell's sensitivities below 0.005 µg cm⁻², respectively.

Each metal obeyed Beer's law over the trace-level range 0.2–1.8 µg mL⁻¹ with correlation coefficients exceeding 0.999. Exploiting the additivity of absorbances, the simultaneous-equation method resolved binary and ternary mixtures with recoveries of 99–101 %. The stoichiometry of the chelates, established by Job's method of continuous variation and the mole-ratio method, was 1:1 for copper and 1:2 for nickel and cobalt, with high conditional stability constants (log K = 8–13). The method was validated for linearity, accuracy, precision, detection and quantification limits, robustness and ruggedness, all parameters meeting accepted criteria. Common foreign ions were tolerated at high levels and the few interfering transition metals were suppressed by selective masking. Application to water, pharmaceutical and alloy samples gave results in excellent agreement with certified or labelled values, with no statistically significant difference by the t- and F-tests. The proposed method therefore offers a genuine, low-cost alternative to atomic and plasma spectrometric techniques for the simultaneous determination of transition metals.