Spectrophotometric study of the chromium(III)/azide system in the ultraviolet region and its analytical application

Abstract

IntroductionAzide and thiocyanate complexes of several metallic cations have been systematically studied in our laboratories with a view to understanding their coordination chemistry (equilibria) [1][2][3] and developing their analytical applications [4][5][6][7][8][9][10][11][12]. There is great analogy between the azide and the thiocyanate ligands (pseudohalides), since they develop the same red color in acid solution containing iron(III). Some of our studies have been of a comparative character, showing that the azide system is much more stable than the thiocyanate one. As part of these continuous investigations, it is expected that chromium(III) will form complexes with the pseudohalide azide in the same way that other transition metals such as iron, copper, nickel, and cobalt.In the literature, studies carried out by Sherif and Orab [13,14] in aqueous medium using the visible region revealed the existence of a relationship between the several free ligand (azide) and chromium(III) concentrations with absorbance, producing colorings ranging from violet to green. Moreover, the solution was reported to develop blue tones under great ligand excess. In the work of Templeton & King [15], the effect of perchloric acid concentration on the formation of pentaaquoazidochromium(III) compounds was also reported. The main objective of the latter work was to determine the kinetics and equilibrium of this system. The authors reported maximum absorption at 270 nm, under highly acid concentrations, which was attributed to interactions between azidochromium(III) and/or hydrogen azidochromium(III) and the perchlorate ion. The Abstract: A sensitive and alternative method for the spectrophotometric determination of chromium(III) based on the formation of chromium(III)/azide complexes was established by investigating a new band in the ultraviolet region. The best experimental conditions for the analytical determination of this metallic ion were: ligand and perchloric acid analytical concentration = 493 and 12.0 mmol L -1 , respectively; aqueous medium; T = 25.0 ºC; contact time = 1 hour. The maximum molar absorptivity coefficient occurred at 287 nm (average 1.481 ± 0.008 × 10 4 L mol -1 cm -1 ), leading to the determination of metal ion concentrations one hundred times lower than the ones formerly determined in the visible region. The system obeys Beer's Law and is suitable for chromium determination in the 0.702-2.81 mg L -1 concentration range (15-65% T, 1.00 cm-width quartz cells).Analytical applications of the current method were tested with a nutritional supplement containing chromium. Results were compared with those obtained with atomic absorption spectrometry.