Spectrophotometric and Analogue Derivative Spectrophotometric Determination of Chromium(III) with 2-(5-Bromo-2-pyridylazo)-5-(N-propyl-N-sulfopropylamino)phenol

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Abstract
A sensitive method for the spectrophotometric determination of chromium(III) with 2-(5-bromo-2-pyridylazo)-5-(N-propyl-N-sulfopropylamino)phenol (5-Br-PAPS) is described. 5-Br-PAPS reacts with chromium(III) to form a 1:2 (metal:ligand) complex in weakly acidic media upon heating at 95 °C. The complex has an absorption maxima at 592 nm and gives a constant absorbance in the pH range of 3.4–6.2. The molar absorptivity and sensitivity for an absorbance of 0.001 are 8.94×104 dm3 mol−1 cm−1 and 0.58 ng cm−2 respectively. The complex, once formed, is not decomposed by the addition of trans-1,2-cyclohexanediamine-N,N,N′,N′-tetraacetic acid (CDTA). Thus, the selectivity of the method can be increased by the use of CDTA as a masking agent. The sensitivity of the method can be increased by employing analogue-derivative spectrophotometry, chromium(III) determination at parts-per-billion levels being feasible.