Highly selective spectrophotometric determination of trace cobalt and development of a reagentless fiber-optic sensor Original Research Article

A highly selective spectrophotometric method for the determination of trace cobalt based upon the reaction of cobalt(III) with 1-(2-pyridylazo)-2-naphthol (PAN) in the presence of peroxodisulfate is described. Following an experimental design procedure based on a central composite design (CCD) method and a specifically orientated univariate optimization, it was found that an alkaline media of pH=11 and controlled peroxodisulfate concentration are crucial factors for enhancing the formation and aqueous solubility of the Co(III)–PAN chelate. Immobilization of PAN into a cellulose acetate membrane was also investigated leading to the construction of a reagentless fiber-optic sensor. According to the proposed reaction mechanism dissolved Co(II) is easily oxidized to Co(III) by peroxodisulfate and then readily complexed with PAN, either in solution or on the membrane surface giving a unique absorbance peak at 645–650 nm. Linear calibration graphs over the concentration range 0.02–0.5 mg l−1 with a 3σ limit of detection of 0.01 and 0.1–2 mg l−1 with a limit of detection of 0.07 mg l−1 were obtained by applying the photometric method and the fiber-optic sensor, respectively. Excellent agreement with the reference atomic absorption spectrometric method was achieved when the proposed methodology was applied for the determination of cobalt in various samples of industrial, environmental and biological importance.