Rapid sensing of molybdenum by combined colorimetric solid-phase extraction—Reflectance spectroscopy

A new and selective colorimetric solid-phase extraction (C-SPE) procedure for the determination of Mo(VI) in aqueous samples is proposed. Phenylhydrazine (PHZ) was used as a coloring reagent for molybdenum and its immobilisation onto a commercially available Amberlite XAD-16 resin was achieved. In this analytical technique analogous to a reflectometric sensor, the analytes in samples were extracted onto a solid sorbent matrix loaded with a colorimetric reagent and then quantified directly on the adsorbent surface by using a miniature reflectance spectrometer. The measurements were carried out at a wavelength of 554.05 nm since it yielded the largest divergence different in reflectance spectra before and after reaction with Mo(VI). The solution pH was controlled at pH 2.2 using phosphoric acid-primary phosphate buffer solution. A large number of metal ions known as common matrix elements for Mo(VI) did not interfere with the determination. The amount of resin-concentrated Mo(VI) was then determined in a few seconds by using a miniature reflectance spectrometer, with a total sample workup and readout time of 5 min using only 1-mL sample volumes. The linear dynamic range of Mo(VI) was found within the concentration range of 0.5–12 ppm with its LOD of 0.2 ppm. The C-SPE from different extractions (n = 5) gave a RSD of 3.3% at 9.6 ppm Mo.