Speciation of sub‐parts per billion levels of Cr(III) in waters by solid‐phase adsorption followed by thin‐layer x‐ray fluorescence spectrometry

A thin‐layer x‐ray fluorescence spectrometric (XRFS) method is described for the speciation of low‐parts per billion levels of Cr(III) in waters. The method is based on the adsorption of Cr(III) on to solid‐phase hydrous ferric hydroxide (HFO), and the applicability of the method is demonstrated using seawaters, a matrix widely regarded as difficult to analyse for contaminant elements. The Cr(III) fluorescence emission intensity from the solid phase vs its initial concentration in the solution phase, to a first approximation, can be described as a Langmuir isotherm, and is linearly proportional to Cr(III) concentrations up to ∼50 µg l −1 . The method detection limit for Cr(III) in seawater, determined by energy‐dispersive (ED)‐XRFS and wavelength‐dispersive (WD)‐XRFS, is 0.10 and 0.88 µg l −1 , respectively. The precision of the ED‐XRFS and WD‐XRFS analyses can generally vary between ∼5 and 10%. This relatively low precision can be attributed to the topography of the amorphous HFO impregnation in the solid phase. The error due to the presence of Cr(VI), as CrO 4 2− or Cr 2 O 7 2− , is within the experimental error, and the method is suitable for the determination of Cr(III) when it is the predominant Cr species present in the sample.