Quantitation of Sulfur Oxyanions in Industrial Metal Refining Solutions with Dispersive and Fourier Transform Raman Spectroscopy

Fourier transform (FT)-Raman and dispersive, visible excitation Raman spectra are reported for standard solutions and highly colored industrial metal refining solutions containing sulfur oxyanions, primarily sulfate and sulfamate. Good-quality spectra are obtained for both standard and industrial solutions. However, quantitation of both sulfur oxyanion concentrations is complicated by self-absorption effects. In the FT-Raman spectra, self-absorption of the incident laser beam yields an exponential correlation between Raman peak intensity, directly proportional to the apparent sulfate concentration, and the actual sulfate concentration. An internal standard, dispersive Raman spectroscopy, and excitation at 647 nm were used in an attempt to apply a self-absorption correction for absorption of the incident laser beam by ratioing the relative Raman intensities of the standard and species of interest. This approach improves the accuracy for sulfate and sulfamate determination to ±6.1% and ±3.4%, respectively. This study represents the most detailed characterization presented so far of the errors expected in quantitative Raman spectroscopy of industrial solutions. These results suggest that Raman spectroscopy is a uniquely powerful technique for the qualitative and quantitative characterization of industrial metal refining solutions.