Quantitative Phosphate Analysis in Industrial Raw Phosphoric Acid Based on Evaluation of Bandshifts in FT-Raman Spectroscopy

We present a new approach for quantitative chemical analysis by Raman spectroscopy with the example of phosphate analysis in highly concentrated industrial raw phosphoric acid. The proposed method is based on the direct measurement of the Raman shift of the v_s(P-OH) band of phosphoric acid, which suffers a blue shift upon increasing concentration. Correlation of the exact band position rather than band intensity with the concentration of the analyte eliminated the need to reference the measured Raman signal to an internal or external standard, which is usually needed to account for changes in laser power or optical throughput. A calibration curve ranging from 100 to 1400 gL⁻¹ phosphoric acid was established (coefficient of variation of the method, V_xo: 0.99%; analysis time: 7 min per sample). The applicability of the developed method to industrial samples was proven by an interference study including sulfate, magnesium, aluminum, and iron(III) as well as by comparing the results to those of gravimetry. The observed peak shift due to changes in phosphate concentration was investigated by two-dimensional (2D) correlation analysis, which provided valuable insight in the inter- and intramolecular interactions in highly concentrated phosphoric acid.