1610 Volume 54, Number 11, 2000 APPLIED SPECTROSCOPY 0003-7028 / 00 / 5411-1610$2.00 / 0 q 2000 Society for Applied Spectroscopy Quantitative Phosphate Analysis in Industrial Raw Phosphoric Acid Based on Evaluation of Bandshifts in FT-Raman Spectroscopy H. ABDERRAZAK, M. DACHRAOUI, M. J. AYORA CAN ˜ ADA, and B. LENDL * Institute of Analytical Chemistry, Vienna University of Technology, Getreidemarkt 9-151, A-1060 Wien, Austria (B.L.); Department of Physical and Analytical Chemistry, University of Jae ´n, Paraje las Lagunillas s/n, E-23071 Jae ´n, Spain (M.J.A.C.); and Laboratoire de Chimie Analytique et Electrochimie, Faculte ´ des Sciences de Tunis, De ´partement de Chimie–Campus Universitaire– 1060 Tunis–Tunisie (H.A., M.D.) 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 n s (P–OH) band of phosphoric acid, which suffers a blue shift upon increasing concentration. Correlation of the exact band posi- tion 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 21 phosphoric acid was estab- lished (coefcient 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 compar- ing the results to those of gravimetry. The observed peak shift due to changes in phosphate concentration was investigated by two-di- mensional (2D) correlation analysis, which provided valuable in- sight in the inter- and intramolecular interactions in highly concen- trated phosphoric acid. Index Headings: Phosphoric acid; Quantitative analysis; FT-Raman spectroscopy; 2D correlation analysis; Hydrogen bonding; Band shifts. INTRODUCTION We report a rapid and accurate method for phosphate analysis in concentrated industrial phosphoric acid based on Fourier transform (FT) Raman spectroscopy. Industri- al raw phosphoric acid is obtained by disintegration of phosphate-containing rocks with sulfuric acid. The con- centration of phosphoric acid in the resulting product reaches up to 1300 g/L, whereas the concentration of re- maining sulfuric acid is generally below 50 g/L. The in- dustrial samples further contain traces of other ions where magnesium, aluminum, iron(III), and uoride dominate. The commercial value of raw phosphoric acid is dened by the concentration of phosphoric acid, which is cur- rently determined by gravimetry. 1 In an effort to nd an alternative to this very time-consuming and labor-inten- sive analytical technique and to provide the basis for on- line process control, we explored the potential of FT- Raman spectroscopy. Recently, Raman spectroscopy has received consider- able interest in process analysis as a versatile, fast, and robust analytical technique that is capable of solving a Received 24 May 2000; accepted 20 July 2000. * Author to whom correspondence should be sent. wide range of analytical problems even in a harsh indus- trial environment. 2–5 By shifting the vibrational spectro- scopic information from the mid-infrared to the visible near-infrared (VIS-NIR) spectral region, one can use high-performance ber-optical systems, enabling multi- plexing of a single instrument as well as remote analysis. 6 Nevertheless, for quantitative analysis by Raman spec- troscopy it is crucial to keep the experimental conditions constant over time. This can be difcult due to uctua- tions in the excitation intensity or slight changes in the alignment of the experimental setup. 7–9 As these varia- tions affect the intensity of the measured signal, they im- pair the accuracy of quantitative analysis. A solution to this common problem is to reference the Raman intensity of the analyte to a Raman signal of an invariant species or part of the optical setup that remains constant over time. As internal references, Raman bands of either the solvent or of an additional standard can be used. 10 In the case of Raman measurements at elevated temperature, such as the determination of the mole fraction of MgCl 2 in molten salt, it was also shown that the shoulder of the Rayleigh line can be used as an internal standard. 7 A further possibility is to reference the analyte signal to a Raman band of the ow cell or the coating of a ber- optic probe. The exact location of Raman frequencies contains in- formation on the chemical structure of the analyte as well as on inter- and intramolecular interactions taking place as, for example, formation of hydrogen bonds. 11 These interactions result in small shifts of Raman bands that can be used to determine physico-chemical parameters as well as latent properties of a sample. In this paper we report the rst successful evaluation of band shifts for quantitative analysis. The advantage of this approach is clear as the analytical result is not inuenced by changes in the band intensities due to slight changes in the ef- ciency of the experimental setup. The observed spectral changes due to changes in the concentration of phospho- ric acid were investigated by two-dimensional (2D) cor- relation spectroscopy, 12 indicating that several processes are responsible for the observed shift of the n s (P–OH) band towards higher frequencies with increased concen- tration. EXPERIMENTAL Reagents. All reagents were of analytical grade and obtained from Fluka (Buchs, Switzerland) or Merck (Ger-