Journal of Solution Chemistry, Vol. 29, No. 3, 2000 A New Investigation of Aqueous Orthophosphoric Acid Speciation Using Raman Spectroscopy Mourad Cherif, 1 Arbi Mgaidi, 1 Naceur Ammar, 1 Ge ´raldine Valle ´e, 2 and Walter Fu ¨ rst 2 * Received December 12, 1998; revised October 18, 1999 The Raman spectrum of aqueous phosphoric acid has been investigated at apparent concentrations of 0.3 to 9.7 mol-dm -3 at 25°C. A quantitative analysis has been made over this concentration range after the determination of the response coefficients of the H 2 PO - 4 and H 3 PO 4 species. In the first step, the spectra were interpreted assuming that only two species (H 2 PO - 4 and H 3 PO 4 ) were present in the system. The dissociation of phosphoric acid obtained in this case is consistent with the values Preston and Adams (1) obtained, and which was also found from Raman spectroscopy. However, a discrepancy exists between the representations from spectra and experimental ones. This discrepancy can be removed if another species, the anionic dimer H 5 P 2 O - 8 is taken into account. Therefore, in the second step, a modified interpretation of the spectra, was used to determine the concentra- tions of the H 2 PO - 4 ,H 5 P 2 O - 8 , and H 3 PO 4 species and to deduce the corresponding degree of dissociation of the acid, as well as the speciation of the solutions as a function of the apparent concentration of phosphoric acid. As in the results Elmore and co-workers, 2 which were deduced from pH measurements, the degree of dissociation reaches a minimum and then increases significantly for apparent phosphoric acid concentrations greater than 1 mol-dm -3 . KEY WORDS: Phosphoric acid; speciation; degree of dissociation; Raman spectroscopy. 1. INTRODUCTION Several methods of phosphoric acid production involve leaching of calcareous phosphorites by sulfuric acid, which produce a solution composed mainly of phosphoric acid and calcium sulfate. The calcium sulfate is also 1 Laboratoire de Physico-Chimie des Mate ´riaux, I.P.E.S.T. BP 51, 2070 la Marsa, Tunisie. 2 Laboratoire Chimie & Proce ´de ´s, ENSTA, 32 Boulevard Victor, 75739 Paris cedex 15, France. 255 0095-9782/00/0300-0255$18.00/0  2000 Plenum Publishing Corporation