Abstract
The Raman spectrum of aqueous phosphoric acid has been investigated at apparent concentrations of 0.3 to 9.7 mol-dm(-3) at 25 degrees C. A quantitative analysis has been made over this concentration range after the determination of the response coefficients of the H2PO4-; and H3PO4 species. In the first step, the spectra were interpreted assuming that only two species (H2PO4- and H3PO4) 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 H5P2O8-, is taken into account. Therefore, in the second step, a modified interpretation of the spectra, was used to determine the concentrations of the H2PO4-, H5P2O8-, and H3PO4 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)