A new investigation of aqueous orthophosphoric acid speciation using Ramanspectroscopy

The Raman spectrum of aqueous phosphoric acid has been investigated at appa rent concentrations of 0.3 to 9.7 mol-dm(-3) at 25 degrees C. A quantitativ e analysis has been made over this concentration range after the determinat ion of the response coefficients of the H2PO4-; and H3PO4 species. In the f irst step, the spectra were interpreted assuming that only two species (H2P O4- and H3PO4) were present in the system. The dissociation of phosphoric a cid 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 experimenta l ones. This discrepancy can be removed if another species, the anionic dim er H5P2O8-, is taken into account. Therefore, in the second step, a modifie d interpretation of the spectra, was used to determine the concentrations o f the H2PO4-, H5P2O8-, and H3PO4 species and to deduce the corresponding de gree of dissociation of the acid, as well as the speciation of the solution s 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 significan tly for apparent phosphoric acid concentrations greater than 1 mol-dm(-3).