RAMAN AND INFRARED SPECTROSCOPIC INVESTIGATIONS OF AQUEOUS ALKALI METAL PHOSPHATE (PO43-) SOLUTIONS

phosphate-water clusters Abstract: Raman and infrared spectra on aqueous Na3PO4, and K3PO4 solutions have been measured. The modes of the PO43- species have been assigned on the basis of Td symmetry. The pK3 value of phosphoric acid has been determined by Raman spectroscopy. In addition ab initio calculations have been carried out on phosphate and phosphate-water clusters. Aqueous solutions of Na3PO4 (0.0060 – 0.592 mol/L)- and K3PO4 (0.0148 – 3.690 mol/L) solutions have been re-investigated by Raman and infrared spectroscopy. It is shown that earlier results are in many respects inadequate (band positions, depolarization degrees etc.) due to the use of non-stoichiometric products or carbonate-contaminated solutions [1, 2]. Although the phosphate ion should have a simple spectrum, there is much confusion in the literature over the assignment of the modes [3, 4]. The “free ” phosphate ion, PO43-, possesses tetrahedral symmetry (Td) and has nine normal modes which span the vibrational representation Γv(Td) = a1 + e + 2f2. All modes are Raman active (ρ(a1) = 0; ρ(e) = ρ(f2) = ), but only the f modes are infrared allowed. The four Raman active modes and the two infrared active modes have been assigned as follows: ν2(e) = 414 ± 2 cm-1, ν4(f2) = 558 ± 2 cm-1 (both depolarized), ν1(a1) = 936.4 ± 0.2 cm-1(ρ = 0.001)