Raman and Infrared Spectroscopic Investigations of Aqueous Alkali Metal Phosphate (po4) Solutions

Raman and infrared spectra on aqueous Na3PO4, and K3PO4 solutions have been measured. The modes of the PO4 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, PO4, 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) = 3⁄4), 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, ν4(f2) = 558 ± 2 cm (both depolarized), ν1(a1) = 936.4 ± 0.2 cm(ρ = 0.001), and ν3(f2) = 1006 ± 2 cm (depolarized). The peak position and full width at half height (fwhh) of ν1 PO4 are concentration dependent: in K3PO4 solutions shifts the peak position with concentration to slightly lower wavenumbers [4]. The two infrared-active f modes coincide in their peak position with the Raman modes as predicted. Substitution of water with heavy water causes a shift of ν1 PO4 to 933.5 ± 0.2 cm, whilst ν3(f2) shifts to = 1000 ± 2 cm [4]. This, and the strong influence on the OD and OH mode, is a clear indication of strong hydrogen bonding, P-O...D(H) (cf. [4, 5]). Phosphate hydrolyses according to eq. (1): PO4 + H2O ↔ HPO4 + OH (1). The strongest mode of the HPO4 at 990 cm, ν1(a1) increases with dilution until it becomes the strongest mode in the Raman spectrum (cf. Figure 1). In the most concentrated K3PO4 solution (mol ratio of salt : H2O = 1 : 12), the tetrahedral symmetry of PO4 is disturbed and the ν1 mode becomes infrared active. In the supercooled hydrate melt (salt : H2O = 1 : 8), a low –frequency asymmetry of the ν1 mode has been noticed [4]. The water content for the most concentrated solution and the melt is not sufficient to hydrate all the ions, and the formation of contact ion pairs is possible. The quantitative Raman spectroscopic determination of the hydrolysis degree, α, and the determination of pK3 of the phosphoric acid in these aqueous Na3PO4and K3PO4solutions has been undertaken [4]. The hydrolysis of PO4 in aqueous solution may be written according to eq.