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Ion hydration and association in aqueous potassium phosphate solutions

Eiberweiser, A., Nazet, A., Hefter, G. and Buchner, R. (2015) Ion hydration and association in aqueous potassium phosphate solutions. The Journal of Physical Chemistry B, 119 (16). pp. 5270-5281.

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Ionic hydration and ion association in aqueous solutions of KH2PO4, K2HPO4, and K3PO4 at 25 °C up to high concentrations have been investigated using dielectric relaxation spectroscopy (DRS). The three phosphate anions were found to be extensively hydrated, with total hydration numbers at infinite dilution of ∼11 (for H2PO4-), ∼20 (HPO42-), and ∼39 (PO43-). These values are indicative of the existence of a second hydration shell around HPO42- and especially PO43-. Two types of hydrating water molecules could be quantified: irrotationally bound (ib, H2O molecules essentially "frozen" on the DRS time scale) and "slow" (loosely bound water molecules with identifiably slower dynamics than bulk water). For H2PO4- over the entire concentration range and for HPO42- and PO43- at concentrations c ≲ 1 mol L-1, only "slow" H2O was detected; however, at higher concentrations of the latter two anions, an increasing fraction of ib water appears, making up ∼50% of the total hydration number close to the saturation limit of K2HPO4. Contrary to common belief, all three salts showed significant ion pair formation, with standard association constants of the 1:1 species increasing in the order: KH2PO40(aq) < KHPO4-(aq) < KPO42-(aq). The main type of ion pair in solution shifted from solvent-shared ion pairs (SIPs) to double-solvent-separated ion pairs (2SIPs) in the same sequence.

Item Type: Journal Article
Murdoch Affiliation: School of Engineering and Information Technology
Publisher: American Chemical Society
Copyright: © 2015 American Chemical Society
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