Zusammenfassung
Ionic hydration and ion assocn. in aq. solns. of KH2PO4, K2HPO4, and K3PO4 at 25 °C up to high concns. have been investigated using dielec. relaxation spectroscopy (DRS). The three phosphate anions were found to be extensively hydrated, with total hydration nos. at infinite diln. of ∼11 (for H2PO4-), ∼20 (HPO42-), and ∼39 (PO43-). These values are indicative of the existence of a second ...
Zusammenfassung
Ionic hydration and ion assocn. in aq. solns. of KH2PO4, K2HPO4, and K3PO4 at 25 °C up to high concns. have been investigated using dielec. relaxation spectroscopy (DRS). The three phosphate anions were found to be extensively hydrated, with total hydration nos. at infinite diln. of ∼11 (for H2PO4-), ∼20 (HPO42-), and ∼39 (PO43-). These values are indicative of the existence of a second hydration shell around HPO42- and esp. PO43-. Two types of hydrating water mols. could be quantified: irrotationally bound (ib, H2O mols. essentially "frozen" on the DRS time scale) and "slow" (loosely bound water mols. with identifiably slower dynamics than bulk water). For H2PO4- over the entire concn. range and for HPO42- and PO43- at concns. c ⪅ 1 mol L-1, only "slow" H2O was detected; however, at higher concns. of the latter two anions, an increasing fraction of ib water appears, making up ∼50% of the total hydration no. close to the satn. limit of K2HPO4. Contrary to common belief, all three salts showed significant ion pair formation, with std. assocn. consts. of the 1:1 species increasing in the order: KH2PO40(aq) < KHPO4-(aq) < KPO42-(aq). The main type of ion pair in soln. shifted from solvent-shared ion pairs (SIPs) to double-solvent-sepd. ion pairs (2SIPs) in the same sequence.
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