title: Ion hydration and association in aqueous potassium phosphate solutions.
abstract: 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 (HPO4(2-)), and ~39 (PO4(3-)). These values are indicative of the existence of a second hydration shell around HPO4(2-) and especially PO4(3-). 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 HPO4(2-) and PO4(3-) 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: KH2PO4(0)(aq) < KHPO4(-)(aq) < KPO4(2-)(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.
authors.affiliation: †Institut für Physikalische und Theoretische Chemie, Universität Regensburg, Universitätsstr. 31, D-93040 Regensburg, Germany.
authors.forename: Andreas
authors.initials: A
authors.lastname: Eiberweiser
authors.affiliation: †Institut für Physikalische und Theoretische Chemie, Universität Regensburg, Universitätsstr. 31, D-93040 Regensburg, Germany.
authors.forename: Andreas
authors.initials: A
authors.lastname: Nazet
authors.affiliation: ‡Chemistry Department, Murdoch University, 90 South Street, Murdoch, WA 6150, Australia.
authors.forename: Glenn
authors.initials: G
authors.lastname: Hefter
authors.affiliation: †Institut für Physikalische und Theoretische Chemie, Universität Regensburg, Universitätsstr. 31, D-93040 Regensburg, Germany.
authors.forename: Richard
authors.initials: R
authors.lastname: Buchner
mesh_terms: D007477:Ions / Q000737:chemistry; D010710:Phosphates / Q000737:chemistry*; D017680:Potassium Compounds / Q000737:chemistry*; D012996:Solutions; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*
chemical_list: D007477:Ions; D010710:Phosphates; D017680:Potassium Compounds; D012996:Solutions; D014867:Water; C013216:potassium phosphate