Ion hydration and association in aqueous potassium phosphate solutions

Area of Science:

• Physical Chemistry
• Solution Chemistry
• Spectroscopy

Background:

• Understanding ionic hydration and ion association is crucial for aqueous solution chemistry.
• Phosphate salts (KH2PO4, K2HPO4, K3PO4) are common in various applications, necessitating detailed study of their solution behavior.
• Previous research has not fully elucidated the extent of hydration and ion pairing in these systems.

Purpose of the Study:

• To investigate ionic hydration and ion association in aqueous solutions of KH2PO4, K2HPO4, and K3PO4.
• To quantify hydration numbers and identify different types of hydrating water molecules.
• To determine the extent and nature of ion pair formation in these phosphate systems.

Main Methods:

• Dielectric relaxation spectroscopy (DRS) was employed to probe water molecule dynamics in the phosphate solutions.
• Experiments were conducted at 25 °C across a range of concentrations, including high concentrations.
• Analysis focused on quantifying hydration numbers and characterizing ion association.

Main Results:

• Phosphate anions (H2PO4(-), HPO4(2-), PO4(3-)) showed extensive hydration, with hydration numbers increasing significantly with anion charge.
• Evidence for a second hydration shell was found for HPO4(2-) and PO4(3-).
• Two types of water molecules were identified: irrotationally bound (ib) and 'slow' water, with ib water becoming prominent at higher concentrations for HPO4(2-) and PO4(3-).
• Significant ion pair formation was observed for all three salts, contrary to prior assumptions.
• Standard association constants for 1:1 ion pairs increased in the order KH2PO4 < KHPO4 < KPO4.
• The dominant ion pair type shifted from solvent-shared ion pairs (SIPs) to double-solvent-separated ion pairs (2SIPs) with increasing anion charge.

Conclusions:

• Phosphate anions in aqueous solutions exhibit complex hydration structures and significant ion pairing.
• The extent of hydration and ion association is strongly dependent on the phosphate anion's charge.
• Dielectric relaxation spectroscopy provides valuable insights into the dynamics of water molecules and the nature of ion interactions in concentrated electrolyte solutions.