In situ ATR-IR observation of nucleation and crystal growth of KH2PO4 in aqueous solution

Abstract

In situ Attenuated Total Reflectance-Infrared (ATR-IR) spectroscopy was used to record and characterize the nucleation and crystal growth of KH₂PO₄ (KDP) from aqueous solution with pH = 1.01–6.96. In the crystallization of KDP, K⁺ and H₂PO₄⁻ ions transform from hydrated ions to clusters and crystalline state. With increasing measurement time, the shift of v₃(PO₄) at 938 and 1145 cm⁻¹ towards lower wavenumbers illustrated the breaking of hydrogen bonding between H₂PO₄⁻ and H₂O. IR bands of v_O–H at 2370 cm⁻¹ and β_O–H at 1274 cm⁻¹ indicated the formation of H₂PO₄⁻ clusters possessing KDP lattice structural characteristics via hydrogen bonding. The disappearance of v₃(PO₄) at 1145 cm⁻¹ indicated the chemical bonding between K⁺ and H₂PO₄⁻ in the nucleation stage of KDP. In KDP supersaturated solution with pH = 2.02–6.96, H₂PO₄⁻ groups with larger hydrated ionic radius initially form a framework structure, and then K⁺ ions with a relatively smaller hydrated ionic radius insert into the H₂PO₄⁻ framework to form KDP. Moreover, the time needed for the appearance of KDP solids in IR measurement can be modified by varying the pH value of KDP supersaturated solution. Such an in situ visualization strategy can characterize the structural variations by spectroscopy, which deepens the insight into the nucleation and crystal growth from aqueous solution during the whole crystallization process.