In Situ IR Spectral Observation of NH4H2PO4 Crystallization: Structural Identification of Nucleation and Crystal Growth

In situ ATR-IR spectroscopy was used to identify the structural variations of NH4+ and H2PO4- during NH4H2PO4 (ADP) crystallization in aqueous solution with different pH values. For supersaturated ADP solution, the time needed for the appearance of ADP solids in IR measurement will be prolonged with increasing or decreasing solution pH value. When ADP crystallizes from the aqueous solution with pH = 0.98-5.00, H2PO4- groups with a larger hydrated ionic radius initially form a framework structure, and then NH4+ groups with a relatively smaller hydrated ionic radius insert into the H2PO4- framework. IR bands of the H-bond at 2387 cm(-1) and a combination of PO4 vibration within the lattice at 1260 cm(-1) indicated the formation of H2PO4- frameworks with ADP lattice structural characteristics via forming hydrogen bonding. The hydrogen bonding between NH4+ and H2PO4- can be indicated by the splitting of v(4) (NH4) at 1400 cm(-1). IR spectra indicated that when the pH value increases up to 6.03, (NH4)(2)HPO4 instead of NH4H2PO4 was crystallized from the aqueous solution. Such an in situ recording strategy is of particular value in identifying the structural characteristics at both nucleation and crystal growth stages during the crystallization process.