Direct in situ ATR-IR spectroscopy of structural dynamics of NH4H2PO4 in aqueous solution

Crystallization of KDP-family crystals depends on the chemical bonding behavior of the crystal constituents in aqueous solution, which are sensitive to solution conditions. We applied in situ ATR-IR spectroscopy combined with a morphology-evolution calibration to declare the structural dynamics of NH4+ and H2PO4− during the NH4H2PO4 crystallization in aqueous solution with different concentrations and pH values. For unsaturated NH4H2PO4 solution, both the H2PO4− stretching vibration mode and NH4+ bending vibration mode are enhanced with increasing concentration. When the NH4H2PO4 solution becomes a saturated and then supersaturated and crystalline state, H2PO4− ions undergo hydrated dimerisation and polymerisation, which can be recorded by the appearance and red shift of the P–O⋯H–O–P in-plane bending vibration mode from 1250 to 1263 cm−1. During this process, hydrated NH4+ ions bind to the (H2PO4−)n frame, reflected by the splitting of the HN4+ bending vibration mode at 1450 and 1400 cm−1. For the supersaturated NH4H2PO4 solution, HPO42− and H2PO4− coexist in solution with increasing pH value up to 6.64, whereas H3PO4 and H2PO4− coexist with decreasing pH value down to 1.52. Such an in situ recording strategy is of particular value in studying system dynamics, and in general to monitor the solution concentrations and compositions before and during the crystallization process.