Direct in situATR-IR spectroscopy of structural dynamics of NH4H2PO4 in aqueous solution

Abstract

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.