In situ FTIR-ATR observation of structural dynamics of H2PO4- in precrystallisation solution

Structural dynamics of H2PO4- in aqueous solution with different supersaturations and pH values was investigated by Fourier transformation infrared attenuated total reflectance (FTIR-AIR) spectroscopy. With increase in the concentration of potassium dihydrogen phosphate (KDP), the variation of H2PO4- configurations can be reflected by increased intensity of v(1)(PO4) and the broadened width of v(3)(PO4) absorption band in FTIR-AIR spectra. With the transition of KDP solution from unsaturated to saturated and supersaturated states, both the red shift of delta(P-O center dot center dot center dot H-O-P) from 1229 to 1259 cm(-1) and the decreased density of stretching vibration of H2O at 1637 cm(-1) illustrate the development of H2PO4- ions from monomers to dimmers and (H2PO4-)(n) (n>2) in aqueous solutions. For saturated KH2PO4 solution, H3PO4 and H2PO4- coexist with decreasing pH value down to 1 center dot 5, whereas HPO42- and H2PO4- coexist in the solution with increasing pH value up to 6 center dot 0. Such an in situ recording strategy is of particular value in studies of structural dynamics in different solution conditions, which can strengthen the understanding of crystallisation mechanisms of KDP family crystals.