Phase transition between two anhydrous modifications of NaHSO4 mediated by heat and water

The phase transition between the two anhydrous modifications of NaHSO4 (α and β) was studied using Raman spectroscopy and differential scanning calorimetry. These measurements indicate that β-NaHSO4 is a metastable phase and readily undergoes phase transition to thermodynamically stable α-NaHSO4 with an exothermic enthalpy change of 3.5 kJ/mol. Both thermal (temperatures >434 K) and chemical (exposure to H2O) pathways were identified for this transition. The transition is irreversible, and α-NaHSO4 is an intermediate phase between β-NaHSO4 and NaHSO4·H2O. The possible mechanism of the phase transition is discussed.