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.