A comprehensive study of performic acid stability and acid-base equilibria in aqueous solution

The growing demand for efficient and sustainable wastewater treatment technologies has intensified interest in alternatives to conventional methods. Performic acid (PFA) has lately emerged as an innovative disinfectant and oxidant that can improve traditional wastewater treatment methods (e.g., chlorination) while complying with stricter regulations on disinfection by-products formation. Although use of PFA as a disinfectant is growing, scientific knowledge about its physicochemical properties remains limited and insufficiently detailed. This study aims to fill this gap through imprehensive experimental assessments of the PFA thermal and acid-base stability, including the evaluation of the protonation constant logKPFA at different temperatures (T) and ionic strengths (I). Experiments on thermal self-decomposition showed that PFA degradation follows Arrhenius kinetics, with an activation energy of 59.7 ± 3.9 kJ⋅mol􀀀 1. Despite its intrinsic instability, storing PFA at T ≤ 0 ◦C allows for sufficient preservation for short- to medium-term applications after synthesis. PFA decomposes quickly converting entirely to H2O2 in alkaline conditions. While it undergoes partial spontaneous degradation at pH 7, decreasing the total oxidant titer. Spontaneous degradation is strictly linked to the PFA protonation constant value logKPFA, which was determined at various temperatures (10 ◦C ≤ T ≤ 30 ◦C) and ionic strengths (0.05 M ≤ I ≤ 1.00 M) in NaClO4(aq) aqueous solution. Extrapolation at infinite dilution resulted in a thermodynamic protonation constant logTKPFA = 7.58 ± 0.01. Overall, the findings offer valuable kinetic and thermodynamic data crucial for predicting the behaviour of PFA in real-world applications and especially in advanced water treatments.