Comparing the suitability of sodium hyposulfite, hydroxylamine hydrochloride and sodium sulfite as the quenching agents for permanganate oxidation

An appropriate quenching agent for potassium permanganate (KMnO₄) is necessary for investigating the oxidation kinetics of contaminants by KMnO₄. In this paper, the suitability of three most commonly used inorganic reductants, including sodium hyposulfite (Na₂S₂O₃), hydroxylamine hydrochloride (NH₂OH·HCl) and sodium sulfite (Na₂SO₃), for quenching KMnO₄ was systematically investigated with phenol as a probe contaminant. Na₂S₂O₃ applied with Na₂S₂O₃/KMnO₄ molar ratio of 20.0 was a good choice for quenching KMnO₄ over the pH range of 2.0-11.0. Quenching KMnO₄ with NH₂OH·HCl was a good alternative with 20-fold excess at pH 4.0-9.0. However, Na₂SO₃ was not recommended to be the quenching agent of KMnO₄ due to the significant change of phenol concentration during quenching reaction. The 3-dimensional UV-vis spectra at different pH were collected with stopped flow spectrometer to investigate the quenching mechanisms of these three quenchers toward KMnO₄, which clearly showed the variation of manganese species with time.