Monitoring the Speciation of Aqueous Free Chlorine from pH 1 to 12 with Raman Spectroscopy to Determine the Identity of the Potent Low-pH Oxidant

Abstract

The speciation of aqueous free chlorine above pH 5 is a well-understood equilibrium of H₂O + HOCl ⇌ OCl⁻ + H₃O⁺ with a p K_a of 7.5. However, the identity of another very potent oxidant present at low pH (below 5) has been attributed by some researchers to Cl₂ (aq) and by others to H₂OCl⁺. We have conducted a series of experiments designed to ascertain which of these two species is correct. First, using Raman spectroscopy, we found that an equilibrium of H₂O + H₂OCl⁺ ⇌ HOCl + H₃O⁺ is unlikely because the “apparent p K_a” increases monotonically from 1.25 to 2.11 as the analytical concentration is increased from 6.6 to 26.2 mM. Second, we found that significantly reducing the chloride ion concentration changed the Raman spectrum and also dramatically reduced the oxidation potency of the low-pH solution (as compared to solutions at the same pH that contained equimolar concentrations of Cl⁻ and HOCl). The chloride ion concentration was not expected to impact an equilibrium of H₂O + H₂OCl⁺ ⇌ HOCl + H₃O⁺, if it existed. These observations supported the following equilibrium as pH is decreased: Cl₂ (aq) + 2H₂O ⇌ HOCl + Cl⁻ + H₃O⁺. The concentration-based equilibrium constant was estimated to be approximately 2.56 × 10⁻⁴ M² in solutions whose ionic strengths were ∼0.01 M. The oxidative potency of the species in low pH solutions was investigated by monitoring the oxidation of secondary alcohols to ketones. These and other results reported here argue strongly that Cl₂ (aq) is the correct form of the potent low-pH oxidant in aqueous free-chlorine solutions.