Pulse radiolysis study of concentrated sulfuric acid solutions. Formation mechanism, yield and reactivity of sulfate radicals

Abstract

In the pulse radiolysis of concentrated sulfuric acid solutions, the absorption spectrum and the molar absorption coefficient (1600 dm³ mol^–1 cm^–1) of the sulfate radical are unchanged up to 10 mol dm^–3 H₂SO₄, suggesting that the sulfate radical exists in the dissociated form (SO^– ₄). Two formation processes for the sulfate radical have been directly demonstrated in sulfuric acid and hydrogensulfate solutions: a fast one completed in the duration of the electron pulse, and a slow one occurring over a microsecond time range. For sulfate solutions only the fast formation process is observed. In sulfuric acid solutions the slow formation process is OH + HSO^– ₄→ H₂O + SO^– ₄(4.7 × 10⁵ dm³ mol^–1 s^–1) and OH + H₂SO₄→ HSO₄+ H₂O → SO^– ₄+ H₃O⁺(1.4 × 10⁷ dm³ mol^–1 s^–1), and the fast formation process is the direct action of radiation on sulfuric acid with a G value of (2.7 ± 0.4)× 10^–2 molecule eV^–1. The yields of (OH + SO^– ₄) and H can be quantified as: G(OH + SO^– ₄)= 2.9f_w+ 2.7f_s and G(H)= 3.7f_w+ 2.7f_s. The yields of SO^– ₄ have also been evaluated and the decay kinetics and reactions of the sulfate radical studied.