Pulsradiolyse des Schwefelwasserstoffs in wäßriger Lösung

Abstract

The reactions of the OH radical with H2S and SH⊖ are very fast (k = 1.1·1010 and 5.4·109 mole-1 liter sec-1, respectively). The hydrated electron reacts with H2S (1.1·1010) but does not react with SH⊖. The rate constant of the reaction CNS+SH⊖ CNS⊖ +SH was found to be equal to 9.8·108. Species absorbing at 3800 Å (e ∼ 104 mole-1 liter cm-1) are formed in these reactions of H2S and SH⊖ The intensity of this absorption is very low in strongly acid solutions, increases with pH, goes through a broad maximum at about pH=7, reaches a lower plateau between pH = 8.5 -10.5 and decreases in strongly alkaline solutions. At pH values below 7, the intensity is dependent on the concentrations of both SH⊖ and free H2S, i. e. at constant SH⊖ concentration a stronger absorption occurs if more free H2S is present. In the plateau below pH = 8.5 -10.5, the intensity is practically independent of the SH⊖ concentration above 4·10-5 mole/l. The decay of the 3800 A absorption after the pulse is first order with τ1/2 = 5 μsec independent of the OH⊖ and SH⊖ concentrations in alkaline solutions.Below pH = 7, the absorption disappears by second order, the rate constant being larger at lower pH values. SH or S⊖ absorb weakly in the near UV and do not contribute to the 3800 Å absorption. This absorption is attributed to the ions H2S2 ⊖ or H2S2⊖ in alkali and to H2S2⊖or higher complexes such as H3S4 ⊖ in acid. The first order decay of 5 μsec is attributed to the spontaneous decomposition of S2H2⊖ into S⊖ +SH⊖. Two S⊖ ions form S2 2⊖ (2 k=2·109 mole-1 liter sec-1). The complex H2S2 ⊖ is formed in the fast reaction SH + SH⊖ ↔ S2H2 ⊖ (dissociation constant about 4·10-5 mole/l). The formation of an intermediate complex SHOH⊖ in the reaction of OH with SH⊖ is discussed. This intermediate reacts with either SH⊖ to form S2H2⊖ or with OH⊖ (or other proton acceptors such as ΗΡΟ4 2Θ) to form SΘ+ H2O+OHΘ.