The high temperature radiosulfur exchange reaction of sulfur dioxide with concentrated sulfuric acid and ammonium bisulfate Public Deposited

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  • The present study has consisted primarily of an investigation of the rates of isotopic sulfur exchange between sulfur dioxide and various sulfur (VI) solutions, both 100% H₂SO₄ and, in particular, related ammonia-containing sulfur (VI) solutions. One of the main objects of the work was to obtain a comparison between exchange rates in such systems and those previously observed in aqueous acid systems. The exchange was studied at temperatures from 167.5°C to 225.3°C, the sulfur (VI) systems examined including: (1) 100% H₂SO₄; (2) fused ammonium bisulfate; (3) mixtures of ammonium bisulfate and 100% H₂SO₄; and (4) a 1:1 mixture of ammonium bisulfate and 84.5% H₂SO₄. The observed rates were all conveniently measurable throughout the indicated temperature range. Previous studies of exchange rates between sulfur dioxide and either concentrated sulfuric acid solutions or fused ammonium bisulfate have shown in both cases the reaction to be first order in the amount of sulfur dioxide and proportional to the fraction of it dissolved in the solution, viz.: Rate = k (SO₂) (1) where (SO₂) represents dissolved sulfur dioxide concentration. The results showed, however, a wide variation of the rate constants for different types of solutions. In an attempt to account for this wide variation, an effort has now been made to interpret both the present results as well as previous data in terms of sulfur dioxide activities rather than concentrations. To this end equation (1) has been rewritten in the form Rate = k[subscript a] γ (SO₂) (2) where γ is the activity coefficient of sulfur dioxide in the solutions. In order to calculate the rate constants based on sulfur dioxide activities (k[subscript a]), the concentration rate constants (k) and activity coefficients (γ) were first calculated. The latter quantities were computed from the relationship: γ = P/P' (3) where P is the actual partial pressure of solute SO₂ over a solution at the composition of interest and P' is the calculated ideal solution (Raoult's law) partial pressure at the same composition. Values of P in equation (3) were calculated directly from sulfur dioxide distribution coefficients between gas phase and solution on the basis of the ideal gas equation, Henry's law having been found valid in these systems. The distribution coefficients were either obtained from the present exchange results or, for some of the previous data, were estimated by interpolation or extrapolation from data measured under slightly different conditions. A comparison between the two types of rate constants, k and k[subscript a], shows that, while the constants, k, vary drastically from one solution to another, the activity constants, k, vary among themselves to only a modest degree. A striking example of this feature is provided by comparison of exchange rates at 225.3°C in fused ammonium bisulfate and in the 1:1 mixture of ammonium bisulfate with 84.5% H₂SO₄. Here the constants k differ by a factor of 32 while the k[subscript a] values differ by only a factor of two. This comparison shows clearly the merits of interpreting the rate as first order in sulfur dioxide activity rather than concentration. Apparent activation energies, frequency factors and activation entropies, both for k values and k[subscript a] values, have been calculated for the systems: (1) fused ammonium bisulfate; (2) 100% H₂SO₄; and (3) 1:1 ammonium bisulfate and 84.5% H₂SO₄. It was found that, even though the k values themselves showed good accord among themselves as compared to the k values, these related kinetic parameters failed to reflect this accord. Thus important variations among the k based parameters appear to persist with the k[subscript a] based parameters. It is suggested that the basis for this lack of concordance among the latter parameters may be related to the fact that the activity coefficients of the sulfur (VI) exchanging species and of the activated complexes have not been adequately taken into account. Since the sulfur (VI) concentration in the solutions varied but little from case to case, it was not possible to test the rate dependence on this factor. However, previous work has demonstrated a first order dependence of rate on bisulfate ion in aqueous acid, and it has been assumed the same dependence prevails in the present solutions. If this assumption is valid, the relative constancy of k[subscript a] values observed, as the ratio of bisulfate ion concentration to unionized H₂SO₄ concentration changes drastically from one solution to another, implies that the rate must be approximately first order in both of these sulfur (VI) species and must have, for exchange with either, approximately the same specific rate constant. Hence it is tentatively proposed that the exchange rate may be, at least approximately, described, for all the solutions here examined by the rate law Rate = k₂ γ (SO₂)[(HSO₄⁻) + (H₂SO₄)]
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