Abstract
Densities of aqueous solutions of sulfuric acid (H2SO4) have been measured by vibrating tube densimetry at molalities m varying from 0.02 to 3.0 mol·kg–1, at temperatures over the range 323.15 ≤ T/K ≤ 523.15 at 10 MPa pressure. These results were used to calculate the corresponding apparent molar volumes Vϕ(H2SO4,aq) and appear to be the first-ever systematic study of the volumetric properties of this important acid at elevated temperatures. At T ≤ 373.15 K, the present results aligned well with recent literature data and with values reported in the 1926 International Critical Tables. No reliable experimental data were found for comparison at higher temperatures. The present results confirm that at lower concentrations and temperatures, sulfuric acid solutions exist as a variable mixture of H+(aq) + HSO4–(aq) + SO42–(aq). However, at higher temperatures, the degree of association increases markedly such that at T ≥ 448.15 K, sulfuric acid solutions behave like a simple 1:1 electrolyte [H+(aq) + HSO4–(aq)] even at low concentrations. The variation of Vϕ(H2SO4,aq) over the entire experimental region was well modelled using a simple Pitzer equation that specifically included (where appropriate) the effects of chemical speciation. Combination of this model with relevant literature data enabled estimation of the standard ionic volume V°(HSO4–,aq) and the standard volume change ΔrV° for the reaction H+(aq) + SO42–(aq) → HSO4–(aq) at temperatures up to 523 K.
Highlights
With major uses in fertilizer manufacture and in the hydrometallurgical processing of non-ferrous metals,[1−3] the annual global production of sulfuric acid at ca. 260 Mt is higher that of all other anthropogenic chemicals
The first dissociation step goes to completion except at very high acid concentrations and/or high temperatures[4] and will not be considered further in this paper
Article densities available for sulfuric acid solutions at elevated temperatures (T > 373 K). In view of this unsatisfactory situation, the first aim of this paper is to extend the existing database for the volumetric properties of H2SO4(aq) up to moderate concentrations and to elevated temperatures
Summary
With major uses in fertilizer manufacture and in the hydrometallurgical processing of non-ferrous metals,[1−3] the annual global production of sulfuric acid at ca. 260 Mt is higher that of all other anthropogenic chemicals. While pure sulfuric acid is rarely employed, its aqueous solutions and/or its salts have a myriad of industrial and other uses. They are of considerable environmental interest due to their presence throughout the biosphere,[4] their roles in acid mine drainage,[5] and in the formation of upper-atmospheric aerosols.[6] In addition to their practical importance, aqueous solutions of sulfuric acid are of fundamental scientific interest because of the status of H2SO4(aq) as the archetypal diprotic acid. Like all multiprotic acids in aqueous solution, sulfuric acid dissociates in a stepwise manner. As is well known, the second dissociation (eq 2) is relatively weak, with a standard equilibrium constant Ka° ≈ 0.01 at nearambient conditions, becoming even weaker with increasing temperature.[4]
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