Abstract

Densities of fifteen aqueous solutions of sulfuric acid (H₂SO₄) have been measured by vibrating-tube densimetry at solute molalities (m) from (0.01 to 3.0) mol·kg⁻¹ over the temperature range 293.15 ≤ T/K ≤ 343.15. These data have been used to calculate the corresponding apparent molar volumes Vϕ(H₂SO₄,aq), which represent a significant expansion of the volumetric database for this industrially-important acid. At 298.15 K the present results agree well with literature data, notably with the century-old values given in the 1926 International Critical Tables. At other temperatures, where comparisons are possible agreement with the present Vϕ values is also very satisfactory. Consistent with earlier studies, Vϕ(H₂SO₄,aq) was found to exhibit an abnormally-large decrease at low concentrations (m ≤ 0.1 mol·kg⁻¹). This effect is consistent with a change in the chemical speciation of H₂SO₄(aq), from an essentially 1:1 electrolyte (H⁺(aq) + HSO₄− (aq)) at higher concentrations to a predominantly 1:2 electrolyte (2H⁺(aq) + SO₄²⁻ (aq)) in dilute solutions. The Vϕ values were modelled using variants of Young’s rule and the Pitzer formalism. Combination of these results with literature values for the standard volume V°(SO₄²⁻,aq) enabled estimation of V°(HSO₄−,aq) and the standard volume change, ΔrV°, for the first protonation of the sulfate ion (H⁺(aq) + SO₄²⁻(aq) → HSO₄−(aq)) as functions of temperature. It is shown that V°(HSO₄−,aq) is sensitive to the value of the first protonation constant and probably cannot be determined to better than ± 0.3 cm³·mol⁻¹ at present.