Chemical speciation effects on the volumetric properties of aqueous sulfuric acid solutions

Abstract

<div lang="en" class="abs"><p class="abs_header">Abstract</p><p>Densities of fifteen aqueous solutions of sulfuric acid (H₂SO₄) have been measured by vibrating-tube densimetry at solute molalities (<em>m</em>) from (0.01 to 3.0) mol·kg⁻¹ over the temperature range 293.15 ≤ <em>T</em>/K ≤ 343.15. These data have been used to calculate the corresponding apparent molar volumes <em>V_ϕ</em>(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 <em>International Critical Tables</em>. At other temperatures, where comparisons are possible agreement with the present <em>V_ϕ</em> values is also very satisfactory. Consistent with earlier studies, <em>V_ϕ</em>(H₂SO₄,aq) was found to exhibit an abnormally-large decrease at low concentrations (<em>m</em> ≤ 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 <em>V_ϕ</em> 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, Δ_rV°, 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.</p></div>