Agreement of electrolyte models with activity coefficient data of sulfuric acid in water

Highlights: • A 3-parameter Pitzer equation for γ_±vs. m is used in analyzing aqueous H₂SO_4. • The equation fits better for aqueous H₂SO₄ as 1–3, than 1–2 or 1–1/1–2 electrolyte. • The 1–2 analysis is the worst, obtained with negative values of the β⁽¹⁾ parameter. • Pitzer model cannot distinguish between electrolyte families, ionization modes. • The DH–SiS model analyzes aqueous H₂SO₄ as 1–3 electrolyte better than Pitzer model. - Abstract: The calculation of thermodynamic properties of many strong electrolytes in solution, including aqueous sulfuric acid, has been performed over the past four decades using so-called thermodynamic models, such as the well-known Pitzer model. I have recently pointed out (Fraenkel, 2012) [15,16] that H₂SO₄ in water appears to follow the mean ionic activity pattern of a strong 1–3 electrolyte, and postulated that this H₃A acid may be H₄SO₅ fully ionizing to 3H⁺ (3H₃O⁺) and HSO₅^3-. This contrasts with the traditional view of the aqueous acid – claimed to be supported by thermodynamic models – according to which H₂SO₄ retains its molecular structure in water and dissociates primarily to H⁺ and HSO₄^-, and at <0.1 M, HSO₄^- dissociates further to H⁺ and SO₄^2-. I now show that a good fit of Pitzer model with the activity coefficients reported by Hamer and Harned can be obtained for the “1–3 H₂SO₄” even by using the simple 3-parameter equation of the model; the best-fit Pitzer parameters are β⁽⁰⁾ = 0.240, β⁽¹⁾ = 4.30 and C_MX = −0.0134, and the standard deviation, σ is 0.0152. With the corrected activity coefficients as proposed in the first reference above, the best-fit parameters are β⁽⁰⁾ = 0.230, β⁽¹⁾ = 3.60 and C_MX = −0.0120, and σ = 0.0081. σ of the analysis of the “1–3 acid” is in both cases considerably lower than that of the “1–2 acid” (σ = 0.049) that provides a best-fit β⁽¹⁾ value of −3.000; a negative β⁽¹⁾ is inappropriate since it is parallel to a negative ion–ion distance of closest approach in Debye–Hückel-type expressions of the activity coefficient