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

• A 3-parameter Pitzer equation for γ±vs. m is used in analyzing aqueous H2SO4.
• 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.

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 HSO53-. 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 SO42-. 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 CMX = −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 CMX = −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.