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The variation of the activity coefficient of aqueous sulfuric acid with acid concentration, solution's temperature, and solvent's (water) dielectric constant, clearly fits a typical strong 1–3 electrolyte behaviour [Fraenkel, J. Phys. Chem. B. 116, 11662 (2012)]. Therefore, the aqueous acid was proposed to have the molecular formula H4SO5, and to dissociate to 3H+ (3H3O+) and 1HSO53−. When the above study was reported, electric conductivity data of the aqueous acid could not be theoretically interpreted to prove or disprove the 1–3 electrolyte behaviour, due to the lack of an appropriate conductance theory. The improved Debye–Hückel–Onsager (DHO) theory, DHO–SiS, recently reported [Fraenkel, Phys. Chem. Chem. Phys. 20, 29896 (2018)], allows, unlike its parent theory, a quantitative analysis of strong 1–1, 1–2 and 1–3 electrolytes. A good-to-excellent theory–experiment agreement is achieved for dilute aqueous H2SO4 at 0–50°C, but only if the acid is considered, as before, as a strong 1–3 electrolyte; the new analysis thus corroborates the previous activity-based findings. In contrast, the conductance of dilute sulfuric acid in pure methanol follows the conventional H2SO4 dissociation pattern, exhibiting a 'mixed 1–1/1–2' electrolyte behaviour. [ABSTRACT FROM AUTHOR]

Single-ion activity coefficients of HCl in MeOH–H 2 O, computed by DHO–SiS from single-ion equivalent conductivities. [Display omitted] Theory–experiment fit for electrolytes in solvents other that pure water has always been challenging. In ionic activity, solvent's permittivity (ε) plays an important role; at ambient pressure, ε is only a function of temperature, but in different solvents, it is also influenced by the nature of the solvent used, being smaller in less polar solvents. Employing the extended Debye–Hückel ion activity theory DH–SiS, HCl in nonaqueous or mixed solvents provided, up to moderate concentration, an excellent theory–experiment agreement even at very low ε , < 10 (Fraenkel, J. Phys. Chem. B 115 (2011) 14634). A parallel analysis is now reported for the ionic electric conductance of HCl in such solvents; specifically, in pure methanol, methanol–water mixtures, and dioxane–water mixtures. The analysis is done using DHO–SiS – a recently presented refined and extended Debye–Hückel–Onsager theory (Fraenkel, Phys. Chem. Chem. Phys. 20 (2018) 29896). In conductance, theory is further complicated by the effect of solvent's viscosity (η) that – like ε – in addition to varying with temperature, varies also with the kind of solvent. However, good-to-excellent theory–experiment fit is achieved with different η values at ε > 30; and the equivalent conductivity proves, as before, to be theoretically interrelated with the activity coefficient. At ε < 30, a fit occurs only with an adjusted ε (ε adj) postulated to reflect the influence of the exerted external electric field on solvent's molecular arrangement. [ABSTRACT FROM AUTHOR]

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 HSO 4-, and at <0.1 M, HSO 4- dissociates further to H+ and SO 42-. 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. [ABSTRACT FROM AUTHOR]