Refinement of the Pitzer–Debye–Hückel Equation for Single Asymmetric Aqueous Electrolyte Systems

The Pitzer–Debye–Hückel equation (PDH) is widely used as the long-range term in electrolyte local composition models to describe the non-ideality of electrolyte solutions in the low concentration range. However, the PDH equation’s derivation typically involves disregarding the third term of the radial distribution function, which leaves uncertainties regarding its impact on asymmetric systems, especially those with high asymmetry. This paper addresses this issue by introducing a trinomial radial distribution function and re-deriving the PDH equation, aiming to evaluate the efficacy of the modified equation in describing various asymmetric electrolyte systems at low concentrations (0–1 mol·kg⁻¹). Initially, the osmotic coefficients of 19 single asymmetric electrolyte systems were fitted using the modified PDH equation (M-PDH). The results demonstrated that the accuracy of the M-PDH equation was significantly higher compared to the original PDH equation, yielding standard deviations (SD) of 0.1812 and 0.4238, respectively. Furthermore, an analysis and recommendation for the distance parameter b were provided. Finally, a comparative analysis was conducted to assess the contributions of the third term of the radial distribution function in contrast to the first two terms to the osmotic coefficients. Overall, this study enhances our understanding of how asymmetry affects the PDH equation in describing the thermodynamic properties of electrolyte systems.