ArticleVolume 4, Spring 2009

Modification of the Debye-Huckel Equation for Concentrated Binary Aqueous Electrolyte Mixtures

Sumit A Rastogi1,

Anil K Rastogi2

1 Columbia University,
2 Real Time Simulation Inc.


In this work, new modifications are made to the Debye-Huckel (DH) equation. The DH equation is used to calculate the activity coefficient of an ion in an aqueous solution and can be used for solutions with an ionic strength, I, of less than 0.1 M. One of the terms in the DH equation is “a”, the distance of closest approach of ions. The “a” term is considered a constant in both the original and extended forms of the DH equation (developed by Debye-Huckel2 and Pitzer8, respectively). For more concentrated electrolyte solutions, the assumption of “a” as a constant is not true, and the DH equation inaccurately predicts ionic activity and solution osmotic coefficients. The new modifications to the DH equation presented in this paper are based on calculating “a” as a function of concentration. A nonlinear regression program, Graph Pad Prism 5, is used to calculate four adjustable parameters present in the modified extended DH equation. The parameter values were obtained using mean molal activity and osmotic coefficient data. Results for 101 binary systems are presented, which include all applicable ionic strength and mean molal activity coefficient ranges. The results for mean molal activity and osmotic coefficients with the proposed modified DH equation show significant improvement over the original DH equation using “a” as a constant. The modified DH equation will be useful in the simulation of industrial processes such as those in metals and biochemical manufacturing, corrosion and electroplating, and the treating of water and sour gases in the petrochemical industry. Key Words: Chemical Engineering, mathematical modeling, simulation, chemical mixtures