Thermodynamics of Electrolyte Solutions
Thermodynamic properties of electrolyte solutions such as steam pressure and freezing point depression can be calculated by using thermodynamically deduced equations, provided that the activity coefficients of the components in the solution are known.
Similarly, the electromotive forces of cells are often theoretically calculated, provided the activity coefficients of the components in their solutions are known. Thus, processing the thermodynamics of electrolyte solutions largely boils down to processing activity coefficients.
Thermodynamic equations and activity coefficients
Reliable activity coefficient equations, which would allow the activity coefficients to be calculated from composition variables of a solution, have proved difficult, however. One important area of our research is to solve these equations for strong electrolytes in an aqueous solution (solutions that are perfect for ions).
In these assays, we use thermodynamic equations that our research group derives as accurately as possible from the activity coefficients. For example, the thermodynamic properties of strong electrolytes in dilute aqueous solutions can be described by simple activity coefficient equations (Hückel-equations) as accurately as it is possible to measure.
Our most recent investigations at different temperatures and in simple electrolyte solutions show, that using the calculation method we use it is possible to generalize for many other cases as well. Examples of these new studies can be found in the articles about hydrochloric acid solution in the Journal of Solution Chemistry [36 2007, 39-59].
Our research also shows that the small additional changes to the Hückel equation we use are also suitable for strong solutions, as our hydrochloric acid studies verify.
Our research in this area is lively, and in the future, simple Hückel equations should in many cases allow us to develop a competitive method of calculation for use, for example, in the process industries, already partially occupied by Pitzer equations. The Hückel method, however, requires further basic research, which involves a significant contribution from our research group.
Thermodynamics and weak acids in aqueous solutions
Another important area of research looks at the thermodynamic properties of weak acid aqueous solutions.
In weak acid aqueous solutions, there is always a dissociation-related equilibrium, which involves a hydrogen ion, an acid anion and a neutral acid molecule (= weak acid generated species).
For this dissociation reaction-related equilibrium constant (= stoichiometric dissociation constant Km) it is possible to determine the ion's activity coefficient equations, which are directly connected using composition variables.
Our research in this area has recently focused on these ion activity coefficient equations, that we have determined for many weak acids in different salt solutions, such as sodium and potassium chloride.
Our method of Km calculation is based on the activity coefficient formula of ions using a Hückel-type equation. To this date, to our knowledge there are no equations published in the previous literature that have been shown to be as reliable.
The ion activity coefficient equations also allow the pH of the solutions to be calculated. For example, acetic acid Km values can be used in our newly developed calibration method for glass electrode cells.
Glass electrodes are measuring devices that most laboratories in the world today base their pH measurements on. Our new calibration method is pH-independent and may lead to scientific breakthroughs in the use of glass electrode cells. Using glass electrode cells with this calibration method in potentiometric titration it is possible to directly measure the hydrogen ion concentration of a solution.
Associate Professor Jaakko Partanen, D.Sc. (Tech)
Tel. +358 40 137 3084