ABSTRACT
Thermodynamic data always refer to a selected standard state. The definition given by IUPAC [1] is adopted in the NEA-TDB project as outlined in the corresponding guideline [2]. According to this definition, the standard state for a solute B in a solution is a hypothetical solution, at the standard state pressure, in which mB=m°=1mol · kg
−1, and in which the activity coefficient γB is unity. However, for many reactions, measurements cannot be made accurately (or at all) in dilute solutions from which the necessary extrapolation to the standard state would be simple. This is invariably the case for reactions involving ions of high charge. Precise thermodynamic information for these systems can only be obtained in the presence of an inert electrolyte of sufficiently high concentration, ensuring that activity factors are reasonably constant throughout the measurements. The objectives of this chapter are to describe and illustrate various methods of extrapolation to zero ionic strength and to describe the methods preferred in the NEA review along with recommended values of the auxiliary parameters. By following these guidelines, the members of the NEA specialist teams performing the review will be assured of using the same theory consistently for the extrapolation to zero ionic strength.
