Introduction

In dealing with the thermodynamic properties of ions we have one difficulty in addition to those encountered in dealing with compounds and elements. For compounds and elements we found that although we could measure absolute values for some properties, others such as enthalpy and the other energy terms contained an undetermined constant. We got around this by using the concept of “formation from the elements.” It would of course be very convenient to also have thermodynamic properties of individual ions, but because positively and negatively charged ions cannot be separated from each other to any significant extent, their individual properties cannot be measured. To get around this, we need an additional convention, while retaining the formation from the elements convention.

In the following section we discuss the problems of activities of ionic species. Following that we discuss the conventions used to obtain numerical values for the state variables of individual ions, and we discuss the theory underlying the two major approaches to systematizing the data on electrolytes, the HKF and the Pitzer models. Because these are essentially equations of state, we introduced them in Chapter 13 (§13.6.2 and §13.6.3).

Activities of electrolyte components

Fugacity versus concentration for electrolytes

An electrolyte is a compound that splits up into charged particles when dissolved in water. For example, halite, NaC1, splits up into Na+ and C1− particles (ions) in solution.