Electrolyte Solutions Are Always Non-Ideal

Electrolyte solutions are important in all branches of chemistry, but especially in analytical chemistry, and biochemistry. These systems by their nature are always non-ideal, and represented an early challenge to theoreticians interested in describing their thermodynamic properties. The solute components are ions, cations, and anions, which carry opposite charges and thus interact very differ- 

In very dilute electrolyte solutions, the most important consideration is ion-dipole interactions. One expects these interactions to be different for cations and anions. This follows from the fact that the solvent molecule is not a simple dipole in the electrostatic sense but instead it has a chemical structure which is different at each end of the molecular dipole. Each ion interacts locally with four to six solvent molecules in its immediate surroundings. In the case of water, the concentration of water molecules in the pure liquid is 55.5 M it follows that the number of water molecules experiencing direct interaction with ions in dilute solutions represents a small fraction of the total number. 

Under some circumstances ion-ion interactions can be more important than ion-dipole interactions. This is especially true when the valence of the ion is greater than one, and the electrolyte concentration is high. Then, the formation of ion pairs and higher aggregates is possible. Two types of ion pairs have been recognized, namely, contact ion pairs in which the cation and anion are in physical contact, and solvent-separated ion pairs in which one or two solvent molecules are situated between the cation and anion. Ion pairing must be considered in developing a complete picture of an electrolyte solution. 

As was seen in chapter 2, both dipole-dipole interactions and hydrogen bonding are important in determining the structure and thermodynamic properties of pure water. In the immediate vicinity of an ion, the solvent structure is disrupted so that local dipole-dipole interactions and hydrogen bonding are different than they are in pure water. These changes are also important because they affect the local permittivity and the strength of ion-ion interactions. 

In the present chapter, the properties of electrolyte solutions in water are discussed in detail. Initially the solvation of ions in infinitely dilute solutions is considered on the basis of the Born theory. Then, the Debye-Hiickel model for 

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