Nonelectrolytes and Activity

Going beyond solutions of electrolytes in water, several other possibilities need consideration electrolytes in nonaqueous solvents, nonelectrolytic behavior in solutions, and nonelectrolytes in nonaqueous solvents. None of the theories proposed for the quantitative prediction of solution behavior has been as successful as that of Debye and Huckel for dilute ionic aqueous solutions. Nevertheless, general trends can be predicted. 

Electrolytes in nonaqueous solvents The most significant work for analytical chemists in this area has been concerned with acids and bases discussion of this topic is reserved for Chapter 4. 

Other than specific effects that result from conventional chemical interactions (such as acid-base or complex formation), the main factors to be considered are hydration of ions, electrostatic effects, and change in dielectric constant of the solvent. For example, hydration of ions of added salt effectively removes some of the free solvent, so that less is available for solution of the nonelectrolyte. The Setschenow equation probably best represents the activity coefficient of dilute solutions (less than 0.1 M) of nonelectrolytes in aqueous solutions of salts up to relatively high concentrations (about 5 M)  

According to (2-29) the logarithm of the activity coefficient should be proportional to the ionic strength. The proportionality constant k is positive if the solute has a lower dielectric constant than the solvent, as is usually the case for aqueous solutions. As a first approximation an equation of this form is valid for many solutes up to ionic strengths of the order of unity. The quantity k, called the salting coefficient, depends on the nature of the solute and of the electrolyte and usually has a value of 0.01 to 0.10. 

From a practical viewpoint we may conclude that molecular solutes have activity coefScients near unity up to an ionic strength of 0.1 and that deviations are moderate even at ionic strengths of the order of unity. In contrast to those of ionic solutes, activity coefficients of molecular solutes usually are slightly greater than unity. 

---