Instant Notes: Analytical Chemistry

passing helium through them, or by boiling them, since gas solubility

decreases as the temperature is increased.

● Liquids. When different liquids are mixed, many types of behavior may

occur. If the molecules in the liquids are of similar size, shape, polarity and

chemical nature they may mix in all proportions. For example, benzene and

methylbenzene (toluene) mix completely. In such ideal solutions, obeying

Raoult’s law, the activity coefficient is close to 1:

a =p/pn=x

If the component molecules differ greatly in polarity, size or chemical nature

(e.g., water and tetrachloromethane) they may not mix at all. This is an

important condition for solvent extraction (Topic D1). The distribution of a

solute between a pair of immiscible liquidsdepends primarily on the

solubility of the solute in each liquid.

● Solids generally follow the ‘like dissolves like’ rule. Nonpolar, covalent

materials dissolve best in nonpolar solvents. Solid triglycerides such as

tristearin are extracted by diethyl ether, but are nearly insoluble in water.

Salts, such as sodium chloride are highly soluble in water, but virtually

insoluble in ether.

Ions in solution The behavior of ions in solution may be summarized as follows.

(i) Solids whose structure consists of ions held together by electrostatic forces

(e.g. NaCl) must be separated into discrete ions when they dissolve. These

ions often gain stability by solvation with molecules of the solvent. Such

solutions are described as strong electrolytes.

(ii) Some covalent molecules, such as ethanoic acid, may form ions by the

unequal breaking of a covalent bond, followed by stabilization by solvation.

This occurs only partially and these are called weak electrolytes.

H : OCOCH 3 [H++-OCOCH 3

(iii) In some cases ions do not separate completely. In concentrated solutions,

oppositely charged ions may exist as ion-pairs, large ions of surfactants

may aggregate into micelles, which are used in capillary electrophoresis

(Topics D8 and D9), and the dissociation of covalent molecules may be only

partial.

(iv) At ‘infinite dilution’, that is, as the concentration approaches zero, ions are

truly separate. However, even at quite low concentrations, the attractions

between ions of opposite charge will cause each ion to become surrounded

by an irregular cloud, or ionic atmosphere. As the solution becomes even

more concentrated, the ionic atmosphere becomes more compact around

each ion and alters its behavior greatly.

In very dilute solutions, the effects of the ionic atmosphere may be approxi-

mated by using the Debye-Hückel theory, which predicts that the mean ionic

activity coefficient, g±is, for an electrolyte with positive ions with charge z+,

negative ions with chargez-, is given by:

log(g±) =-A (z+. z-) √|(I)

where I is the ionic strength=^1 ⁄ 2 S(cizi^2 ) for all the ions in the solution.

For more concentrated ionic solutions, above 0.1 M, no general theory exists,

C1 – Solution equilibria 57