Instant Notes: Analytical Chemistry

to the solvent. The solvent structure is also disrupted by the presence of species

different in size, shape and polarity from the solvent molecules.

Ideally, the behavior should depend on the concentration m(in molarity,

mole fraction or other units), but often this must be modified and the activity, a,

used:

a=mg=p/pn

where gis called the activity coefficient. The vapor pressure of the solution is p,

and that in the standard state is pn. Activities are dimensionless.

Solvents, such as water, with high dielectric constants (or relative permittivi-

ties) reduce the force F between ions of charges z 1 e and z 2 e a distance rapart:

F=z 1 z 2 e^2 /eoerr^2

where eois the permittivity of free space. Also, they will solvate ions more

strongly and thus assist ion formation and separation.

Hexane, diethyl ether and tetrachloromethane (CCl 4 ) all have low dielectric

constants and are nonpolar. They are very poor at ionizing solutes. However,

they are very good solvents for nonpolar substances.

Solubility The equilibrium amount of solute which will dissolve in a given amount of
solvent at a given temperature and pressure is called the solubility.The solu-
bility may be quoted in any units of concentration, for example, mol m-^3 ,
molarity, mole fraction, mass per unit volume or parts per million (ppm).
There is a general ‘rule of thumb’ that ‘like dissolves like’. For example, a
nonpolar hydrocarbon solvent such as hexane would be a very good solvent for
solid hydrocarbons such as dodecane or naphthalene. An ester would be a good
solvent for esters, and water or other polar solvents are appropriate for polar
and ionic compounds.

● Gases dissolve in solvents according to Henry’s Law, provided they do not

react with the solvent:

pB=xBK

where xBis the mole fraction of solute gas B which dissolves at a partial pres-

surepBof B, and Kis a constant at a given temperature. This is analytically

important for several reasons. For example, nitrogen is bubbled through

solutions to decrease the partial pressure of oxygen in electrochemical experi-

ments. Similarly, air is removed from liquid chromatography solvents by

56 Section C – Analytical reactions in solution

Table 1. Properties of some solvents

Solvent Boiling point (∞C) Density, Dielectric

(g cm-^3 ) constant, er

Water 100 1.00 78.6

Ammonia - 34 0.68 22.0

Ethanol 78 0.79 24.3

n-hexane 69 0.66 1.88

Diethyl ether 34 0.71 4.33

Note: density at 25∞C or at BP; dielectric constant =relative permittivity