Instant Notes: Analytical Chemistry

Section C – Analytical reactions in solution

C1 SOLUTION EQUILIBRIA

Solvents The use of solvents for analytical work is determined by their properties, as
shown in Table 1.
Solvents with high dielectric constants (er> 10), for example, water and
ammonia, are referred to as polarand are ionizing solvents, promoting the
formation and separation of ions in their solutions, whereas those where eris
about 2, such as diethyl ether, tetrachloromethane and hexane are nonpolar and
are nonionizing solvents. There are also many solvents whose behavior is inter-
mediate between these extremes.
The solution process in a liquid may be represented by a general equation:

+=

The action of solution changes the properties of both solute and solvent. The

solute is made more mobile in solution, and its species may solvateby attraction

B(sol)

solution

B

solute

A(l)

solvent

Key Notes

The major component of a solution is referred to as the solvent, and there

is a wide range of inorganic and organic solvents used in analytical

chemistry. Their properties determine their use.

When a substance called the solute is dissolved in a solvent to form a

solution, its behavior is often altered. Reactions in solution are faster than

in the solid state. The amount of substance that can dissolve in a given

amount of solvent at a stated temperature and pressure is called the

solubility and is determined by the nature of the materials and the laws

governing the solubility equilibrium.

Some substances form ions, which are species possessing a charge. These

behave in a distinct way in solution. They may attract molecules of

solvent, may associate together, and may react with other species to form

complexes or a precipitate.

Since concentrations vary over a very wide range, they are often

represented by the logarithmic pX notation where pX = -log(X), where X

is the concentration or activity of an ion, or an equilibrium constant.

The laws of thermodynamics govern the behavior of all species in

solution. Every reaction depends upon the thermodynamic properties of

the species involved. Where those properties are changed by the solvent

by association, by reaction or temperature, the behavior will alter.

Physical and chemical equilibria in solution are most important.

Related topics Other topics in Section C Separation techniques (D1-D9)

(C2-C10)

Solvents

Solubility

Ions in solution

The pX notation

Equilibria in

solution