Weak Electrolyte
A compound which remains significantly undissociated in solution.
'p' Notation
Defined by
and used for convenience in handling quantities which vary in magnitude through many powers of ten.
For example, the H 3 O+ concentration of an aqueous solution may vary from 10 mol dm–^3 to 10–^14 mol
dm–^3. Expressed in the 'p' notation this becomes pH = –1 to pH = 14. Similarly the dissociation constant
for acetic acid Ka = 1.75 × 10 –^5 or 10–4.76 becomes pKa = 4.76.
3.1—
Chemical Reactions in Solution
The rate of a chemical reaction and the extent to which it proceeds play an important role in analytical
chemistry. The fundamental problem which faces the analyst arises because thermodynamic data will
indicate the position of equilibrium that can be reached, but not the time taken to reach that position.
Similarly, a compound may be thermodynamically unstable because its decomposition will lead to a net
decrease in free energy, whilst a high activation energy for the decomposition reaction restricts the rate
of decomposition. In practical terms such a compound would be stable, e.g. NO. It is thus essential to
consider all analytical reactions from both thermodynamic and kinetic viewpoints.
Equilibrium Constants
An equation may be written for a generalized reaction
where a, b, c, x, y, z are numbers indicating its stoichiometry. The position of equilibrium is expressed
by the thermodynamic equilibrium constant Ke which is defined by
In this expression, the square brackets refer to the activity of the component although it is more
convenient to use its concentration. This approximation is generally satisfactory, except at very high
concentrations, and is particularly suitable for analytical use. Where it is necessary to distinguish
between the constant obtained using concentrations and the true thermodynamic equilibrium constant
Ke, the former may be termed the equilibrium quotient and assigned the symbol Q. The exact relation
between Ke and Q has been the subject of much investigation and speculation. In this