Page 360 - Instrumentation Reference Book 3E

Chemical analysis:

electrochemical techniques

W. G. GUMMINGS and K. TORRANCE, edited by I. VERHAPPEN

Acids and alkalis when added to water, but weak alkalis such as
ammonium hydroxide (NH40H) are only slightly
In order to appreciate electrochemical techniques ionized in water and produce much smaller con-
of chemical analysis it is necessary to have an centrations of hydroxyl ions.
understanding of how substances dissociate to As with weak acids, the strength of a weak base
form ions. is indicated by its dissociation constant
All acids dissociate when added to water to
prodiice hydrogen ions in the solution, e.g., nitric K= [B+l[OH-I
acid: IBOHI
HN03 + H+ + NO: where [B’] is the concentration of alkaline ions,
[OH-] is the concentration of hydroxyl ions, and
The extent to which dissociation takes place var- [BOH] is the concentration of undissociated
ies from acid to acid, and increases with increas- alkali.
ing dilution until, in very dilute solutions, almost Strong electrolytes have no dissociation con-
all the acid is dissociated. stant; the expression for strong acids
According to the ionic theory, the characteris- [A-][H+]/[HA] and the corresponding expression
tic properties of acids are attributed to the hydro- for alkalis vary considerably with change in con-
gen ions (H’) which they produce in solution. centration. With strong acids and alkalis the
Strong acids (nitric, sulfuric, hydrochloric) are apparent degree of ionization can be taken as a
those that produce a large concentration of measure of the strength of the acid or base.
hydrogen ions when added to water. As a result So far it has been assumed that the effective
the solutions are excellent conductors of electri- concentrations or active masses could be
city. Weak acids like carbonic acid (H2C03) and expressed by the stoichiometric concentrations
acetic acid (CH3COOH) when dissolved in water but, according to modern thermodynamics, this
produce small concentrations of hydrogen ions, is not strictly true. For a binary electrolyte
and their solutions are poor conductors of elec- AB e A+ + B- the correct equilibrium equation
tricity. is:
The strength of a weak acid is indicated by its
dissociation constant K which is defined as
where LZA+~B- and AB represent the activities of
A’, B- and AB and K, is the thermodynamic
dissociation constant. The thermodynamic quan-
where [A-] is the molar concentration of the tity “activity” is related to concentration by a
acidic ions, [H’] is the concentration of hydrogen factor called the activity coefficient, Le.,
ions, and [HA] is the concentration of undisso- activity = concentration x activity coefficient.
ciated acid. Using this concept, the thermodynamic activity
The dissociation constant K varies with tem-
perature but. at a given temperature, if a little coefficient is
more acid is added to the solution, a portion of
it dissociates immediately to restore the relative
amount of ions and undissociated acid to the
original value. where f refers to the activity coefficients and the
Similarly, the typical properties of alkalis in square brackets to the molar concentrations. The
solution are attributed to hydroxyl ions (OH-). activity coefficients of unionized molecules do not
Strong alkalis such as sodium hydroxide (NaOH) differ much from unity, and so for weak electrolytes
produce large concentrations of hydroxyl ions in which the ionic concentration, and therefore the