Page 61 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

DISSOCIATION OF POLYPROTIC AClDS 2.14

The basic dissociation constant K, is given by:

Since [H+] [OH-] = Kw (the ionic product of water), we have

The values of Ka and K, for different acids and bases Vary through many
powers of ten. It is often convenient to use the dissociation constant exponent
pK defined by

The larger the pKa value is, the weaker is the acid and the stronger the base.
For very weak or slightly ionised electrolyes, the expression cc2/(1 - cc)V = K
reduces to cc2 = KV or cc = &f since cc may be neglected in comparison with
unity. Hence for any two weak acids or bases at a given dilution V (in L), we
have cc, = KV and cc, = ZV, or a,/a, = fi/&. Expressed in
words, for any two weak or slightly dissociated electrolytes at equal dilutions, the
degrees of dissociation are proportional to the square roots of their ionisation
constants. Some values for the dissociation constants at 25 OC for weak acids
and bases are collected in Appendix 7.

2.14 DISSOCIATION OF POLYPROTIC AClDS

When a polyprotic acid is dissolved in water, the various hydrogen atoms
undergo ionisation to different extents. For a diprotic acid H,A, the primary
and secondary dissociations can be represented by the equations:

If the acid is a weak electrolyte, the Law of Mass Action may be applied, and
the following expressions obtained:

K, and K, are known as the primary and secondary dissociation constants
respectively. Each stage of the dissociation process has its own ionisation
constant, and the magnitudes of these constants give a measure of the extent
to which each ionisation has proceeded at any given concentration. The greater
the value of K, relative to K,, the smaller will be the secondary dissociation,
and the greater must be the dilution before the latter becomes appreciable. It
is therefore possible that a diprotic (or polyprotic) acid may behave, so far as
dissociation is concerned, as a monoprotic acid. This is indeed characteristic of
many polyprotic acids.
A triprotic acid H3A (e.g. phosphoric(V) acid) will similarly yield three
dissociation constants, K,, K,, and K,, which may be derived in an analogous
manner: