Page 74 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

2 FUNOAMENTAL THEORETICAL PRINCIPLES OF REACTIONS IN SOLUTION

solution of ammonium formate:
pH = 7.0 + 1.88 - 2.37 = 6.51
(Formic acid: Ka = 1.77 x mol L-'; pKa = 3.75)
i.e. the solution has a slightly acid reaction.

2.20 BUFFER SOLUTIONS
A solution of hydrochloric acid (0.0001 mol L-') should have a pH equal to 4,
but the solution is extremely sensitive to traces of alkali from the glass of the
containing vesse1 and to ammonia from the air. Likewise a solution of sodium
hydroxide (0.0001 mol L-'), which should have a pH of 10, is sensitive to traces
of carbon dioxide from the atmosphere. Aqueous solutions of potassium chloride
and of ammonium acetate have a pH of about 7. The addition to 1 L of these
solutions of 1 mL of a solution of hydrochloric acid (1 mol L-') results in a
change of pH to 3 in the former case and in very little change in the latter. The
resistance of a solution to changes in hydrogen ion concentration upon the
addition of small amounts of acid or alkali is termed buffer action; a solution
which possesses such properties is known as a buffer solution. It is said to
possess 'reserve acidity' and 'reserve alkalinity'. Buffer solutions usually consist
of solutions containing a mixture of a weak acid HA and its sodium or potassium
Salt (A-), or of a weak base B and its Salt (BH'). A buffer, then, is usually a
mixture of an acid and its conjugate base. In order to understand buffer action,
consider first the equilibrium between a weak acid and its salt. The dissociation
of a weak acid is given by:

and its magnitude is controlled by the value of the dissociation constant Ka:

The expression may be approximated by writing concentrations for activities:

This equilibrium applies to a mixture of an acid HA and its salt, Say MA. If
the concentration of the acid be ca and that of the Salt be c,, then the
concentration of the undissociated portion of the acid is (ca- CH+]). The
solution is electrically neutral, hence [A-] = c, + [H +] (the Salt is completely
dissociated). Substituting these values in the equilibrium equation ( l8), we have:

This is a quadratic equation in [H +] and may be solved in the usual manner.
It can, however, be simplified by introducing the following further approximations.
In a mixture of a weak acid and its salt, the dissociation of the acid is repressed
by the common ion effect, and [H'] may be taken as negligibly small by