Page 374 - Instrumentation Reference Book 3E

The concept of pH 357
Between 55 and 95°C 17.4.6 Common ion effect
pH = 4.000 + 1/2[(t - 15)2/100] - (t - 55)/500 All organic acids and the majority of inorganic
acids are weak electrolytes and are only partially
Other standard buffer solutions are given in Sec- dissociated when dissolved in water. Acetic acid,
tion 17.4.7. for example, ionizes only slightly in solution, a
The emf. E, is measured and likewise the e.m.f. process represented by the equation
E1 and E2 of similar cells with solution X replaced
by standard solutions S1 and S2, so that El and E2 HAC * H+ + Ac-
are on either side of and as near as possible to Ex.
The pH of the solution X is then obtained by Its dissociation constant at 25°C is only
assuming linearity between pH and E, Le., 1.8 x Le.,
(pHX - pH Si)/(PH S2 - PH SI) ([H+][Ac-])/[HAc] = 1.8 x mollliter
= (Ex - ElY(E2 - El)
or
17.4.4 Neutralization [H+][Ac-] = 1.8 x lO-'[HAc]
When acid and base solutions are mixed, they Therefore in a solution of acetic acid of moderate
combine 1.0 form a salt and water, e.g., concentration, the bulk of the acid molecules will
hydrochloric sodium sodium water be undissociated, and the proportion present as
acid + hydroxide = chloride + HOH acetic ions and hydrogen ions is small. If one of
H'Cl- i Na+OH- = NafC1- + (largely the salts of acetic acid, such as sodium (NaAc) is
(dissociated) (dissociated) (dissociated) undissociated)
added to the acetic acid solution, the ionization of
Thus, if equal volume of equally dilute solutions the acetic acid will be diminished. Salts are, with
of strong acid and strong alkali are mixed, they very few exceptions, largely ionized in solution,
yield neither an excess of H+ ions nor of OH- and consequently when sodium acetate is added
ions and the resultant solution is said to be neu- to the solution of acetic acid the concentration of
tral. The pH value of such a solution will be 7. acetic ions is increased. If the above equation is to
continue to hold, the reaction H' + Ac-' i HAC
must take place, and the concentration of hydro-
17.4.5 Hydrolysis gen ions is reduced and will become extremely
Equivalent amounts of acid and base when mixed small.
will produce a neutral solution only when the acids Most of the acetic ions from the acid will have
and bases used are strong electrolytes. When a recombined; consequently the concentration of
weak acid or base is used, hydrolysis occurs. When unionized acid will be practically equal to the
a salt such as sodium acetate, formed by a weak total concentration of the acid. In addition, the
acid and a strong base, is present in water, the concentration of acetic ions in the equilibrium
solution is slightly alkaline because some of the mixture due to the acid will be negligibly small,
H+ ions from the water are combined with acetic and the concentration of acetic ions will, there-
radicals in the relatively undissociated acetic acid, fore, be practically equal to that from the salt.
leaving an excess of OH- ions, thus: The pH value of the solution may, therefore, be
regulated by the strength of the acid and the ratio
sodium acetate + water + aceticacid + sodiumhydroxide [salt]/[acid] over a wide range of values.
Na+Ac- + HOH-HAC + Na+OH-
(dissociated) (iargely undissociated) (dissociated) Just as the ionization of a weak acid is dimin-
ished by the addition of a salt of the acid, so the
The pH value of the solution will therefore be ionization of a weak base will be diminished by
greater than 7. Experiment shows it to be 8.87 the addition of a salt of the base, e.g., addition of
in 0.1 mol/liter solution at room temperature. ammonium chloride to a solution of ammonium
Similarly, ammonium chloride (NHdCl), the hydroxide. The concentration of hydroxyl ions in
salt of a weak base and a strong acid, hydrolyzes the mixture will be given by a similar relationship
to form the relatively undissociated ammonium to that obtained for hydrogen ions in the mixture
hydroxide (NHdOH), leaving an excess of H+ of acid and salt, i.e.,
ions. The pH value of the solution will therefore
be less than 7. Experiment shows it to be 5.13 at [OH-] = K[alkali]/[salt]
ordinary temperatures in a solution having a con-
centration of 0.1 mollliter. 17.4.7 Buffer solutions
A neutralization process therefore does not
always produce an exactly neutral solution when Solutions of a weak acid and a salt of the acid
one mole of acid reacts with one mole of base. such as acetic acid mixed with sodium acetate and