198 DIY Science: Illustrated Guide to Home Chemistry Experiments
LABORATORY 11.2:
dETERmINE THE pH of AqUEoUS SALT SoLUTIoNS
When an acid reacts with a base, it forms a salt
and water. For example, reacting hydrochloric
acid with sodium hydroxide produces the salt
sodium chloride (common table salt) and water.
Reacting nitric acid with potassium hydroxide
produces potassium nitrate and water. Reacting
acetic acid with aqueous ammonia produces
ammonium acetate and water. And so on. Such
reactions are referred to as neutralizing the
acid with the base (or vice versa).
RIREEqU d EqUIpmENT ANd SUppLIES£ goggles, gloves, and protective clothing£ beaker, 150 mL£ pH meter£ ammonium acetate, 0.1 m (100 mL)£ ammonium chloride, 0.1 m (100 mL)£ sodium acetate, 0.1 m (100 mL)£ sodium chloride, 0.1 m (100 mL)SBSTITUTIU oNS ANd modIfICATIoNS- You may substitute a foam cup or similar container
for the beaker.
Unfortunately, the word “neutralize” is a common source of
confusion among beginning chemists, many of whom assume
that reacting equivalent amounts of an acid and base should
yield a solution that contains only the “neutral” salt that is neither
acidic nor basic, and therefore has a pH of 7.0. That’s not how it
works.
The pH of a neutralized solution depends on the particular acid
and base that are reacted. Reacting equivalents of a strong acid
with a strong base in fact does produce a salt solution that has a
pH at or near 7.0, as does reacting a weak acid with a weak base.
But if the strengths of the acid and base are very different—as
occurs, for example, if you react a strong acid with a weak base
or vice versa—the pH of the neutralized solution will not be 7.0.
The greater the difference in the strengths of the acid and base,
the greater the difference in the pH of the neutralized solution
from 7.0.
For example, if you neutralize hydrochloric acid (a very strong
acid) with aqueous ammonia (a relatively weak base), the
resulting solution of ammonium chloride will have a pH less than
7.0. Conversely, if you neutralize acetic acid (a relatively weak
acid) with sodium hydroxide (a very strong base), the resulting
neutralized solution of sodium acetate will have a pH greater
than 7.0.
In this lab, we’ll determine the pH of aqueous solutions of
various salts.
SBSTITUTIU oNS ANd modIfICATIoNS- You may substitute narrow-range pH testing paper
for the pH meter, with some loss of accuracy.
(Typical inexpensive pH meters are accurate to 0.1 or
0.2 pH, while typical narrow-range pH testing paper
is accurate to 0.4 or 0.5 pH.) - You may reduce the quantities of the solutions,
depending on the size and type of probe used by
your pH meter, and use smaller containers. If your
meter has a very thin probe (or if you are using pH
paper), you can reduce the quantities from 100 mL
to 10 mL and use test tubes. - If you have bench solutions of any of the required
chemicals, you can make up the required solutions
by diluting the bench solutions appropriately. - You can make 100 mL of 0.1 M ammonium acetate by
mixing 10 mL of 1.0 M aqueous ammonia with 10 mL
of 1.0 M acetic acid and 80 mL of water. - You can make 100 mL of 0.1 M ammonium chloride
by mixing 10 mL of 1.0 M aqueous ammonia with 10
mL of 1.0 M hydrochloric acid and 80 mL of water. - You can make 100 mL of 0.1 M sodium acetate by
mixing 10 mL of 1.0 M sodium hydroxide with 10 mL
of 1.0 M acetic acid and 80 mL of water. - You can make 100 mL of 0.1 M sodium chloride by
mixing 10 mL of 1.0 M sodium hydroxide with 10 mL
of 1.0 M hydrochloric acid and 80 mL of water.