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TITRATION ANALYSES 263

A similar process occurs when we spread a thick paste of zinc and
Some campaigners
castor oil on a baby’s bottom each time we change its nappy. The
believe the AlCl 3 pro-
‘zinc’ is in fact zinc oxide, ZnO, which, being amphoteric, reacts
duced by Equation
with the uric acid in the baby’s urine, thereby neutralizing it.
(6.40) hastens the
onset of Alzheimer’s
Worked Example 6.9 But how much stomach acid is neutralized disease. Certainly, the
by a single indigestion tablet? The tablet contains 0.01 mol of MOH, brains of people with
+
where ‘M’ is a monovalent metal and M its cation. this nasty condition
contain too much alu-
We first consider the reaction in the stomach, saying it proceeds with minium.
1 mol of hydrochloric acid reacting with 1 mol of alkali:

+
+
MOH (s) + H 3 O (aq) −−−→ M (aq) + 2H 2 O (6.41)
M is merely a cation. We say Equation (6.41) is a 1:1 reaction, occurring with a 1:1
+
stoichiometry. Such a stoichiometry simplifies the calculation; the 3:1 stoichiometry in
Equation (6.40) will be considered later.
From the stoichiometry of Equation (6.41), we say the neutralization is complete
after equal amounts of acid and alkali react. In other words, we neutralize an amount
n of hydrochloric acid with exactly the same amount of metal hydroxide, i.e. with
1 × 10 −2 mol.
The tablet can neutralize 0.01 mol of stomach acid.

This simple calculation illustrates the fundamental truth under-
The experimental tech-
lying neutralization reactions: complete reaction requires equal
nique of measuring out
amounts of acid and alkali. In fact, the primary purpose of a
the amount of acid and
titration is to measure an unknown amount of a substance in alkali needed for neu-
a sample, as determined via a chemical reaction with a known tralization is termed a
amount of a suitable reagent. We perform the titration to ascer- titration.
tain when an equivalent amount of the reagent has been added to
the sample. When the amount of acid and alkali are just equal,
The amounts of acid
we have the equivalence point, from which we can determine the and alkali are equal at
unknown amount.
the equivalence point.
In a typical titration experiment, we start with a known volume The linguistic similar-
of sample, call it V (sample) . If we know its concentration c (sample) , ity between these two
we also know the amount of it, as V (sample) × c (sample) . During the words is no coinci-
course of the titration, the unknown reagent is added to the solu- dence!
tion, usually drop wise, until the equivalence point is reached (e.g.
determining the endpoint by adding an indicator; see p. 273ff).
At equivalence, the amounts of known and unknown reagents are We need equal num-
the same, so n (sample) = n (unknown) . Knowing the amount of sample bers of moles of acid
and alkali to effect
and the volume of solution of the unknown, we can calculate the
neutralization.
concentration of the unknown.