CL-10 Plus – User Manual
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ECPUS vers 0.2 eng
3.2.2
Titration curve and Buffer Power measurement.
•
Titration curve.
Titration is used to determine the amount of acid in a given solution. In this procedure a
measured volume of sample is titrated with a solution of a base, usually sodium
hydroxide (NaOH), the concentration of which is known. The NaOH is added in a small
increments until the acid is neutralised, as determined with an indicator or a pH meter.
From the volume of NaOH added at known concentration, it is calculated the
concentration of acid in the solution titrated.
From the titration of a sample containing weak acids (pK between 3 and 6) additional
information are obtained, measuring the pH of the acid after each increment of NaOH
added, to the point of neutralisation. A plot of pH of the solution vs. the amount of NaOH
added to this point is called ‘titration curve’.
Let us take the
titration curve of acetic acid
, a typical weak acid. At the beginning, (see
figure) before any NaOH is added, the acetic acid is slightly ionised.
When aliquots of NaOH are added, the added OH
-
will combine with free H
+
in the
solution to form H
2
O. As soon as free H
+
is removed, some of undissociated acetic acid
immediately dissociates. At the midpoint of the titration, one-half of the original acetic
acid has undergone dissociation, so that the proton-donor concentration will be equal to
the proton-acceptor concentration.
As the titration is continued by adding further increments of NaOH, the remaining
undissociated acetic acid is gradually converted into acetate.
The titration curve is a reversible reaction (adding H
+
back to the system, acetate is
converted back to the initial state).
NaOH added
pH
7
pK of
Acetic Acid
3
