Chemistry Reference
In-Depth Information
acid), then the percentage of the compound ionised increases to about
90%. If the pH increases to two units above the p
K
a
(or two units below
for a base), the percentage ionised increases to 99%, since both pH and
p
K
a
are logarithmic relationships, and so on to 99.9%, 99.99% etc. This
approximate 'rule of thumb' is summarised below.
For weak acids:
pH
p
K
a
compound is approximately 50% ionised
pH
p
K
a
1
compound is approximately 90% ionised
pH
p
K
a
2
compound is approximately 99% ionised
pH
p
K
a
3
compound is approximately 99.9% ionised
pH
p
K
a
4
compound is approximately 99.99% ionised
For weak bases:
pH
p
K
a
compound is approximately 50% ionised
pH
p
K
a
1
compound is approximately 90% ionised
pH
p
K
a
2
compound is approximately 99% ionised
pH
p
K
a
3
compound is approximately 99.9% ionised
pH
p
K
a
4
compound is approximately 99.99% ionised
This relationship is hugely important and well worth committing to
memory. It will reappear many times in this topic, in many different guises,
and will allow the readers to impress colleagues (particularly medical
colleagues) with their uncanny understanding of pH and ionisation of
drugs.
In the case of predicting the pH at the end point of titrations, most
acid-base reactions are considered over when the ratio of ionised form to
unionised form is 1000 to 1, i.e. when
99.9
pH
p
K
a
log —-
0.1
From the rules above, this point is reached when the pH of the solution is
three units above the p
K
a
of the acid (or three units below the p
K
a
of the
conjugate acid of the base), and this allows an appropriate indicator to be
chosen.
For example, if the acid being titrated has a p
K
a
of 4.7, then the end
point pH will be 4.7
7.7, and an indicator that changes colour
between pH 7.0 and 8.0 should be chosen. Similarly, for a base with a p
K
a
of 8.5, the end point pH will be 8.5
3
5.5, and an indicator with a
pH range of 5.0 to 6.0 should be used. The pH ranges of many common
indicators are shown in Chapter 6 (p. 144).
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