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ACID–BASE INDICATORS 273

We can split the fraction term in Equation (6.51) by employing the laws of logarithms,
to yield
−
[A ]
+
log 10 K a = log [H 3 O ] + log 10 (6.52)
10
[HA]
+
The term ‘log 10 K a ’ should remind us of pK a (Equation 6.52), and the term log [H 3 O ]
10
will remind us of pH in Equation (6.20), so we rewrite Equation (6.52) as
−
[A ]
−pK a =−pH + log 10 (6.53)
[HA]
which, after a little rearranging, yields the Henderson–Hasselbach equation,
Equation (6.50).

6.5 Acid–base indicators

Thename‘litmus’
What is ‘the litmus test’?
comes from the Old
pH indicators Norse litmosi,which
derives from litr and
Litmus is a naturally occurring substance obtained from lichen. It mosi, meaning dye and
imparts an intense colour to aqueous solutions. In this sense, the moss respectively.
indicator is a dye whose colour is sensitive to the solution pH.
If the solution is rich in solvated protons (causing the pH to be
Much of the litmus
less than 7) then litmus has an intense red colour. Conversely, a
in a laboratory is
solution rich in hydroxide ions (with a pH greater than 7) causes pre-impregnated on
the litmus to have a blue colour. dry paper.
To the practical chemist, the utility of litmus arises from the way
its colour changes as a function of pH. Placing a single drop of
litmus solution into a beaker of solution allows us an instant test of Litmus is an indicator.
the acidity (or lack of it). It indicates whether the pH is less than To avoid ambiguity,
7 (the litmus is red, so the solution is acidic), or the pH is greater we shall call it an
‘acid–base indicator’
than 7 (the litmus is blue, so the solution is alkaline). Accordingly,
or a ‘pH indicator’.
we call litmus a pH indicator.
In practical terms, we generally employ litmus during a titration.
The flask will contain a known volume of acid of unknown con- Litmus often looks
centration, and we add alkali from a burette. We know we have purple–grey at the
reached neutralization when the Litmus changes from red (acid neutralization point.
in excess) and just starts changing to blue. We know the pH of This colour tells us we
the solution is exactly 7 when neutralization is complete, and then have a mixture of both
note the volume of the alkali, and perform a calculation similar to the red and blue forms
Worked Example 6.11. of litmus.
The great English scientist Robert Boyle (1627–1691) was the
first to document the use of natural vegetable dyes as acid–base indicators.