Chemistry, Third edition

ACID–BASE INDICATORS 301

Acid–base indicators are

available in easy to use strips.

Multiplying by 1,

log [H 3 O(aq)]logKa(T)log

CA

Cs

Remembering that

log

CA

log

Cs

Cs CA

we arrive at the relationship

pHpKalog

Cs

CA

which is known as the Henderson–Hasselbalchequation (Exercise 16L).

Acid–base indicators

Acid–base indicators, such as methyl orange, phenolphthalein and litmus, show two

extreme colours, one at lower pH and the other at higher pH.

For example, in the case of methyl orange, the two extreme colours are red (lower

pH) and yellow (higher pH). As with most indicators, the change in colour does not

occur over a very small change of pH. The range of pH over which an indicator

changes colour is called its pH range. The pH range of methyl orange is 3.2–4.4. Below

pH 3.2 methyl orange is red, but as the pH is increased above3.2, the observed colour

contains an increasing amount of yellow until, beyond pH 4.4, it is entirely yellow.

Indicators are sometimes absorbed into paper strips (e.g. litmus paper). The indi-

cator known as ‘universal indicator’ is a mixture of selected indicators which displays

different colours at different pH values. (To demonstrate this, place a large crystal of

tartaric acid in a small flask. Add dilute universal indicator solution and one drop of

NaOH solution. Upon gently swirling the flask, the acid slowly dissolves, and the

indicator passes through an impressive sequence of colours.)

16.5

Using the Henderson–

Hasselbalch equation

A buffer solution contains

0.10 mol of one of the acids

listed in Table 16.2 mixed

with 0.10 mol of its

potassium salt. The

observed pH of the buffer

was 4.18. What is the

name of the acid?

Exercise 16L