Chemistry, Third edition

16 · ACID–BASE EQUILIBRIA

How acid–base indicators work

Acid–base indicators consist of molecules that have different molecular structures at

lower pH and at higher pH. These higher-pH and lower-pH forms have different

colours.

Consider methyl orange. Above pH 4.4, methyl orange exists entirely as the ion

This ion is the higher-pH form. It is coloured yellow. It is represented as R–SO 3.

Below pH 3.2, methyl orange exists entirely as

in which one of the nitrogen atoms has been bonded to a proton (i.e. protonated) as

a result of reaction with H 3 O(aq). This ion is the lower-pH form. It is coloured red

and is represented as HR–SO 3 . These changes are summarized by the equilibrium

equation

R–SO 3 (aq)H 3 O(aq)\===\HR–SO 3 (aq)H 2 O

yellow red

All acid–base indicators undergo similar molecular changes with changing pH. The

general equation representing these changes is

higher-pH form H 3 O(aq)\===\lower-pH form H 2 O

(‘acid form’)

NNSN O 3

H 3 C

H 3 C

+

H

NNSN O 3

H 3 C

H 3 C

302

Colours of acid–base indicators

Table 16.4 shows the pH range of selected acid–base

indicators. Use this table to help you decide the colour of

phenolphthalein and bromocresol green at a pH of

(i)2, (ii)3.7, (iii)5,

(iv)6, (v) 8 (vi)11.

Table 16.4Acid–base indicators

Indicator Extreme colours pH range

Lower pH Higher pH

Alizarin yellow colourless yellow 10.1–12.0

Bromocresol green yellow blue 4.0–5.6

Litmus red blue 4.7–8.2

Methyl orange red yellow 3.2–4.4

Methyl red yellow red 4.8–6.0

Phenolphthalein colourless pink 8.2–10.0

Exercise 16M