Indicators

Indicators

• Weak acids, HIn, that have different colours in the undissociated form, HIn and the disscociated form In-.

• HIn(aq) H+(aq) + In -

(aq)

HIn(aq) H+(aq) + In -

(aq)

Add H+(aq) equilibrium to left red

Methyl Orange

Add OH-(aq) equilibrium to right yellow

Indicators

• Different indicators change colour at different pH values

• Investigate the colours of different indicators in different solutions

Indicator Colour in Acid

Colour in Alkali

pH range for colour change

Indicator Colour in Acid

Colour in Alkali

pH range for colour change

Methyl orange Red Yellow 2.9 – 4.0

Methyl red Red Yellow 4.2 – 6.3

litmus Red Blue 5.0 – 8.0

Bromothymol blue

Yellow Blue 6.0 – 7.6

Phenolphthalein colourless pink 8.2 – 10.0

Phenol red Yellow Red 6.6 – 8.0

Choosing Indicators for Titrations

• For an indicator to work the colour change must occur within the steep part of the pH curve.

Strong Acid / Strong Base

phenolphthalein

Methyl orange

Any indicator in pH range 3 – 11

suitable

Weak Acid / Strong Base

phenolphthalein

Methyl orangeAny indicator in pH range

7 – 11 suitable

phenolphthalein

phenolphthalein

Strong Acid / Weak Base

Methyl orange

Any indicator in pH range 3 – 7

suitable

Methyl orange

Weak Acid / Weak Base

phenolphthalein

Methyl orange

No rapid pH change at

equivalence – no suitable indicators