Indicators. Many chemicals, such as the pigments of pelargonia, have different colours depending on whether they are in the acid form or its conjugate.

Indicators

Many chemicals, such as the pigments of pelargonia, have different colours depending on whether they are in the acid form or its conjugate base.

This is the basis of indicators.

Demonstration• 1) Pipette out 25ml of acid into a

conical flask.• 2) Fill a burette with the base.• 3) Add a few drops of universal

indicator.• 4) Titrate until the colour change

indicates pH 7 has been achieved. Note the volume.

• 5) Repeat using phenolphthalein.• 6) Repeat using methyl orange.

Indicators as weak acids.

Indicators are weak acids.

HIn H⇌ + + In-Acid

Conjugate base

In acid the eqm is to the LHS

HIn ⇌ H+ + In-

Solution has the colour of the undissociated acid form.

In alkali equm is to the RHS.

HIn ⇌ H+ + In-

Solution has the colour of the conjugate base.

Phenolphthalein

C

C

O

O

HO OH

pH > 10.0

C

C

O

O

-O OH

+ H+

pH < 8.2

Eg; Phenolphthalein

• Acid form – colourless, Conjugate base - pink

Methyl orange

NN

+

CH3

CH3

O- Na

+

O

O

SN

H

+ H+NN

CH3

CH3

N S

O

O

O- Na

+

pH < 3.2

pH > 4.4

Methyl orangeMethyl orange

• Acid form – red, conjugate base – yellow.

Bromothymol blue

• Under acid conditions (LHS) bromothymol blue is yellow.

• Under basic conditions (RHS) it is blue.

End points• At the end point;

• [In-] = [HIn]• So the colour will be half way between the

two.• The end point should be sharp, such that the

addition of a small amount of acid or base causes a complete colour change.

• NB The contribution of the indicator to the pH of a solution is minute as only a few drops are used.

pH and endpoint

• HIn ⇌ H+ + In-

• Kin = [H+] [In-] / [HIn]• pKin = pH + log ([HIn] / [In-]) • pH = pKin - log ([HIn] / [In-])• At the end point [HIn]= [In-]• pH = pKin – log1• log 1 = 0• So pH = pKin

• It is assumed that the “acid” colour is totally

obscured by the “base” colour when [In-] = 10 x [HIn]

• Substituting;

• pH = pKin - log ([HIn] / [In-])

• pH = pKin - log([HIn] / 10[HIn])

• log 0.1 = -1

• pH = pKin – (-1)

• pH = pKin + 1

• Similarly the “base” colour will be obscured when;

• [HIn] = 10 [In-] • Subsituting;• pH = pKin - log ([HIn] / [In-])• pH = pKin - log(10[In-] / [In-]) • log 10 = 1• pH = pKin – (+1)• pH = pKin - 1

Effective range of an indicator

• Generally the effective range of an indicator is;

• pKin +/- 1

• ie 2 pH units.• For a titration the end point of the indicator

must correspond with the equivalence point. • The point where neutralisation has taken

place and the reaction has finished.

Indicator Acid colour

Base colour

pKinpH range

Methyl orange

red yellow 3.71 3.2 – 4.4

Methyl red yellow red 5.1 4.8 - 6

Bromo thymol blue

yellow blue 7 6 – 7.6

Phenol phthalein

colourless pink 9.4 8.2 - 10