1. Colour I Colour II Acid-Base Eqm (5): Acid-Base Indicators C. Y. Yeung (CHW, 2009) p.01 Acid-Base Eqm (5): Acid-Base Indicators An indicator (HIn) is a WEAK ACID whose conjugate base is a different colour. HIn + H 2 O H 3 O + + In - [H 3 O + (aq)][In - (aq)] [HIn(aq)] K In = In acid, the excess H 3 O + shift the eqm BW, so the colour seen is colour I. In alkali, H 3 O + is used up and the eqm shifts FW, so the colour seen is colour II.

2. p.02 Methyl Orange (pK In = 3.70) YELLOW RED Phenolphthalein (pK In = 9.3) PINK COLOURLESS Examples:

3. p.03 Phenolphthalein (pK In = 9.3) HPh + H 2 O H 3 O + + Ph - When does it change colour? (1) [H 3 O + (aq)][Ph - (aq)] [HPh(aq)] K In = pK In = [Ph - (aq)] [HPh(aq)] - log pH PINK COLOURLESS this ratio controls the colour shown! Colourless when [Ph - (aq)] [HPh(aq)]  1 10 Red when [Ph - (aq)] [HPh(aq)]  1 10

4. p.04 when [Ph - (aq)] [HPh(aq)]  1 10 When does it change colour? (2) When [H 3 O + ] , eqm shifts BW, [Ph - ] << [HPh]. HPh + H 2 O H 3 O + + Ph - Colourless ! At that time … pH = [Ph - (aq)] [HPh(aq)] + log pK In =110 + log 9.3 =8.3 when [Ph - (aq)] [HPh(aq)]  10 1 When [H 3 O + ] , eqm shifts FW, [Ph - ] >> [HPh]. PINK ! PINK ! At that time … pH = [Ph - (aq)] [HPh(aq)] + log pK In = 10 1 + log 9.3 = 10.3

5. p.05 Phenolphthalein pH 7 8.15 9.15 10.15 colourless pale pink pink pH 8.15 to 10.15 = “Working Range” of phenolphthalein. Methyl orange pH 7 2.70 3.70 4.70 red orange yellow pH 8.15 to 10.15 = “Working Range” of methyl orange. “Working Range” [where colour change is observable] of Indicators

6. p.06 Important note about the use of Indicator … Indicator is actually an acid (a very very weak acid), only a minimal amount is added during a titration, so that the pH of the solution would not be affected. Next …. Titration Curve (Book 2 p. 168 – 169)