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Colour I

Acid-Base Eqm (5): Acid-Base Indicators. p.01. H 3 O + + In -. HIn + H 2 O. [H 3 O + (aq)][In - (aq)]. Colour II. Colour I. K In =. [HIn(aq)]. In alkali, H 3 O + is used up and the eqm shifts FW, so the colour seen is colour II.

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Colour I

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  1. Acid-Base Eqm (5):Acid-Base Indicators p.01 H3O++ In- HIn + H2O [H3O+(aq)][In-(aq)] Colour II Colour I KIn = [HIn(aq)] In alkali, H3O+ is used up and the eqm shifts FW, so the colour seen is colour II. In acid, the excess H3O+ shift the eqm BW, so the colour seen is colour I. Acid-Base Eqm (5):Acid-Base Indicators An indicator (HIn) is a WEAK ACID whose conjugate base is a different colour. C. Y. Yeung (CHW, 2009)

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

  3. p.03 H3O++ Ph- HPh + H2O COLOURLESS PINK [H3O+(aq)][Ph-(aq)] KIn = this ratio controls the colour shown! [HPh(aq)] 1 1  [Ph-(aq)] [Ph-(aq)] [Ph-(aq)] Colourless when - log pH pKIn = 10 10 [HPh(aq)] [HPh(aq)] [HPh(aq)]  Redwhen When does it change colour? (1) Phenolphthalein (pKIn = 9.3)

  4. p.04 When [H3O+], eqm shifts FW, [Ph-] >> [HPh]. When [H3O+], eqm shifts BW, [Ph-] << [HPh]. H3O++ Ph- HPh + H2O   PINK ! Colourless ! when when [Ph-(aq)] [Ph-(aq)] + log + log pKIn pKIn pH = pH = At that time … [HPh(aq)] [HPh(aq)] 1 + log 9.3 = 10 10 10 1 + log 9.3 = [Ph-(aq)] [Ph-(aq)] 1 10 1 [HPh(aq)] [HPh(aq)] = 10.3 = 8.3 When does it change colour? (2) At that time …

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

  6. p.06 Important note about the use of Indicator … Next …. Titration Curve (Book 2 p. 168 – 169) 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.

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