Lec' 26 Indicators

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Fred J. Grieman
Lec. 26 Indicators Polyprotic
Acids Lewis Acids/Bases HW 8 64, 68, 69, 72
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Indication of end point ? equivalence point

• pH Meter Measure pH as a function of Volume of
  Titrant
• Plot titration curve and graphical
  determine endpoint
• Use an indicator weak organic acid that changes
  color
• over
  small pH range
• HIn(aq) H2O(l) ? H3O(aq)
  In-(aq)
• Where it changes color depends on Ka(HIn)
• H3O HIn
• Ka
  In-
• Ratio dictates dominate form

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Phenophthalein used in Chem 1a Lab
The color change in phenolpthalein is due to a
change in structure of the molecule.  In acid,
the molecule is in its HIn form containing a
central 5 membered ring which is somewhat
strained. Clear (absorbs in ultraviolet) In base
the In- structure opens up and becomes
flatter. Pink (absorbs in the visible) Ka 8 x
10-10
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Phenolphthalein Ka 8 x 10-10 ? 10-9 at pH
9 H3O 10-9 HIn
1 Pink begins to show Ka
10-9 In-
Why did we select this Indicator?
pH at endpoint of our rxn? Hydrogen Phthalate
HP-OH- Kb ------- 2.56 x
10-9 P2- P2- H2O
? HP- OH- P2--x
x x at equil. P2-
.030 M Calculate pH at endpoint. x2
----- 2.56 x 10-9 x 8.76 x 10-6
OH- .030 x pOH 5.063 pH 14
- 5.063 8.937
Polyprotic acids on board
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Lewis acid / base interaction
Ligand ? L
M ? positive center
Lewis acid (electron pair acceptor)
Lewis base (electron pair donor)
Coordinate covalent bond (e- pair from ligand
only) H3N H ? NH4 But also,
H3N Co2 ? Co(NH3)2 6 times
6 H3N Co2 ? Co(NH3 )62