Acid-Base Titration

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Acid-Base Titration
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Acid-Base Titration
HnA n OH- n A- n H2O
Equivalence Point The point at which the
reaction is just complete End Point The place
where experimentally the reaction is observed to
be complete
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At Equivalence Point A solution initially
containing A- moles of HA, now contains
an equivalent amount of A-. The amount of base
needed to reach the equivalence point is
independent of the strength of the acid.
THE WHOLE BASIS OF TITRATION RELYS ON THIS FACT!
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Although, the location of the equivalence
point does not change, the shape of the curve
does change.
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Observing the equivalence point

• There are as many equivalence points as there
  are replaceable hydrogens in an acid.
• Some equivalence points MAY
• occur without a noticeable rise in pH!
• Two requirements are necessary
• for a jump in pH to occur.
• The successive Ka values must
• differ by several orders of
• magntude
• 2. The pH of the equivalence point
• must not be very high or very low.

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Observing the equivalence point I knew there
was a use to studying calculus!
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Detecting the equivalence point
pH Measurements are the most direct way of
measuring the equivalence point
Less exacting methods include using an
indicator dye, which is it self an acid or a
base. The protonated and deprotonated forms
have different colors, and the pKa should
be close to the pH expected at the equivalence
point.
The observed color changes of an indicator
changes slowly over 1.5 to 2 pH units.
Indicators are therefore only useful in
titrations of acid and bases that show
definite breaks in the titration curve.
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Common acid-base indicators
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Phenolphthalein
H2In
In2-
In(OH)3-
lt 0 0-8.2
8.2-12.0
gt12.0
colorless
pink to fuchsia
colorless