Ch' 12 AcidBase Titrations

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Ch. 12 Acid-Base Titrations
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Acid-Base Titrations

• Usually, we are interested in how much acid or
  base is present in a sample
• However, by analyzing a titration curve, we can
  also determine pK values for multiple acid/base
  components in the sample

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Titration of a Protein
Ribonuclease enzyme is composed of 124 amino
acids. 16 can be protonated by acid and 20 can
lose a proton to base. Finding the pKs can give
information about the environment of each AA in
the protein.
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Review of Titrations (Strong/Strong)
For a titration of strong acid with strong
base, remember that there are three regions of
the titration curve before, after, and at the
equivalence point. The equivalence point in this
situation will be pH7 because H from a strong
acid reacts stoichiometrically with OH-. You
should always calculate the equivalence volume
(Ve), the amount of titrant expected to reach the
equivalence point. We introduced shortcut
calculations in Ch. 7 for determining pH before
and after Ve added.
50.00 mL of 0.02 M KOH 0.1 M HBr
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Titration Curve Data for OH/HBr
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Titrations with Weak Acids/Bases

• Now that weve learned about equilibria, we can
  look at titrations involving strong/weak
  combinations of acids and bases
• These behave differently than strong/strong
  examples
• Most notably, the equivalence point will be
  dependent on Ka or Kb, and thus not at pH7

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Weak Acid/Strong Base

• What would the titration curve look like for the
  titration of 50 mL of 0.02000 M MES
  2-(N-morpholino)ethanesulfonic acid with 0.1000
  M NaOH? (pKa of MES 6.15)
• This is the reverse of the Kb reaction for A-,
  so
• Since K is large, we can say that the reaction
  goes to completion after each addition of OH-
• Lets calculate Vb (Ve)

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Weak Acid/Strong Base (cont)

• The titration curve has 4 regions in this case
• Before any base is added
• After base is added, but before the equivalence
  point
• At the equivalence point
• After the equivalence point

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Before Base is Added

• Prior to addition of base, we simply have a
  solution of weak acid
• We can just calculate pH based on the equation

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Before Equivalence Point

• Once we add OH-, the solution is a mixture of HA
  and A-. This is essentially a buffer, so we can
  use Henderson-Hasselbalch to find pH. Lets find
  pH with 3 mL OH- added
• Remember that for the H-H equation, we just need
  relative concentrations of HA/A-
• Whats the pH when volume added 1/2 Ve?

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At the Equivalence Point

• At Ve, we have added enough OH- to consume HA.
  The solution now contains only A-, so we can
  write the reaction for the weak base
• But, the formal concentration is not 0.020 M,
  because we have diluted by adding titrant

Dilution Factor
Initial Concentraion
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After the Equivalence Point

• After Ve, weve added excess OH-. At this point,
  we assume that OH- is a much stronger base than
  A-. Lets calculate pH when weve added 10.1 mL
  of OH-, just 0.1 mL past Ve.

Dilution Factor
Initial Concentraion
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The Titration Curve
The steepest point of the curve is still the
equivalence point. At Ve/2, pHpKa, the point
where the slope of the curve is lowest. This
also means that this is the point at which you
have maximum buffer capacity, or the solution is
most resistant to changes in pH.
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Weak Base/Strong Acid

• Titration of a weak base with a strong acid is
  the reverse of the previous example
• Before acid is added, pH is determined by the Kb
  reaction
• After acid is added, but before the equivalence
  point, we have a buffer and can use H-H
• At the equivalence point, B has been converted to
  BH
• After the equivalence point, excess acid
  determines pH

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Example

• 0.1067 M HCl is used to titrate 25.00 mL of
  0.08364 M pyridine (Kb1.69x10-9)
• What is the pH when Va 4.63 mL?
• First find Ve
• So weve added some acid, but are still before
  the equivalence point. We are in the buffer
  region

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Multiprotic Systems
Behavior of multiprotic acids and bases extend
directly from what we just saw with monoprotics.
Before addition of acid, we just have a weak base
equilibrium
After A, but until first equivalence point
(C), solution behaves as a buffer
At the first equivalence point, we have the
intermediate form, and pH1/2(pK1pK2) (Worst
choice for a buffer)
After C, but until 2nd equivalence point
(E), solution behaves as a buffer around the 2nd
eq
At the 2nd eq. point, we have a solution
which behaves just as if we dissolved the
conjugate base
After the 2nd equivalence point, we have added
excess titrant, which determines the pH
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Titration Curve, Phosphoric Acid w/ 0.1 M base
(OH-)
pKa312.15
pKa27.2
pKa12.15
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Finding the End Point

• The end point of the titration can be determined
  with a pH meter or color indicator
• The signal from a pH meter can be input directly
  to a computer for monitoring the titration
• Indicators change color within a certain pH range
  . We always want an indicator whose transition
  overlaps the steepest part of the titration curve.

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Indicators
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Primary Standards-Acids
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Primary Standards-Bases
NaOH/KOH are not primary standards! They
absorb CO2 to form HCO3-
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Strong Acids-Which are Strongest?

• The strongest acid that can exist in water is
  H3O and the strongest base is OH-
• Because of this, strong acids behave with the
  same strength in water. This is called the
  leveling effect--they all dissociate to H3O
• But do you think that HCl, HClO4, HNO3 are
  actually of equal strengths?

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Determining Acid Strength

• We can determine the relative strengths of strong
  acids by putting them in a non-aqueous solvent
• In acetic acid solvent
• In acetic acid, HClO4 is a stronger acid than HCl

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Titration in Non-Aqueous Solvent
A titration curve for several acids against
tetrabutylammonium hydroxide in methyl isobutyl
ketone solvent. Again, we see that HClO4 is a
stronger acid than HCl. We can use
non-aqueous solvents to titrate species that we
cant in water. For instance, species that are
too weak (Low K) to titrate w/ H in water, might
titrate with HClO4 in other solvents.