Chapter 9 PowerPoint

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Buffers
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Buffers and Henderson-Hasselbalch
Equation
• Slns of a weak acid and its conjugate base (or a
weak base and its conjugate acid) are called
buffers.
• They are buffers as they keep the pH from
fluctuating widely upon the addition of small
amounts of a strong acid or base.
• So if a buffer is properly made, you can add
some HCl or NaOH and only have the pH of
the buffer sln change slightly.
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Buffers Cont.
• Buffers are common in nature
• Lakes, rivers, and oceans have a
carbonate buffer system.
• Our blood uses several buffer systems,
including a carbonate buffer (which is in
equilibrium with CO2) and a phosphate
buffer.
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What’s a Good Buffer?
• A good buffer system has fairly high
concentrations of the weak acid and base:
around 0.010M to 0.10M is considered
concentrated.
• The concentrations of the weak acid and
its conjugate base are ideally roughly
equal: [HA] = [A-]
• Although this means that the ideal buffer is 50/50,
buffers can be used where the ratio is 90/10 to 10/90.
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Buffer Capacity
• We can measure how good a buffer system is
by measuring the buffer capacity.
• The buffer capacity is simply the amount of
strong acid or base which can be added to the
buffer without changing the pH greatly.
• The more strong acid or base the buffer can
absorb (or neutralize), the higher the buffer
capacity.
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Buffer Capacity, Cont.
• So the higher the buffer concentration,
the higher the buffer capacity.
• The more equal the weak acid and
conjugate base concentrations are, the
higher the buffer capacity.
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Henderson-Hasselbalch Equation
• If we know that we have a buffer system,
we can use a simple equation to calculate
the weak acid and base concentrations.
• pH = pKa + log([base] / [acid])
• This is the Henderson-Hasselbalch eq.
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Henderson-Hasselbalch Equation
• Time for math!
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Buffer Review
• What is a buffer?
– A buffer is a solution of a weak acid and its
conjugate base (or a weak base and its
conjugate acid).
• What is the purpose of a buffer?
– It is a pH moderator, keeping the pH stable with
small additions of H+ or OH-
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Buffer Review
• How does a buffer keep the pH stable?
– When you add OH-, the weak acid reacts
with it to make the weak conjugate base
– When you add H+, the weak base reacts with
it to make the weak conjugate acid.
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Buffer Preparation
• How is a buffer made?
• There are actually several ways to make a
buffer sln, 2 of which are common.
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Buffers: Preparation Method 1
• You have a desired volume, pH, &
concentration: 500.00 mL of a 0.0500M
sln of TRIS buffer with a pH of 8.1.
• What’s TRIS?
• TRIS base is a crystalline solid with a
MW = 121.14 amu.
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Buffers: Preparation Method 1 cont.
• You calculate how many g TRIS you need
to make the desired volume & M.
• You weigh it out, completely transfer to a
beaker, add about 75% of the final
volume and measure the pH.
• Since TRIS is a base, the pH at this point
will be high compared to what we want.
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Buffers: Preparation Method 1 cont.
• So we need to add HCl until we get to the
desired pH of 8.1. (You are MAKING the
conjugate acid of TRIS base!)
• Start with 3 or 6M HCl, then when you
get close to 8.1, switch to 1M HCl, drop
by drop addition.
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Buffers: Preparation Method 1 cont.
• When you have the desired pH,
completely transfer to a volumetric flask.
• Rinse the beaker thoroughly with DI
water.
• Dilute to the mark with DI water.
• Done!
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Buffers: Preparation Method 1 cont.
• How would you modify the above if you
are starting with an acid like ammonium
chloride or boric acid?
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Buffers: Preparation Method 2
• Method 2 is not nearly as common, but it
is used.
• Let’s say you want a buffer system of
piperidine (which is a base) and its
conjugate acid piperidine hydrochloride.
• Piperidine and its conjugate acid are both
crystalline solids.
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Buffers: Preparation Method 2 cont.
• You want 250.00 mL of a 0.100M sln at a
given pH.
• You will be weighing both the piperidine
and the piperidine hydrochloride out so
you want to weigh enough piperidine to
make 250.00 mL of a 0.050M piperdine
sln.
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Buffers: Preparation Method 2 cont.
• You also want to weigh enough piperidine
hydrochloride to make 250.00 mL of a
0.050M piperdine sln.
• Weigh both solids, add together to a
beaker, dilute with about 75% of the
desired volume, adjust pH, transfer and
dilute to final volume.
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Buffer Capacity
• The buffer capacity is a measure of how well
the buffer resists changes in pH.
• The more strong acid or base the buffer can
absorb (or neutralize), the higher the buffer
capacity.
• The buffer capacity will be at its highest when
pH = pKa or the ratio of A-/HA = 1.
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Buffer Capacity, Cont.
• We also want relatively high buffer
concentrations (relative to a typical
addition of strong acid or base) or at least
as high as is practical.
• A 1M buffer would have a higher buffer
capacity than the same buffer which is
just 0.1 M.
• You just have more moles of HA and A-.
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Buffer Capacity, Cont.
• A common way to measure the buffer
capacity is by measuring pH upon the
addition of strong acid or base.
• The larger the pH, the weaker the buffer
capacity.
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Picking the Right Buffer
• To pick the right buffer, you need to know
the desired pH of your system.
• Then pick a buffer that has a pKa as close
to that pH as possible.
• The pKa should be within 1 unit of the
desired pH.
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Buffers & Temperature
• As equilibrium constants depend on
temperature, a buffer system depends
on temperature.
• Changing temperature changes both the
pH and the pKa.
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Buffers & Ionic Strength
• Using just H-H as given, dilution of a
buffer should not change the pH of the
buffer.
• But this is simplistic, ignoring activities
due to ionic strengths.
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Buffers & Ionic Strength
• Diluting a buffer lowers the ionic
strength of the buffer, and this in turns
changes (raises) the pH.
• This is Chapter 12.
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