### Chapter 9 PowerPoint

```Buffers
1
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.
some HCl or NaOH and only have the pH of
the buffer sln change slightly.
2
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.
3
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.
4
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.
5
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.
6
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.
7
Henderson-Hasselbalch Equation
• Time for math!
8
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-
9
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.
10
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.
12
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
volume and measure the pH.
• Since TRIS is a base, the pH at this point
will be high compared to what we want.
13
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!)
get close to 8.1, switch to 1M HCl, drop
14
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!
15
Buffers: Preparation Method 1 cont.
• How would you modify the above if you
are starting with an acid like ammonium
chloride or boric acid?
16
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.
17
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.
19
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.
20
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-.
21
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.
22
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.
23
Buffers & Temperature
• As equilibrium constants depend on
temperature, a buffer system depends
on temperature.
• Changing temperature changes both the
pH and the pKa.
24
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.
25
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.
26
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