1 buffers. 2 buffers and henderson-hasselbalch equation slns of a weak acid and its conjugate base...

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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 ΔpH upon the addition of strong acid or base.

• The larger the ΔpH, 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.