Buffering action
Addition of small amounts of acid to most solutions causes a dramatic change
in pH. Addition of one drop (about 0.05cm3) of 0.1 mol dm-3
HCl produces a change in pH of about 3 units in 25cm3 of pure water
(pH7).
Example:
Show the pH change that results from addition of
0.05cm3 of 0.1 mol dm-3 hydrochloric acid to 25cm3
water (pH=7).
Total moles of acid = 0.05 x 10-3 x 0.1 = 5 x 10-6
moles
Concentration = moles/litres = 5 x 10-6 / 0.02505 = 1.996
x 10-4 mol dm-3
final pH = -log[H+] = 3.69
However, some solutions resist changes in pH when small amounts of acid or
base are added. On addition of acid, the hydrogen ions get removed by one of
the components of the mixture and on addition of base, the hydroxide ions get
removed by one of the components of the mixture. The effect is called buffering
action. Solutions that behave this way are called buffers.
There are two types of buffer.
- 1 Weak acid and the salt of the same
weak acid, (for example a solution containing ethanoic acid and sodium ethanoate).
This gives a buffer solution with a pH less than 7
- 2 Weak base and the salt of a the same
weak base (for example ammonia and ammonium chloride solution). This gives
a buffer with a pH greater than 7
The first (acidic) buffer works in the following way.
If an acid is added it combines its free hydrogen ions with the ions from the
salt of the weak acid, making the molecular form of the weak acid, which cannot
affect the pH.
If a base is added the OH- ions from the base react with the H+ ions that are
present from the weak acid dissociation. Having been removed from the solution
this stimulates the weak acid to produce more H+ ions (Le Chatelier's Principle)
and the original pH is re-established.
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Acidic buffers
Acidic buffers, i.e. buffers with a pH of below 7,
are formed from a solution containing a weak acid and one of its salts. The
most common example is a mixture of ethanoic acid and sodium ethanoate.
Two equilibria are established:
Equilibrium 1
CH3COOH ⇋ CH3COO- + H+
The first equilibrium lies 99% to the left hand side (i.e. there is a large store of ethanoic acid molecules)
Equilibrium 2
CH3COONa ⇋ CH3COO- + Na+
The second equilibrium lies almost 100% to the right hand side (i.e. there is a large store of ethanoate ions)
Hence, the mixture has large quantities of both ethanoic acid molecules [CH3COOH]
from the first equilibrium, and ethanoate ions [CH3COO-]
from the second equilibrium.
Addition of small quantities of acid (H+)
The H+ ions added react with the excess ethanoate ions in equation 2 and are
removed from the solution as ethanoic acid molecules (these have no effect
on the pH).
Hence the pH stays the same.
Addition of small quantities of base (OH-)
In this case, the OH- ions react with the hydrogen ions from the
first equilibrium removing them from the right hand side.
There is, however, a large reservoir of ethanoic acid on the left hand side
of this equilibrium able to dissociate and make more hydrogen ions, restoring
the pH.
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Basic buffers
A basic buffer is made from a solution containing a weak base and one of its salts. The most common example is a solution of ammonium chloride (salt of a weak base) and ammonia solution (weak base).
In this case there are two equilibria:
Equilibrium 1:
NH3 + H2O ⇋ NH4+ + OH-
This equilibrium lies very much to the left hand side (i.e. there is a large reserve of free ammonia molecules)
Equilibrium 2:
NH4Cl ⇋ NH4+ + Cl-
This equilibrium lies 100% to the right hand side (i.e. there is a large reservoir of ammonium ions)
Addition of small quantities of acid (H+)
The H+ ions from the acid react with the OH- ions from equilibrium 1 and remove
them.
However, there is a large reserve of ammonia molecules available to dissociate from the left hand side making more OH- ions restoring the pH (remember that the value of [H+][OH-] must be constant under all conditions except a change of temperature)
Addition of small quantities of base (OH-)
Adding more OH- ions that can react with the free ammonium ions (from equilibrium
2) producing more ammonia (as in equilibrium 1) and effectively being removed
from the system.
The ammonia molecules have no effect on pH an therefore the pH remains the
same.
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Worked examples
Q861-01 A "buffered"
solution:
- contains an insoluble solid.
- resists a change in its pH.
- contains a salt of a strong acid and a strong base.
- contains a concentrated strong acid
Answer
| By definition, a buffer solution is one whose pH resists small additions
of acid or base. - Response B |
Q861-02 The body needs to
maintain a constant pH of around about 7.4 to operate correctly. It does this
by using a weak acid, carbonic acid (H
2CO
3) and another
chemical. Which of the following substances could be suitable for creating a
buffer in conjuction with carbonic acid in the blood.
- sodium phosphate
- sodium hydrogen carbonate
- sodium sulfate
- sodium chloride
|
The 'other' substance must be the salt of the weak acid, carbonic
acid. The only one in the list that could be formed from carbonic
acid is sodium hydrogen carbonate NaHCO3. Response
B
|
Q861-03 Which mixture is a
buffer solution?
- 0.10 M HI + 0.10 M KI
- 0.10 M KCl + 0.10 M NaCl
- 0.10 M NaCN + 0.10 M HCN
- 0.10 M NaOH + 0.10 M KOH
Answer
|
Buffers consist of a weak acid with the salt of the weak acid.
The only weak acid in the choices is hydrocyanic acid, HCN making
the question very easy. - Response C
|
Q861-04 Which of the following
mixtures would produce a buffer solution when dissolved in 1.0 dm
3
of water?
- 0.50 mol of CH3COOH and 0.50 mol of NaOH
- 0.50 mol of CH3COOH and 0.25 mol of NaOH
- 0.50 mol of CH3COOH and 1.00 mol of NaOH
- 0.50 mol of CH3COOH and 0.25 mol of Ba(OH)2
Answer
|
The concentration of each of the components is proportional to the
number of moles, as they are dissolved in the same volume of solution.
- In A, all of the acid and base are neutralised and only the salt
remains. Not a buffer.
- In B, there is excess acid, therefore the product contains both
weak acid and the salt of the acid. Buffer
solution.
- In C, there is excess base so the final product contains strong
base and the salt (of the strong base and weak acid). Not a buffer.
- In D, the total moles of acid are neutralised by the base exactly
as the base is dibasic (contains two OH groups per formula unit).
Not a buffer.
|
Q861-05 Which of the following
combinations will produce a buffer solution?
- I. 20cm3 0.10 mol dm-3 CH3COOH
and 10cm3 0.10 mol dm-3 CH3COONa
- II. 20cm3 0.10 mol dm-3 CH3COOH
and 10cm3 0.10 mol dm-3 NaOH
- I only
- II only
- Both I and II
- Neither I nor II
Answer
|
Choice I has a weak acid (ethanoic acid) + the salt of ethanoic acid,
sodium ethanoate - buffer solution
Choice II has a weak acid and a strong base. However, these react
together to make a salt of the weak acid and, as the weak acid is
in excess, there will still be some weak acid left at the end of the
reaction. It is, therefore, also a buffer.
|
Q861-06 Which of the following
is a buffer solution?
- I. 0.01 mol dm-3 HCl, 0.01 mol dm-3 NaCl
- II. 0.01 mol dm-3 CH3COOH, 0.01 mol dm-3
CH3COONa
- I only
- II only
- Both I and II
- Neither I nor II
Answer
|
Choice I contains a strong acid and cannot be a buffer.
Choice II contains a mixture of a weak acid and a salt of the same
acid - it is a buffer solution.
|
Q861-07 A buffer solution
can be prepared by adding which of the following to 50cm
3 of 0.10
mol dm
-3 CH
3COOH(aq)?
- I. 50cm3 of a 0.10 mol dm-3 CH3COONa(aq)
- II. 25cm3 of a 0.10 mol dm-3 NaOH(aq)
- III. 50cm3 of a 0.10 mol dm-3 NaOH(aq)
- I only
- II only
- II and III only
- I, II and III
Answer
|
Choice I is the salt of a ethanoic acid. This will make a buffer
solution with ethanoic acid.
Choice II is sodium hydroxide which reacts with ethanoic acid making
the salt of ethanoic acid. The ethanoic acid is in excess and so some
remains at the end of the reaction. The result is a mixture of the
salt and the acid together, a buffer solution.
Choice III - in this case there are as many moles of NaOH as there
are of ethanoic acid - it will all be neutralised leaving only the
salt in the products. It cannot be a buffer.
|
Q861-08 Which of the following
must be mixed with ammonia solution to produce a solution with buffer action?
- Ammonium hydroxide
- Sodium hydroxide
- Sodium chloride
- Ammonium chloride
Answer
|
To make a basic buffer, ammonia solution must be mixed with a salt
of ammmonia, e.g. ammonium chloride - response
D.
|
Q861-09 Which of the following
will produce a buffer solution when mixed with 25cm
3 of 0.1 mol dm
-3
ammonia solution?
- 25cm3 of 0.1 mol dm-3 hydrochloric acid
- 25cm3 of 0.1 mol dm-3 ethanoic acid
- 15cm3 of 0.1 mol dm-3 hydrochloric acid
- 15cm3 of 0.1 mol dm-3 sodium hydroxide
Answer
|
Something must be added to the ammonia that will produce a mixture
of ammonia and the salt of ammonia.
In choice A, the acid will completely neutralise the ammonia leaving
only the salt. In choice B, there is a weak acid that will also completely
neutralise the ammonia. In choice C
the hydrochloric acid reacts with some of the ammonia solution, making
ammonium chloride and leaving some ammonia solution unreacted - this
is a buffer solution. In choice D the sodium hydroxide is a base and
will not react with ammonia solution.
|
Q861-10 Which of the following
combinations produce a buffer solution when equal volumes are mixed?
- I. 0.1 M HCl and 0.1 M NH4Cl
- II. 0.1 M HCl and 0.2 M NH3
- III. 0.1 M NH3 and 0.1 M NH4Cl
- I only
- III only
- II and III only
- I, II and III
Answer
|
The first choice has a strong acid (HCl) with a salt and therefore
cannot be a buffer.
The second choice has an excess of weak base which neutralises all
of the acid and produces a mixture of weak base and the salt of the
weak base. It is a buffer.
The third choice is a weak base and the salt of the weak base, it
is also a buffer.
Correct response = C
|
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