Buffer solutions

Abstract

Buffers
are compounds or mixtures of compounds that, by
their presence in solution, resist changes in pH upon the
addition of small quantities of acid or alkali The resistance
to a change in pH is known as buffer action

Full text

Buffer solutions
Physical Pharmacy
2nd stage –1st semester
2019-2020
Hassanien S. Taghi
Phd , pharmaceutics

Buffers are compounds or mixtures of compounds that, by
their presence in solution, resist changes in pH upon the
addition of small quantities of acid or alkali. The resistance
to a change in pH is known as
buffer action

Biological Uses
In biological systems (saliva, stomach, and blood) it is essential
that the pH stays ‘constant’in order for any processes to work
properly. Most enzymes work best at particular pH values.
•the pH of blood is normally about 7.4
•If the pH varies by 0.5 it can lead to unconsciousness and coma
•carbon dioxide produced by respiration can increase the acidity of
blood by forming H+ ions in aqueous solution
the presence of hydrogen carbonate ions in blood removes excess H+

In other words, a buffer solution resists (or buffers) a change in its pH.
That is, we can add a
small amount of an acid or base to a buffer solution and the pH will
change very little. Two common
types of buffer solutions are:
[1] A weak acid together with a salt of the same acid with a strong base.
These are called
Acid buffers e.g.
, CH3COOH + CH3COONa.
[2] A weak base and its salt with a strong acid. These are called
Basic
buffers
.
e.g.
, NH4OH + NH4Cl.

Buffers usually contain mixtures of a weak acid and one of its salts, although
mixtures of a weak base and one of its salts may be used.
The latter suffer from the disadvantage that arises from the volatility of
many bases.
The action of a buffer solution can be appreciated by considering a simple
system such as a solution of acetic acid and sodium acetate in water. The
acetic acid, being a weak acid, will be confined virtually to its undissociated
form because its ionization will be suppressed by the presence of common
acetate ions produced by complete dissociation of the sodium salt.
A buffer solution has to contain things which will remove any hydrogen ions or
hydroxide ions that you might add to it - otherwise the pH will change. Acidic
and alkaline buffer solutions achieve this in different ways.
How do buffer solutions work?

We'll take a mixture of acetic acid and sodium acetate as typical.
acetic acid is a weak acid, and the position of this equilibrium will be well to
the left:
Adding sodium acetate to this solution, it mean we add a lots of extra acetate
ions. According to Le Chatelier's Principle, that will tip the position of the
equilibrium even further to the left.
Acidic buffer solutions

The solution will therefore contain these important things:
1. lots of un-ionized acetic acid ;
2. lots of acetate ions from the sodium acetate ;
3. enough hydrogen ions to make the solution acidic.
Adding an acid to this buffer solution
The buffer solution must remove most of the new hydrogen ions
otherwise the pH would drop markedly.
Hydrogen ions combine with the acetate ions to make acetic acid.
Although the reaction is reversible, since the acetic acid is a weak
acid, most of the new hydrogen ions are removed in this way.
Since most of the new hydrogen ions are removed, the pH won't
change very much - but because of the equilibria involved, it will fall a
little bit.

Adding an alkali to this buffer solution
Alkaline solutions contain hydroxide ions and the buffer
solution removes most of these.
Removal by reacting with ethanoic acid
The most likely acidic substance which a hydroxide ion is going to collide
with is an acetic acid molecule. They will react to form acetate ions and
water.

Removal of the hydroxide ions by reacting with hydrogen ions
Remember that there are some hydrogen ions present from the ionization.
Hydroxide ions can combine with these to make water. As soon as this
happens, the equilibrium tips to replace them. This keeps on happening
until most of the hydroxide ions are removed.

Common ion effect is defined as the suppression of the degree of
dissociation of a weak electrolyte containing a common ion.
For example, if both sodium acetate and acetic acid are dissolved in the
same solution they both dissociate and ionize to produce acetate ions.
Sodium acetate is a strong electrolyte so it dissociates completely in
solution. Acetic acid is a weak acid so it only ionizes slightly.
According to Le Chatelier's principle, the addition of acetate ions from
sodium acetate will suppress the ionization of acetic acid and shift its
equilibrium to the left. Thus the percent dissociation of the acetic acid
will decrease and the pH of the solution will increase. The ionization
of an acid or a base is limited by the presence of its conjugate base or
acid
Common ion effect

If the acid is weak acid and ionizes only slightly, the expression [HAc]
may be considered to represent the total concentration of acid, and it is
written simply as [Acid]. In the slightly ionized acidic solution, the
acetate concentration [Ac-]can be considered as having come entirely
from the salt, sodium acetate. Hence, equation is written as

The Buffer Equation for a Weak Base and Its Salt
and using the relationship [OH-] =
K
w/[H3O+], the buffer equation is
obtained

Example :
pH and [Salt]/[Acid] Ratio
What is the molar ratio, [Salt]/[Acid], required to prepare an acetate buffer of pH 5.0? Also
express the result in mole percent.
Therefore, the mole ratio of salt to acid is 1.74/1. Mole percent is mole fraction multiplied by
100. The mole fraction of salt in the salt–acid mixture is 1.74/(1 + 1.74) = 0.635, and in mole
percent, the result is 63.5%.
One desires to adjust a solution to pH 8.8 by the use of a boric acid–sodium borate
buffer. What approximate ratio of acid and salt is required?
Question

What is the pH of a solution containing 0.10 mole of ephedrine and 0.01
mole of ephedrine hydrochloride per liter of solution? Since the p
K
bof
ephedrine is 4.64,
Example

If large amounts of acid or base are added to a buffer then changes
in the log salt/acid term become appreciable and the pH alters. The
ability of a buffer to withstand the effects of acids and bases is an
important property from a practical point of view.
This ability is expressed in terms of
buffer capacity (β
). This can be
defined as
being equal to the amount of strong acid or strong base,
expressed as moles of H+ or OH~ ion, required to change the pH of
1 litre of the buffer by one pH unit.
Buffer Capacity

In addition, the capacity is also affected by the ratio of the
concentrations of weak acid and its salt, maximum capacity (
β
max)
being obtained when the ratio of acid to salt = 1.
In such circumstances pH =
pKa
of the acid and
β
max =
0.576 (total buffer concentration).
When selecting a suitable buffer the
pKa
value of the acid should be
close to the required pH and the compatibility of its components
with other ingredients in the system should be considered. The
toxicity of buffer components must also be taken into account if the
solution is to be used for medicinal purposes.

buffer capacity, β.It is also known as buffer efficiency, buffer index,
and buffer value.
the concept of buffer capacity and defined it as the ratio of the
increment of strong base (or acid) to the small change in pH brought
about by this addition. For the present discussion, the approximate
formula
can be used, in which delta, Δ, has its usual meaning, a
finite change
,
and Δ
B
is the small increment in gram equivalents (g Eq)/liter of
strong base added to the buffer solution to produce a pH change of Δ
pH.
Equation ……..1

The buffer capacity calculated from previous equation 1 is only approximate. It gives
the average buffer capacity over the increment of base added. Koppel and Spiro1
and Van Slyke2 developed a more exact equation,
A More Exact Equation for Buffer Capacity
where C is the total buffer concentration, that is, the sum of the molar concentrations of the
acid and the salt. Equation (2) permits one to compute the buffer capacity at any hydrogen ion
concentration—for example, at the point where no acid or base has been added to the buffer
Equation ……..2

An equation expressing the maximum buffer capacity can be derived from the buffer
capacity formula of Koppel and Spiro1 and Van Slyke,2 equation (2). The maximum
buffer capacity occurs where pH = pKa, or, in equivalent terms, where [H3O+] = Ka.
Substituting [H3O+] for Ka in both the numerator and the denominator of equation (2)
gives
Maximum Buffer Capacity
Equation ……..3
Example
Maximum Buffer Capacity
What is the maximum buffer capacity of an acetate buffer with a total concentration
of 0.020 mole/liter? We have

Buffer Capacity
The buffer capacity depends on
(a) the value of the ratio [Salt]/[Acid], increasing as the ratio approaches unity, and
(b) the magnitude of the individual concentrations of the buffer components, the buffer
becoming more efficient as the salt and acid concentrations are increased.
Key Concept