Acid Buffer Solution

Topics: Acid dissociation constant, Buffer solution, PH Pages: 7 (1070 words) Published: October 20, 2013
BUFFERS

By: Luis P. Bazan, RPh., Ph.D.

A buffer solution is a solution of:
1. A weak acid or a weak base and
2. The salt of the weak acid or weak base

Both must be present!
A buffer solution has the ability to resist changes in pH upon the addition of small amounts of either acid or base.
Consider an equal molar mixture of CH3COOH and CH3COONa
CH3COOH (aq)

H+ (aq) + CH3COO- (aq)

Adding more acid creates a shift left IF enough acetate
ions are present

16.3

Which of the following are buffer systems?
(a) KF/HF
(b) KCl/HCl,
c) Na2CO3/NaHCO3

(a) KF is a weak acid and F- is its conjugate base
buffer solution
(b) HCl is a strong acid
not a buffer solution
(c) CO32- is a weak base and HCO3- is it conjugate acid
buffer solution

16.3

Buffers
• are solutions that have the property of
resisting changes in pH when acids or bases
are added to them
• this property results from the presence of a
buffer pair which consists of either:
 weak acid and some salt of a weak acid/its
conjugate base
Ex. Acetic acid and sodium acetate
 weak base and some salt of a weak base/its
conjugate acid
Ex. Ammonium hydroxide and ammonium chloride

BUFFERS IN BIOLOGIC SYSTEMS
 Blood is maintained at a pH about 7.4 by the so-called primary buffers in the plasma and the secondary buffers in the
erythrocytes.
 The plasma contains carbonic acid/bicarbonate and acid/alkali sodium salts of phosphoric acid as buffers
 Plasma proteins, which behaves as acids in blood can combine with bases and so act as buffers.
 In the erythrocytes, the two buffer systems consist of
hemoglobin/oxyhemoglobin and acid/alkali potassium salts of
phosphoric acid.

Pharmaceutical Applications
(1) Preparation of such dosage forms as
injections and ophthalmic solutions which
are placed directly into pH-sensitive body
fluids
(2) Manufacture of formulations in which the
pH must be maintained at a relatively
constant level to ensure maximum product
stability
(3) Pharmaceutical tests and assays requiring
adjustment to or maintenance of a specific
pH for analytic purposes

Henderson-Hasselbalch Equation
 aka: Buffer Equation
 has two forms:
 For weak acid:
pH = pKa + log [salt]
[acid]

Ex. What is the pH of a buffer solution prepared with 0.05 M sodium borate and 0.005 M boric acid?(Ka of boric acid = 5.8x10-10) pH = pKa + log [salt]
[acid]
= 9.24 + log 0.05
0.005
= 9.24 + log 10
= 9.24 + 1
= 10.24, answer

 For weak bases:
pH = pKw – pKb + log [base]
[salt]
Ex. What is the pH of a buffer solution with 0.05 M
ammonia and 0.05 M ammonium chloride? The Kb
value of ammonia is 1.80 x 10-5 at 25 o C.
Kb = 1.80 x 10-5
pKb = - log Kb
= - (1.80 x 10-5)
= - (-4.74)
pH = 14 – 4.74 + 0.05
0.05
= 9.26 + log 1
= 9.26

To calculate the Molar Ratio of Salt/Acid for a Buffer
System of Desired pH
Example:
What molar ratio of salt/acid is required to prepare a
sodium acetate-acetic acid buffer solution with a pH
of 5.76? The pKa value of acetic acid is 4.76 at 25o C.
pH = pKa + log [salt]
[acid]
log [salt] = pH - pKa
[acid]
= 5.76 – 4.76 = 1
Antilog of 1 = 10
Ratio = 10/1 or 10:1, answer

• Percentage of the ionized (salt) species in the
buffer system is taken as:
% ionized (salt) = ___salt__ x 100
salt + acid
Ex. The molar ratio of salt to acid is 10:1. What
is the percentage of the ionized species in the
buffer system?

% ionized (salt) = ___10__ x 100
10 + 1
= 90.91%

• Percentage of the unionized (acid) species in the
buffer system is taken as:
% unionized (acid) = ___acid__ x 100
acid + salt
Ex. The molar ratio of salt to acid is 10:1. What is
the percentage of the unionized species in the
buffer system?
% unionized (ACID) = ___1__ x 100
1 + 10
= 9.09%

Buffer Capacity
 aka:
–buffer action
– buffer efficiency
– buffer index
– buffer value
 is the ability of a buffer solution to resist
changes in pH upon addition of an acid/alkali
...
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