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Acid dissociation constant Essay

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 specificp H 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 = 5.8×10-10) 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
 Approximate formula
β = ∆B
∆pH

Exact Formula/Koppel-Spiro-Van Slyke’s Equation
β = 2.3C _Ka [H3O+]__
(Ka + [H3O+])2
Maximum Buffer Capacity
 occurs when pH = pKa
βmax = 0.576 C
in which C is the total buffer concentration Ex. What is the maximum buffer capacity of an acetate buffer with a total concentration of 0.020 mole per liter?
βmax = 0.576 x 0.020
= 0.01152 or 0.012

ASSIGNMENT
1. What is the pH of a solution containing
0.30 M HCOOH and 0.52 M HCOOK?
2. What molar ratio of salt/acid
3. Percentage of the unionized (acid) species in the buffer system
4. Percentage of the ionized (SALT) species
in the buffer system

QUANTITY OF COMPONENTS IN A BUFFER SOLUTIONS TO YIELD A
SPECIFIC VOLUME

Ex. The molar ratio of sodium acetate to acetic acid

in a buffer solution with a pH of 5.76 is 10:1.
Assuming the total buffer concentration is 2.2
x10-2 mol/L, how many grams of sodium acetate
(m.w. 82) and how many grams acetic acid
(m.w.60) should be used in preparing a liter of
the solution?

SOLUTION:
m.f. sodium acetate = 10/1+10 or 10/11
m.f. Hac

= 1/1+10 or 1/11

Total buffer conc. = 2.2 x10-2 mol/L

Conc of NaAc = 10/11 x (2.2 x 10-2)
= 2 x 10-2 mol/L
Conc of HAc = 1/11 x (2.2 x 10-2)
= 0.2 x 10-2 mol/L
2 x 10-2 mol/L or 0.02 x 82 = 1.64 g of NaAc per liter of solution. 0.2 x 10-2 mol/L or 0.002 x 60 = 0.120 g of HAc per liter of solution

Change in pH with addition of Acid or base
Ex. Calculate the change in pH after adding 0.04
mol of NaOH to a liter of a buffer solution
containing 0.2 M conc. Of NaAC and Hac. The
pKa value of acetic acid is 4.76 at 250C.

pH = pka + log salt/acid
= 4.76 + log 0.2 M/0.2
= 4.76 + log of 1
= 4.76 + 0
= 4.76 (before adding NaOH)

pH = pka + log salt + base/acid – base
pH = pka + log 0.2 + 0.04/0.2 – 0.04
= pka + log 0.24/0.16
= 4.76 + 0.1761 = 4.9361 or 4.94
= 4.94 – 4.76 = 0.18 unit (after adding
NaOH)

ASSIGNMENT
1. What molar ratio of salt to acid would be required to prepare a buffer solution with a pH of 4.5? The pKa value of the acid is 4.05 at 25o C.
2. What is the change in pH on adding 0.02 mol of sodium
hydroxide to a liter of a buffer solution containing 0.5 M of sodium acetate and 0.5 M acetic acid? The pka value of acetic acid is 4.76 at 25 degrees centigrade.
3. The molar ratio of salt to acid needed to prepare a sodium acetate-acetic acid buffer solution is 1:1. Assuming that the total buffer concentration is 0.1 mol/L, how many grams of
sodium acetate should be used in preparing 2 liters of the
solution?
4. What is the change in pH with the addition of 0.01 mol HCl to a liter of buffer solution containing 0.05 M of ammonia and 0.05 M of ammonium chloride? The kb of ammonia is 1.80 x 10-5 at
25 degrees centigrade.


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