Pka value of weak acid and strong bases by titrations method contains following content ☆aim ☆objective ☆defination of buffers ☆buffer capacity ☆necessity of buffers ☆determination of ph ☆POTENTIOMETRY ☆application of buffer ☆Henderson hasselbalch equation ☆requirement, procedure, formula, calculation, observation table , graph and results 珞Thanku......

1. ~By. Pankaja p
Pka value of weak acid

2. AIM :
■ TO DETERMINE Pka VALUE OF WEAK ACID BY TITRATING IT AGAINST
STRONG BASE USING POTENTIOMETRE
OR
■ To determine the maximum buffer capacity of weak acid (Acetic
acid) and its salt of strong base.

3. OBJECTIVES
1) To learn concept of buffer capacity and its utility.
2) To develop concept of potentiometry and handling of
potentiomter.

4. Pka —》Dissociation constant.
Buffer consists —》weak acid and weak base
Note : weak acid don’t dissociate completely

5. DEFINATION:- Buffer
" Buffers are mixtures of compound that
by their presence in the solution resists
change in the PH upon the addition of
small quantities of acid or alkali. "

6. BUFFER CAPACITY
"The buffer capacity of a buffer solution
is a measure of magnitude of its
resistance to change in PH on an
addition of an acid or a base."

7. Buffer capacity is represented by= B (Beta)
Also referred as:‐
1) Buffer index
2) Buffer value
3) Buffer efficiency
4) Buffer coefficient

8. PH of Body fluids = 1-8
PH of Blood = 7.4
PH of stomach = 1-3
PH of Intestinal fluid = 6-7
PH of skin = 6-6.5

9. NECESSITY OF BUFFER
■ If solution come in contact with air , it will absorb co2 and become
acidic.
■ If solution is stored in glass bottle, alkaline impurities from glass may
alter the PH.
● Due to these reasons , pharmaceutical solutions are buffered as the
buffer solution are capable of maintaining PH at some constant value when
even small amount acid or base are added.

10. Determination of PH
■ Electromeric method .
- glass electrode or calomel electrode.
■ Calorimetric method.
a) by PH paper
b) by standard buffer solution and different ph range

11. POTENTIOMETRE
Potentiometre is a device to determine the PH
Potential it is equivalent of strong acid or strong
base that should bring about a net change in the
PH .

12. APPLICATION OF BUFFERS
1) In biological systems :-pH of Blood = 7.4
a) Primary buffers :- Present in plasma.
b) Secondary Buffers :-Present in Erythrocytes
2) pharmaceutical preparation ( injections)
– opthlamic preparation
–ointments and creams

13. Henderson- Hasselbalch equation
It gives PH of buffer solution of a weak acid and it’s salt.
for acid
PH = Pka + [ salt ]/ [ acid]
For base
POH= Pkb + [ salt ]/[ base ]

14. Requirements
Chemicals: - distilled water, sodium hydroxide,
acetic acid, buffer solution, phenolphthalein.
Apparatus: -potentiometer, beakers, burette,
pipette, funnel, clamp etc.

16. PROCEDURE
1) Switch on the pH meter and allow it to stabilize for 10
minute.
2) Wash the electrode in distilled water and wipe with clean
and dry tissue paper.
3) Dip the electrode in the buffer solution of known pH ( 4,
7,9). If the pH is different from standard one then adjust the
pH meter to give exact pH reading. Now pH meter is
standardize and ready to use.|

17. PROCEDURE
4) Take 20ml of 0.4M acetic acid in a beaker and pH
is recorded+10ml of 0.2N NaOH and pH is again
recorded.
5) When there is sudden increase in pH note the
reading and continue for 4-5 readings.
6) Plot a graph of β (Buffer capacity) against pH.

21. Formula
B=dB/dPH
B Is small increment in gram equivalent Per liter strong acid or
base added to solution to produce PH change PH .
Pharmaceutical preparation should have a low buffer
capacity.

22. dB = 2/25× V
1)For V= 25
dB = 2/25×V
= 2/25×25
= 2
2) For V = 26
dB = 2/25×26
= 2.08
Calculation

23. B= dB/dPH
1) dB = 2.08 dPH = 0.3
B= 2.08/0.3
= 6.933
2) dB =2.16 dPH = 0.2
B= 2.16/0.2
= 0.8

24. Amount of 0.4N NAOH (ml) volume in beaker (ml) PH dPH dB B=dB/dPH
0 25 3.5 0 2 0
1 26 3.8 0.3 2.08 6.933
2 27 4 0.2 2.16 10.8
3 28 4.2 0.2 2.24 11.2
4 29 4.5 0.3 2.32 7.33
5 30 4.7 0.2 2.4 12
6 31 4.9 0.2 2.48 12.4
7 32 5.4 0.5 2.56 5.12
8 33 5.8 0.4 2.64 6.6
9 34 6.5 0.7 2.72 3.88
10 35 9.3 2.8 2.8 0.1
11 36 9.8 0.5 2.88 0.576

25. 0
6.933
10.8
11.2
7.33
12
12.4
5.12
6.6
3.88
0.1
0.576
0
2
4
6
8
10
12
14
3.5 3.8 4 4.2 4.5 4.7 4.9 5.4 5.8 6.5 9.3 9.8
B=dB/dPH
B=dB/dPH

26. RESULT
The maximum buffer capacity of weak acid and
it’s salt of strong base and As it’s half
neutralization point at PH 9.3