This document describes an experiment to determine the potency of paracetamol tablets using UV spectrophotometry. Standard solutions of known paracetamol concentrations were prepared and their absorbances measured to generate a calibration curve. The absorbance of a sample tablet solution was then measured and used to calculate its concentration and potency. The calculated potency of 180.3% exceeded the specification range of 100% ± 10%, suggesting an experimental error occurred. Precautions are needed to properly handle equipment and solutions.

1. Name of the Experiment: Determination of the potency of Paracetamol (Napa) tablet (Beximco
Company) by UV-Spectrophotometric method.
Principle:
When a beam of radiation (light) passes through a substance or a solution, some of the light may
be absorbed and the remainder transmitted through the sample. The ratio of the intensity of the
light entering the sample (Io) to that exiting the sample (I) at a particular wavelength is defined
as the transmittance (T). The absorbance (A) of a sample is the negative logarithm of the
transmittance.
A = - log (T)
Figure:Absorption of light by a sample
Beer-Lambert Law
The absorbance of a sample at a given wavelength is proportional to the absorptivity of the
substance (a constant at each wavelength), the path length (the distance the light travels
through the sample) and the concentration of the absorbing substance. In these cases the Beer-
Lambert Law holds:
A = a * b * c where a = the absorptivity of the substance
b = path length
c = concentration of the substance
When working in concentration units of molarity, the Beer-Lambert law is written as:
A = ε * b * c
where ε is the wavelength-dependent molar absorptivity coefficient with units of M-1 cm-1.
As, Ɛ and b, both are constant, then it can be written that, A ∝ c
If, absorbance (A) is plotted on Y-axis against concentration (c) on X-axis, a linear curve is
obtained. It is called standard calibration curve. It follows the equation of a straight line, y =
mx+c.
If the absorbance of an unknown sample is then measured, the concentration of the absorbing
component can be determined from this graph.

2. Paracetamol: Paracetamol is a widely used over-the-counter analgesic (pain reliever) and antipyretic
(fever reducer). Paracetamol is classified as a mild analgesic.
Figure: Structure of Paracetamol
Indication:
Paracetamol is used to treat many conditions such as headache, muscle aches, arthritis, backache,
toothaches, colds, and fevers. It relieves pain in mild arthritis but has no effect on the underlying
inflammation and swelling of the joint.
Reagents:
 Standard paracetamol powder
 Napa Tablet (Beximco company)
 Distilled water
Apparatus:
 UV Spectrophotometer
 Test tube
 Pipette
 Measuring cylinder
 Volumetric flask
 Electronic balance
 Mortar and pestle
y = x
R² = 1
0
2
4
6
8
10
12
0 2 4 6 8 10 12
Absorbance
Concentration (µg/ml)
Standard Calibration Curve

3.  Filter paper
Procedure:
Preparation of stock solution:
1. 5 mg paracetamol powder were weighed by electronic balance
2. This powder was taken in a 50 ml volumetric flask and was filled with distilled water upto the
mark.
3. The flask was shaked well until the paracetamol powder was dissolved. This solution was the
stock solution.
Preparation of working standard:
1. 9 test tubes were taken, were marked serially from 1-9 and were kept in a test tube holder.
2. Test tube marked 1 was taken and was added with 10 ml distilled water. No stock solution is
added in this test tube.
3. Test tube marked 2 was taken and was added with 0.5 ml stock solution and 9.5 ml distilled
water.
4. The dilution process was conducted for the rest of the test tubes.
Test Tube
Number
Stock Solution
(ml)
Distilled Water
(ml)
Concentration
(µg/ml)
1 0 10 0
2 0.5 9.5 5
3 1 9 10
4 1.5 8.5 15
5 2 8 20
6 2.5 7.5 25
7 3 7 30
8 3.5 6.5 35
9 4 6 40
Preparation of Sample Solution:
1. Take 5/10 tablets and measure the individual weight of each tablet by electronic balance
2. Determine the average weight which is 565 mg
3. Take the tablets and crush it in mortar and pestle
5 mg paracetamol is present in 5.65 mg of powder
4. Weigh 5.65 mg powder from the mortar and pestle by electronic balance and keep it in a 50 ml
a volumetric flask
5. Add distilled water upto the mark
6. Then the solution is filtered
7. Take 1 ml of the filtrate in the test tube; add 9 ml distilled water.

4. Calibration of UV Spectrophotometer:
1. Turn on the UV spectrophotometer and wait for the calibration to be completed
2. Fill 2/3rd of the cell with distilled water, set the wavelength at 249 nm and press the auto-zero
button
Determination of absorbance for the working standard and the sample solution:
1. Take test tube 1 and fill the cell with it
2. Place the cell into the sample holder and measure the absorbance
3. Same way, determine the absorbance of the working standard and the sample
Calculation:
Calculation of the experimental/calculated value:
From the standard curve, the following equation is obtained:
Concentration Absorbance
0 0
5 0.199
10 0.378
15 0.570
20 0.649
25 0.894
30 1.059
35 1.317
40 1.443
Sample(18.03) 0.651
Experimental concentration value, x = y-c/m
= (0.651-0.002)/0.036
=18.03
Calculation of the theoretical value:
50 ml stock solution contains 5 mg Paracetamol
 1 ml stock solution contains 5/50 mg Paracetamol
So, concentration of the stock solution = 5mg/50 ml = 0.1 mg/ml = (0.1*1000) µg/ml = 100 µg/ml
Again,
5/10 ml diluted solution contains 100 µg Paracetamol
 1 ml diluted solution contains 100/5 or 100/10 µg Paracetamol
= 20 or 10 µg/ml

5. So, Theoretical concentration value = 20 or 10 µg/ml
Determination of Potency:
Potency = Experimental value/Theoretical value *100
= (18.03/10)*100
=180.3%
Result: The potency of supplied paracetamol tablet is 180.3%
Comment: According to USP specification of potency for tablet should be 100±10. In our experiment,
potency of paracetamol tablet was 180.3% which is not comply with specification. This study provides
the knowledge of R2 value. R2 value of this study is 0.9948 which is near to 1. But drug didn’t pass
the test.
Reason:
 Drug didn’t pass the test due to experimental error.
 UV-spectrophotometer didn’t work properly.
 There may be some error to prepare the sample solution.
Precaution:
 Apron must be worn.
 All equipments and instruments were handled properly.
 Standard and sample ingredients were weighed and handled properly.