S4. SPECTROPHOTOMETRIC ANALYSIS OF PARACETAMOL TABLETS
PURPOSE
To determine the paracetamol content of analgesic tablets by ultraviolet/visible
spectrophotometry.
PREWORK
Prepare a flow chart of the sample progress from the grinding of the sample through to the
analysis.
Prepare a flow chart of the standard preparation
PROCEDURE
1. Determine the average weight of an analgesic tablet by weighing five tablets and dividing
the result by 5.
2. Grind up the five tablets using a clean mortar and pestle.
3. Accurately weigh three portions of the ground tablets, and determine the average mass of
a tablet
4. Using your ground powder weigh three samples each roughly equal in weight to the
average tablet weight, and quantitatively transfer into 500 mL volumetric flasks. Label the
Samples 1, 2 and 3.
5. Accurately weigh about 0.15g (record the mass) of pure standard paracetamol and
quantitatively transfer to a 500 mL volumetric flask. Label it Standard A.
6. Add 125 mL of 0.1 M NaOH to each of the flasks.
7. Swirl the flasks for 15 minutes to dissolve the active ingredient of the tablets. Make up to
volume with purified water.
8. Mix the contents of each flask well and filter each through fluted #1 filter papers into
clean beakers. Discard the first 50 mL of each filtrate (used to sacrificially rinse your
equipment). For the standard, filter another 50 mL; for the unknowns, another 20 mL. You
do not need to filter the entire contents.
9. Wash each of the 500 mL volumetric flasks that contained the original samples.
10. For the samples, pipette 5 mL of the filtrate into the rinsed flasks from step 9. Add 25 mL
of 0.1 M NaOH and make up to the mark with purified water.
11. From the filtrate of the Stock Standard solution, pipette 5 mL back into the rinsed
Standard A flask. Pipette 10 mL and 20 mL aliquots into two other 500 mL volumetric
flasks labelled Standard B and Standard C respectively. Add 25 mL (by measuring
cylinder) of 0.1 M NaOH to each flask and make up to the mark with purified water.
12. Prepare a blank by diluting 10 mL of 0.1 M NaOH to 100 mL in a volumetric flask. There
should now be six 500 mL and one 100 mL volumetric flasks
13. Zero the spectrophotometer by running a baseline, using the blank solution between 200
and 400nm.
14. Rinse the sample cell with Standard B and record the spectrum between 200-400 nm.
Determine the wavelength of maximum absorbance.
15. Set the wavelength to that of the maximum absorption found in step 13 and record the
absorbance (not the spectra) for all the samples and standards.
CALCULATIONS
1. Calculate the concentration (in mg/L) of the original standard solution made in step 6.
2. Calculate the concentrations of the diluted standards A, B and C, (use your prework
flowchart to assist).
3. Calculate the absorptivity coefficient for paracetamol, using the results from each
standard
4. Plot a calibration graph of absorbance vs concentration of standards.
5. Determine the concentration of paracetamol in each of the sample solutions
6. Calculate the average paracetamol content in the tablets as %w/w, as follows (there
are a number of methods to calculate the answer):
• for each sample solution, apply the appropriate dilution factor to determine the
original concentration of the solutions made in step 3.
• for each sample solution, determine the mass of paracetamol in each flask.
• to determine the %w/w, divide the mass of paracetamol in each flask by the mass of
ground tablet that was used and multiply by 100.
• calculate the average value and the relative precision
OR
•
Use your flow chart to determine the amount of paracetamol in the original flask
7. Calculate the average paracetamol content in the tablets as mg/tablet by using the
%w/w calculated above and the average mass of a tablet.
QUESTIONS
1. Why is the blank used in this experiment not ideal?
2. Why is it necessary to prepare three or four standards for spectroscopic analyses?
3. Why was it better to carry out dilution of the standards and samples rather than simply
weighing out less material initially?
4. Why does the method not just dissolve a tablet in each flask?
5. Find the structure of paracetamol. What groups in it would cause the absorption of
ultraviolet radiation?
6. Find the structure of aspirin. Do you think it could be analysed in a similar manner to
paracetamol?
S4. RESULTS SHEET
Date of analysis
Identity of instruments
Measurement
Mass (g)
5 tablets
Average tablet
Sample 1
Sample 2
Sample 3
Mass of standard
paracetamol
Solution
Blank
Standard A
Standard B
Standard C
Sample 1
Sample 2
Sample 3
Have you?
Completed the instrument log
Completed the sample register
Completed the standard register
Teachers signature
Conc (mg/L)
0.00
Absorbance
Date
Signature