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UV SPECTROPHOTOMETRIC DETERMINATION OF THE
ACETYLSALICYLIC ACID CONTENT IN A PAINKILLER TABLET
1
THEORY
During this UV spectrophotometric determination of the acetylsalicylic acid content in a painkiller
tablet, the acetylsalicylic acid (ASZ) is hydrolyzed in basic medium to form salicylate. This hydrolysis
of acetylsalicylic acid (ASZ) occurs by excess NaOH at room temperature and is complete and rapid.
O
O
C
C
O-H
O
OH
O-C-CH3
+ 2 OH
O
+ CH3-C-O
+ H2O
O
acetylsalicylzuur (ASZ)
salicylaat
The UV absorption of this salicylate is measured. A calibration curve is drawn up based on salicylic
acid standard solutions in a basic medium. The salicylic acid (SZ) in the calibration solutions was also
hydrolyzed in a basic medium to salicylate.
O
O
C
C
O-H
O
OH
OH
+ OH
salicylzuur (SZ)
+ H2O
salicylaat
Note that this method - although it is an acetylsalicylic acid (ASZ) determination - still uses salicylic
acid (SZ) as the primary standard. Salicylic acid dissolves completely (albeit with difficulty) in water
when heated. Acetylsalicylic acid, on the other hand, hardly dissolves in water and can therefore not
be used here as a primary standard.
A UV spectrum is recorded from salicylate to determine the wavelength of maximum absorbance. At
this wavelength of maximum absorbance the calibration solutions and the sample measurement
solution are measured.
Analytical chemistry and Instrumental analysis lab C 2
Trial12 – ASZ: UV spectrophotometric
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2
SOLUTIONS
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stock standard solution of salicylic acid (0.1 mg SZ / mL): Accurately weigh approximately
0.1000 g of salicylic acid in a weighing cup. Salicylic acid is difficult to dissolve in water.
O
O
C
C
O-H
O-H
OH
O-C-CH3
O
-
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salicylzuur (SZ)
acetylsalicylzuur (ASZ)
MM = 138,12 g / mol
MM = 180,16 g / mol
Therefore, it is dissolved in warm distilled water. After cooling to room temperature, the
contents of the weighing beaker are quantitatively transferred to a 1000.0 mL volumetric
flask that is brought up to volume with demineralized water.
0.1 M NaOH solution.
PROCEDURE
-
-
-
-
Preparation of the salicylate standard calibration series:
 From the stock standard solution of salicylic acid (0.1 mg SZ / mL) pipette
successively 10.00; 20.00; 30.00; 40.00 and 50.00 mL in 100.0 mL volumetric flasks.
 Add 1 mL of 0.1 M NaOH to each of the five volumetric flasks and also to a sixth
100.0 mL volumetric flask that serves as a blank.
 Bring the six volumetric flasks to volume 100.0 mL with deionized water.
Crush a painkiller tablet (unweighed) in a cup.
Dissolve the tablet in 20 mL of 0.1 M NaOH.
Transfer quantitatively to a 250.0 mL volumetric flask. Rinse the pestle and mortar
thoroughly with 0.1 M NaOH (not with demineralized water).
Adjust the volumetric flask to 250.0 mL with 0.1 M NaOH (not with demineralized water).
Homogenize by shaking. A light precipitate may form at the bottom of the volumetric flask.
This is the binding agent from the tablet. However, the sample solution should therefore not
be filtered.
Pipette 2.00 mL of the sample solution from the 250.0 mL volumetric flask (without
aspirating precipitate) into a 100.0 mL volumetric flask and bring the volumetric flask to
100.0 mL with demineralized water (not with 0.1 M NaOH). This is the sample measurement
solution.
Record a UV absorption spectrum of the calibration solutions and the sample measurement
solution against the blank (in quartz cuvettes - QS).
Read the absorbances of the calibration solutions and the sample measurement solution at
the wavelength with maximum absorbance.
Analytical chemistry and Instrumental analysis lab C 2
Trial12 – ASZ: UV spectrophotometric
3
4
CALCULATIONS
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-
-
-
-
Present the pipetting scheme schematically.
Display the spectrum.
Draw up a calibration curve with in the
 Y-axis: the absorbances of the salicylate standard solutions
 - in mg SZ / 100 mL).
Calculate the best fitting line (with the correlation coefficient).
Determine by linear interpolation for the sample measuring solution: either: the number of
ml of 0.1 mg SZ / mL salicylic acid stock standard solution corresponding to the amount of
salicylate in the 100.0 mL volumetric flask of the sample measuring solution, or: the
concentration of salicylic acid in the 100.0 volumetric flask mL of the sample measuring
solution (in mg SZ / 100 mL).
Calculate the mass of salicylic acid (in mg SZ) in the 100.0 mL volumetric flask of the sample
measuring solution. Using the molar masses of salicylic acid and acetylsalicylic acid, convert
to the mass of acetylsalicylic acid in the 100.0 mL volumetric flask of the sample measuring
solution.
Convert to the mass of acetylsalicylic acid (in mg ASZ) in the painkiller tablet by taking the
pipetting scheme into account.
Report the acetylsalicylic acid content as the number of mg acetylsalicylic acid / tablet.
Note: For pharmaceutical preparations it is customary to express the content as mg active
component / tablet (and not as m% active component). This is the reason why the tablet has
not been weighed (analytically).
Calculate that during the hydrolysis of the salicylic acid in the salicylate standard solutions
and of the acetylsalicylic acid in the sample solution, an excess of NaOH was indeed added so
that complete hydrolysis is guaranteed.
Also prove from the pipetting diagram that the concentration of NaOH in the sample
measuring solution is almost the same as that in the blank and in the salicylate standard
calibration solutions.
Analytical chemistry and Instrumental analysis lab C 2
Trial12 – ASZ: UV spectrophotometric