Spectrophotometric Analysis of Aspirin in Commercial Tablet

Presented to
Tutor’s name
Institution

Prepared by
Student’s name
Course title

Date of Submission

Spectrophotometric Analysis of Aspirin in Commercial Tablet
Objectives
This experiment is aimed determining the percentage of the active ingredient ASA Eguate tablet. Among other objectives are to enable the student to:
1. Accurately prepare standard solutions
2. Use Beer’s law to construct a calibration curve
3. Determine molar absorptivity using Beer’s Law.
4. Describe the fundamental principle underlying spectrophotometric analysis.
Introduction
Acetyl salicylic acid is a synthetic drug that was first synthesised by Bayer Company in Germany. The drug which is also branded as ‘Aspirin,’ is one of the most popular drugs that have remained in the market over a long period of time (Anne & Andermann 1996). It is mainly used to depress fever and as a pain killer. Study shows that when low dosages are taken in daily routine chances of contracting heart attack is minimised (Jeffreys 2005). Even though in the past decades other drugs such as Panadol and ibuprophen captured the market, ASA still is most important and widely used medicine.
Apart from the active compound present in the drug, other inactive ingredients are also present. Such as dyes, fillers and drying agents among others. The active ingredient contained in a given tablet is always stated on the outer surface of the package. In this experiment, the percent active ingredient in a commercial aspirin tablet is determined. It should be known that Aspirin is the common trade name of the ASA. The acetyl salicylic acid present in the tablet is reacted with Iron (III) chloride to form a violet complex. The UV/VIS spectrophotometer will then be used to determine the concentration of this complex. The results of concentration obtained will then be used to determine the weight and percent weight (%weight) of the acetylsalicylic acid in the Aspirin tablet.
Materials and Instrumentation
The instrument used in this experiment is UV/VIS spectrophotometer. Other apparatuses used are hot plate, polystyrene cuvettes, 2-100ml volumetric flasks, 6-10ml volumetric flasks, 10ml measuring cylinder, 1ml pipette, 150ml beaker and a watch glass.
The materials required for the experiment include salicylic acid reagent, 1.0M caustic soda, 0.02M Iron (III) chloride, Eguate tablet.
Spectrophotometric Determination of Acetylsalicylic acid
The acetylsalicylic acid is an ethanoate ester of 2-hydrohybenzoic acid. In a basic medium, the acetyl ester is readily hydrolysed to form a salicylate anion. He de-esterification process is followed by addition of concentrated sulphuric acid before addition of FeCl3. The reaction results into formation of a violet complex.

The violet solution of Iron salicylate is fed into the Spectrophotometer, and a spectrum is transmitted. The maximum absorption is obtained slightly above 500nm (Skoog et al. 2007) where measurements of concentration are taken.
In order to determine the concentration of the iron salicylate complex, the absorbance of several known concentration of the iron salicylate complex solution is measured, and a calibration curve obtained. The solutions used in the calibration are obtained by first preparing a stock solution of the complex solution. Five different standard solutions of known concentrations are prepared by diluting the stock solution. The absorbance of each of the five standard solutions is obtained and then plotted against the concentration to obtain a linear curve. Finally, the absorbance of the solution of the commercial aspirin solution is obtained before using the calibration graph to obtain its concentration.
Standard Solution and Beer’s Calibration Curve
Mass of ASA sample = 0.159g
Molar mass of ASA = 180.2g/mol
Moles of ASA in the sample = mass/molar mass
= 0.159/180.2 = 0.0008824mol
Concentration of ASA in the stock solution after transferring the ASA into a 100ml volumetric flask
= 0.0008824 x 1000/100
= 0.008824 mol ASA.

M1 V1 = M2 V2
0.008824 x 0.5 ml/ 10ml = 0.0004412 mol/L
The absorbance values of solutions A-E are recorded in table 1.

Table 1: Table showing corresponding absorbance and concentration of the five standard solutions.
Solution Concentration
(mol/L) Absorbance
A 0.0004412 0.4815
B 0.0003530 0.4034
C 0.0002647 0.2924
D 0.0001765 0.1938
E 0.0000882 0.0915

For this data, the graph of Absorbance against Concentration is as follows:

Figure 1 Salicylic acid calibration graph

Absorbance
Concentration
(x 10-3mol/L)
Determination of Aspirin
Mass of Aspirin tablet = 0.382g
Absorbance of the diluted solution = 0.2480
Concentration of ASA obtained from the calibration graph = 0.0002680 mol/L
Concentration of the unknown ASA prepared by dilution of 0.3ml of solution in 10ml
= 10 x 0.0002680/0.3 = 0.008935 mol/L
The Eguate tablet was dissolved and then hydrolysed before diluting in 100ml of solution, therefore;
Moles of ASA in solution = 0.008935 x 100/1000 = 0.0008935 mol/L
Mass of ASA = 0.0008935 x 180.2
= 0.16g.
% ASA in the Eguate tablet = 0.161/0.382 x 100
= 56.5%

Precautions
Precaution should be taken to avoid contact of caustic soda with the skin. This can be achieved by wearing gloves and safety glasses during the experiment. While heating the mixture of sodium hydroxide and ASA during hydrolysis, the content should be covered with a watch glass to prevent the solution from spattering.
Disposal
The remaining reagents such as sodium hydroxide and Iron salicylate solutions should not be discarded directly to the environment but should first be neutralised before disposal.
Questions
1. To determine the absorption of the sample in the spectrum, the reading should be determined in the boundary between the blue, green and yellow regions.
2.
3. Iron III chloride rather than water is used to dilute Aspirin because their reaction gives a complex of violet colour which is photosensitive. The violet light has a specific wavelength that can be used to determine the absorbance of the sample in the complex. Distilled water does not give a coloured complex and so cannot give desirable results in the spectrophotometer.
4. The graphed calibration graph of Absorbance vs. Concentration is a straight line that exhibits linearity. The absorbance varies directly with the concentration.

Bibliography
Anne, A. & Andermann, J. 1996, ‘Physicians, Fads, and Pharmaceuticals: A History of Aspirin,’ McGill Journal of Medicine, vol. 2, no. 2, pp. 456-461.
Jeffreys, D. 2005, Aspirin: The Remarkable Story of a Wonder Drug. New York: Bloomsbury Publishing.
Skoog, et al. 2007, Principles of Instrumental Analysis (6th ed.). Belmont: Thomson Brooks.