Analysis of Aspirin Tablets


The objective of this experiment was to determine the percentage of 2-ethanoylhydroxybenzoic acid in Aspirin tablets.

Theoretical Background

2-ethanoylhydroxybenzoic acid, also known as acetylsalicylic acid, is an analgesic (pain-killer) drug available under a commercial name of Aspirin. It is an ethanoate ester of salicylic acid, which is obtained from willow bark.

Aspirin inhibits the production of prostaglandin synthase, an enzyme essential for production of prostaglandins, that cause swelling, fever, and are responsible for pain transmission to the brain [1].

Figure 1. Structural formula of 2-ethanoylhydroxybenzoic acid.

The acetylsalicylic acid makes up only a percentage of Aspirin tablets. The aim of this lab session is to determine the percentage. The method chosen involves an analytical technique of volumetric analysis called back titration.


- weighing bottle

- semi-analytical, electronic balance

- 25cm3-pipette

- four 250cm3-conical flasks

- magnetic stirrer

- 250cm3-standard flask

- burette with a stand

Table 1. Reagents used.

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Firstly, one aspirin tablet was weighed on a semi-analytical, electronic balance, its mass being recorded. Subsequently, the tablet was hydrolyzed using an excess (25cm3) of solution of 1.0 moldm-3 sodium hydroxide. 25cm3 of distilled water were added. The obtained mixture was then gently heated on a magnetic stirrer for a period of 10 minutes to ensure that all the acetylsalicylic acid was hydrolyzed. After cooling, the mixture (50cm3) was transferred to a standard flask and made up with distilled water to the 250cm3 mark. Finally, a series of three titrations was conducted, in each one 25cm3 of hydrolyzed solution were transferred to ...

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