Aspirin Determination Using Colorimetric Determination And Back Titration

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The aim of this investigation was to determine the aspirin content in aspirin tablets using colorimetric determination (colorimetry) and back titration. Two different brands of tablets were used in this investigation, these were “Tesco” tablets and “Numark” tablets.

After looking at the results, it was seen that colorimetric determination gave more accurate results. This was because the back titration gave results which were consistently lower than the mass stated on the package even with the tablets being in date. The colorimetry results were more accurate, as even though they were further away from the value on the packet, as it provided results which would have been more likely to be in the tablets. E.g. the tablets would have probably had a higher value than stated on the tablet to increase shelf life.

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In the back titration, it was found that the Tesco tablets contained 296mg of aspirin and the Numark tablets contained 297mg of aspirin. This value was below the stated mass on the package (which was 300mg). There was also supposedly 1566mg of aspirin in 1508mg of pure aspirin. This again shows the degree of inaccuracy of the technique.

The colorimetric determination provided results which were more realistic for tablets. The Tesco tablets contained 387mg of aspirin and the Numark tablets contained 308mg of aspirin. This was compared to a value of 300mg on the package.


General Underlying Chemistry

Aspirin has a formula of:

This, therefore, means that it has a molecular formula of C9H8O4, thus giving it a GFM of 180 (this value was used in the calculations later).

Aspirin (also known as acetylsalicylic acid) is an aromatic compound which contains various functional groups. Aspirin is most commonly used as a drug, e.g. as a pain reliever or to reduce a fever. The three defining functional groups in aspirin consists of a benzene ring, a carboxyl group and an ester link.

Benzene is an aromatic hydrocarbon. It is a colourless liquid which has a formula of C6H6. In a benzene molecule, there is sp2 hybridisation taking place. Each carbon atom is sp2 hybridised, this is formed from one s orbital and two p orbitals. There are three half-filled sp2 orbitals in a benzene molecule, and the electrons are more likely to be found in the larger lobe. A diagram of a sp2 orbital can be found below:

This therefore means that an electron is left occupying a p orbital on each carbon atom with each of these p orbitals overlapping side on with other p orbitals on the neighbouring carbon atoms. This is what forms the π electron cloud in a benzene ring. In a benzene molecule, there are 12 sigma (σ) bonds and the pi (π) electron cloud. The six electrons which are in the π electron clouds are considered delocalised. This is because they are not tied to one carbon atom, but instead they are shared between all six of these carbons.

There is also hydrogen bonding in aspirin molecules which allows the aspirin to be slightly soluble in water. This is because the polar hydroxyl groups in the aspirin can bond with the very polar water.

Overall in an aspirin molecule, there is 17 σ bonds, 1 π electron cloud and 2 π bonds. Sigma bonds are produced by end on overlap of atomic orbitals and pi bonds are created through the side on overlap of atomic orbitals.

Back Titration

A back titration, or also know as an indirect titration, is a method which allows the concentration of an unknown solution to be calculated using a know excess of a reagent. The quantity of this excess of reagent is calculated by titration with a second reagent. For example, when the sample of aspirin of accurate known mass was reacted with a known excess amount of sodium hydroxide solution. Firstly, the hydrolysis of aspirin was catalysed by the alkali where ethanoic acid and salicylic acid (2-hydrobenzoic acid) were produced. The alkali then neutralised these acids.

As an excess of sodium hydroxide was used, the amound of sodium hydroxide remaining was determined by titrating it against a standard solution of sulphuric acid:

2NaOH + H2SO4 2H2O + Na2SO4

This method was used in the determination of aspirin since aspirin is insoluble in water, so it therefore cannot be determined by a direct titration.


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