Recrystallized salicylic acid
We used analytical techniques to test the purity of both salicylic acid and recrystallized. Melting point and TLC were used on all three samples. We recrystallized the salicylic acid because this would then allow us to see the differences in purity between normal salicylic acid and the recrystallized. The recrystallized in theory should be more pure because the crystallisation processes purifies the sample. This would show up on the TLC and the melting point.
We recrystallized salicylic acid so that it could be compared with the aspirin samples. From the recrystallization we worked out the percentage recovery. The calculation consisted of the original weight of acid (2g) divided by the weight of the acid after recrystallization which was 1.3g. Then it should be multiplied by 100 to get a percentage. We achieved 51.5% after recrystallization. This was considerably lower than we had expected. Although we knew that the weight of the acid would be reduced we did not expect it to be so dramatic.
Recrystallized substances are normally purer than standard samples because the impurities or contaminants get removed by the filtering, evaporating and separation that occurs during the recrystallization.
Results
After recrystallizing the salicylic acid we calculated the percentage recovery. The result we achieved was not what we had expected. We had expected some of the acid to have been lost and the recovery be reduced. The results we got showed that only 51.5% of the acid remained after the crystallisation. This was surprising because almost half of the original substance of salicylic acid had gone. The original mass was 2 grams, after the crystallisation it was 1.03 grams. The reason I believe that the recovery was only 51.5% was because when filtering the solution some of the crystals would have been left behind on the filter paper or the sides of the beaker and flask. This would have reduced the final weight of the acid. Also the length of time that the acid was left to cool would affect the percentage recovery. The reason for this is that when the solution of distilled water and salicylic acid are cooled the acid begins to form crystals. If the solution was not allowed to cool for long enough then some of the acid would have passed through the filter paper as it would still be dissolved in the solution.
Factors that affected purity
The purity of the salicylic acid would be affected by the equipment used, the effectiveness of the filter paper, the age of the sample and how it is stored.
Accuracy of method used
The method that we used to test the purity of the salicylic acid, aspirin and recrystallized salicylic acid was effective and allowed up to calculate the results we needed. However there are some areas that could be improved to make the results more effective and repeatable.