Evaluatation of the accuracy of the techniques I used to estimate purity
Melting point
The melting point of pure aspirin is between 135o C and 139 o C, if the melting point of the synthesised aspirin that you made is less than the pure aspirin then it contains impurities, if the melting point of the synthesised aspirin is way more or less then the synthesised aspirin is not aspirin. If it is the same then it is pure aspirin. Mel-temp apparatuses are used to determine the temperature at which your synthesized crystals melt. the melting point of 2-Hydroxybenzoic acid is 158-160o C.
This is fairly reliable as If your synthesised aspirin is impure, it will be lower than the literature value by an amount that is roughly proportional to the amount of impurities present. This therefore shows you how much Impurities is in you aspirin if it is much lower than the literature value then you have not made aspirin. This does not however show you what impurities are present just how much.
When melting the aspirin you could burn the synthesised aspirin or miss when the synthesised aspirin melted this is human error. This could be improved by using an electronic piece of equipment which records the melting point of the aspirin accurately rather than judging it by eye. Therefore there is less room for human error.
Boiling point
Pure substances have a distinct boiling temperature and this can be used to determine the purity of the substance. The boiling point of Ethyl Ethanoate is 77 o C; if the synthesised ethyl ethanoate is higher than the literature value then this means that there may be traces of ethanol.
This is fairly reliable as if your synthesised ethyl ethanoate is impure it will be higher than the literature value if it is lower than the literature value then you may have made another substance. You do not however know for sure what the impurities are; in this case it could be ethanol, as this boils around the same time as pure ethyl ethanoate sometimes higher depending on the amount present.
When boiling the synthesised ethyl ethanoate you may boil the substance before you have time to record the boiling temperate this would be due to human error and would mean that you could not repeat the experiment as you may have evaporated your synthesised ethyl ethanoate, this could probably mean that you have not made a pure sample or something completely different or you could had missed the boiling temperature. This could be improved by using an electronic piece of equipment which records the boiling point of the ethyl ethanoate accurately rather than judging it by eye. Therefore there is less room for human error.
Chromatography
Thin Layer Chromatography is a solid-liquid technique. Thin layer chromatography is used to determine if two compounds are identical by separating the mixtures. If the compound that was made (aspirin) has a spot in the same place as the pure sample then this is the same but if other spots are present the compound that you synthesised is likely to have impurities present. Impure, recrystalised and a pure sample of aspirin are compared. Thin layer chromatography is a rapid separation technique, which means that a pure substance gives one spot and if extra spots are observed as well as the pattern of the known compound then impurities are likely to be present in the sample. You can also work out the Rf values by the dividing the distance travelled by the component by the distance travelled by the solvent. If the spots are identical then you have made pure aspirin, because the substances are white or colourless you will need to develop the plate before you can see what has happened. When the thin layer chromatography is complete observe the plate under a short wavelength UV lamp and lightly mark with a pencil any spots observed after this place the plate in a beaker containing a few iodine crystals, place a cover on the beaker and warm it gently on a steam bath until the spots begin to appear this will make the spots permanent so that you see them if possible. You may also run an infrared (IR) spectrum of the product and compare it with the spectrum of pure aspirin.
Chromatography techniques are used in industry because they can be controlled very precisely and use very small amounts of substance. Therefore this test is to work out the purity of your synthesised product as it is more accurate and can also identify the impurities and you can compare the impurities to known impurities which may be present so that you know what was present in your final product this is particularly important in the medical industry. This can be combined with the boiling and melting temperature of the substance to make sure that it is definitely pure.
Thin line chromatography in the lab is not as accurate as in the industry as there is more room for human error as everything is done by eye judgement and small calculations where as in the industry electrical equipment is used to make sure the product is 100% pure.
