Salicylic Acid Investigation

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1. Aims
a) To find the molar absorptivity of salicylic acid by plotting a graph of absorbance against the concentration of salicylic acid, using the standard solution.
b) To find the concentration of aspirin at the various time intervals using the molar absorptivity value in the previous part, initial concentration of aspirin and the concentration of salicylic acid obtained from the decomposition of the aspirin pill.
c) Lastly, to calculate the rate constant of the decomposition of aspirin by plotting a graph of ln[aspirin] against time and making use of the gradient of the best fitted line along with the standard deviation.
2.Data Treatment and Analysis

In order to find the molar absorptivity of salicylic acid, a UV spectroscopy is carried
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Iron(III) solution was added to the salicylic acid to form a organometallic complex. This makes use of the reaction between the phenol functional group in salicylic acid and ferric ions which allows for visibility due to its violet hue(1). The absorbance is directly proportional to the concentration of salicylic acids. This means that the higher the concentration of salicylic acid, the higher the amount of salicylate-iron complex formed, resulting in higher violet intensities and hence a higher absorbance, as seen from table 1. Since the Fe3+ ions react with the singular phenol functional group in salicylic acid, the amount of Fe3+ added should be in a 1:1 ratio with the concentration of salicylic acid.
Fig 2. Plot of concentration of standard salicylic acid against
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This is only possible because aspirin decomposes to salicylic acid in a 1:1 ratio. ln[aspirin] was also calculated so that the plot obtained will show a linear relationship with time and allows us to analyse trends between the two variables.
Although the data is a little sporadic, there is a visible trend of a gradual increase of salicylic acid concentrations and a decrease in aspirin concentrations.
Fig 4. Plot of ln[Aspirin] against time The plot in Fig. 4 shows a strong linear relationship between ln[aspirin] and time. The calculated r2 value is 0.99236 while the gradient is -6.94931 x 10-5. The standard deviation of the regression line is as small as 2.73×10-6, as expected from the properties of ln values. The gradient, which is essentially the change in concentration divided by the change in time, follows the differential rate law for first order reactions where:
Rate = (-d[aspirin])/dt=
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