Purity of Aspirin by Spectrophotometry

The mass of acetylsalicylic acid was determined using a analytical balance. Sodium hydroxide (NaOH) was added to the acetylsalicylic acid and heated in order to hydrolyze acetylsalicylic acid. Iron chloride was added to each volumetric flask as the purity of the aspirin can be tested by addition of Fe+3 to a suspension of the product. Phenols such as salicylic acid react with iron chloride to form coloured (violet) complexes Aspirin easily absorbs green light and green is the compliment of violet, which allows a determination of the amount of aspirin present.

The percentage purity of aspirin was found to be 90. 62%. Aspirin tablets (about 5-grams) are usually compounded of about 0. 32 g of acetylsalicylic acid, which indicates that aspirin should be about 94% pure. The calculated value for aspirin content was close to the true value and indicates that aspirin sample tested was very pure. .Introduction Aspirin is a member of a family of chemicals called salicylates. This chemical can be converted (changed) by the body after it is eaten to another chemical, salicylic acid.

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Salicylic acid may be irritating because it is an acid; therefore one of the acidic parts was covered up with an acetyl group, converting it to acetylsalicylic acid (ASA). ASA not only could reduce fever and relieve pain and swelling, but better for the stomach and worked even better than salicylic acid. Salicylic acid produces a salicylate dianion, when dissolved in water, which reacts with an acidic solution Fe (NO3)3(aq), to produce a strong violet tetraaquosalicylatroiron (III) complex.

The violet color of the complex results from the fact that the complex strongly absorbs green light.

When this green is removed from normal white light, we observe violet (therefore, green is the compliment of violet). This absorption of green light can be used to quantitatively determine the amount of aspirin present in the solution. The more green light that is absorbed, the more violet the solution, and hence, the more salicylate is present. If green light is directed into a solution that contains this aspirin complex, some of the green light will be absorbed. The intensity of the green light leaving the sample, I, is less than the original intensity of the green light, I0.

We can talk about the fraction of light that was transmitted through the sample, transmittance (T); or we can talk about the amount of light that was absorbed by the sample, absorbance (A). Transmittance is inversely proportional to absorbance: the more light that passes through the sample, the higher the transmittance and the lower the absorbance; conversely, the less light that passes through the sample, the lower the transmittance and the higher the absorbance. Several standard solutions of the salicylate complex are prepared.

The absorbance of each standard solution is measured at the wavelength of maximum absorption of green light (530 nanometer) using a spectrophotometer. A graph of these absorbance values versus the concentration of each of the standards should yield a straight line. This relationship is known as Beers' Law: A = a b c A is the absorbance of the solution, a, is the molar absorptivity (a constant for this complex), b is the path length of cuvette (in cm), and c is the molar concentration of the solution being measured. The absorbance of a solution is directly proportional to the concentration of that solution.

Therefore, the molar concentration, c, of a solution can be determined by simply measuring the absorbance, A, of that solution. Since we are actually measuring the absorbance of the complex, the stoichiometry of the reaction producing the complex is 1:1. So, if we know the concentration of the complex, we know the concentration of the aspirin. The concentration, the amount of acetylsalicylic acid and purity of aspirin sample will be determined by using Beer’s Law plot. The concentration and absorbance of sodium salicylate will be found, thus being able to find the percentage purity of aspirin.