04MANasaVISNIR - Mass Percent of ASA Revised 2/12/10 MASS...

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Mass Percent of ASA Revised 2/12/10 1 MASS PERCENT OF ASA IN ASPIRIN OBJECTIVES To determine the mass percent of ASA in an aspirin tablet by visible spectroscopy requires the following: An absorption spectra to find max by measuring absorbance of a Fe 3+ -ASA standard solution as a function of wavelength. The absorbances of five Fe 3+ -ASA solutions of known concentration are measured to create a Beer’s Law Plot. The treatment of an aspirin tablet with NaOH and iron (III) in acidic solution to produce a colored Fe 3+ -ASA solution. The absorbance of this solution is measured and related to the Beer’s Law plot to find its Fe 3+ -ASA concentration (and, therefore, ASA concentration). By taking into account the dilutions that occurred in the reaction of the aspirin tablet, the mass percent of ASA in the aspirin tablet can be calculated. SAFETY PRECAUTIONS Safety goggles and aprons must be worn at all times. Sodium hydroxide is corrosive and can cause burns; prevent contact with eyes, skin and clothing. Wash affected areas thoroughly with cold water. Similar care should be taken with FeCl 3 solution (which has been acidified to pH < 2 with hydrochloric acid). The aspirin tablets used as unknowns may be contaminated through student activities and are NOT for internal use. When using a pipet, always use a pipet bulb to provide suction, never pipet by mouth. INTRODUCTION "Aspirin", first isolated from willow bark and the meadowsweet plant, has been used to reduce pain, fever and tissue swelling. An aspirin tablet contains caffeine, buffers, starch or other binding agents, and acetylsalicylic acid (ASA, the active ingredient for combating pain). The structure of ASA is shown in Figure 1.
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Revised 2/12/10 2 C O O C O O CH 3 H Figure 1. Acetylsalicyclic Acid (ASA) A solution of ASA is colorless and, therefore, does not absorb light in the visible region between 400 and 700 nm. After hydrolysis with a strong base, a deprotonated ASA is able to bind with Fe 3+ ions in acidic solution. A solution of the resulting cationic Fe 3+ -ASA complex is deeply colored and stable at pH < 2. The stoichiometric ratio of ASA to Fe 3+ -ASA is 1:1. ASA Fe 3+ -ASA Figure 2. Synthesis of Fe 3+ -ASA When white light (light containing all wavelengths from 400 to 700 nm) passes through a Fe 3+ - ASA solution, wavelengths corresponding to green light are strongly absorbed and the solution appears purple. When white light encounters the Fe 3+ -ASA complex, the green light is absorbed as a result of the excitation of valence electrons to higher energy electron orbitals. Therefore, the intensity of light at the absorbed wavelengths is reduced passing through solution; the amount of reduction is dependent on the concentration of the absorbing species and the distance the light must travel through the solution (path length). This linear dependence is known as the Beer- Lambert Law (or "Beer's Law"): (1) A = C l A = absorbance (no units) = molar absorptivity coefficient (units = L/mol-cm)
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04MANasaVISNIR - Mass Percent of ASA Revised 2/12/10 MASS...

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