If one looks at Beer’s Law, it may appear that you only need to measure the absorbance of a solution of unknown concentration, and then using Beer’s Law, you could use this measurement to determine the concentration (direct method). Unfortunately, one term in Beer’s Law, the molar absorptivity, ε , is a function of instrumental and environmental factors. Setting the wavelength of light, λ , which passes through the medium, to the correct value is critical as ε varies considerably with λ . Therefore, one cannot obtain accurate values of ε from a reference book. If one measures, under specified instrumental and environmental conditions, the absorbance of several solutions (standard solutions) of known concentration containing the same analyte as the unknown, a plot of absorbance vs. concentration can be made. This is sometimes called a calibration curve, even though it is usually a straight line. If the absorbance of a solution of unknown concentration is measured under the same instrumental and environmental conditions as were used in making the calibration curve, the concentration of the unknown can be determined by locating where the absorbance measured with the unknown is found on the calibration curve and reading the associated concentration, or by solving the equation of the line fitted to the data for concentration. In part A of this experiment you will be provided with “solution S”, a solution of known concentration of the salicylate ion. You will combine solution S with a FeCl 3 solution to prepare a series of solutions of the purple salicylato iron(III) complex (formula and structure discussed in pre-lab lecture) of known concentrations. You will set the colorimeter to a wavelength of 565 nm ( λ max – discussed in the Beer’s Law experiment in CHEM 1033), so that all the absorbance measurements will be accurate. You will measure the absorbance of the series of solutions, and graph the (absorbance, concentration) data using Excel to prepare a calibration a calibration curve, to be used in Part B. When you look at the label on a bottle of aspirin, it should tell you how much acetylsalicylic acid (aspirin) there is in each tablet. Is the manufacturer telling the truth? In Part B of this experiment you will prepare solutions as you did in Part A, but use an aspirin tablet in place of solution S. The absorbance of a salicylato iron(III) solution will be measured and, using your Beer’s Law plot from Part A, you will determine its concentration and then “back calculate” to find the number of milligrams of acetylsalicylic acid in the aspirin tablet. This value will be compared with that on the label. SAFETY/HYGIENE/WASTE DISPOSAL 1. Always wear your goggles! 2. Caution: many chemicals are corrosive. You should wear gloves and be sure your skin is not exposed. Discard the gloves after use, and wash your hands. Chemical exposure may result in an itchy sensation. If you have any sensation of itching, burning, or tingling, thoroughly flush the area with water. Continue flushing until several minutes after the sensation has subsided. Inform your lab instructor.
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- Spring '10