Making AND USING VISIBLE ABSORPTION lab 2 - Sharma1 Making...

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Sharma1 Making AND USING VISIBLE ABSORPTION MEASUREMENTS By-Arashjot Sharma CHM-241Lab Dr. s. klarreich September 17, 2014 Introduction
Sharma2 The main theory behind this experiment is that by using a spectrophotometer the visible spectrum of an absorbing species can be obtained. Using these findings the analytical wavelength can be determine. By collecting the absorbance data a Beer’s law plot can be constructed. Using this plot the concentration of the absorbing species in a solution containing an unknown concentration of the species can be determined. OBJECTIVE The objective of this experiment is to use a spectrophotometer to obtain the visible spectrum of an absorbing species. Another objective is to determine the analytical wavelength, the absorption, and the percent transmittance and use this data to create a Beer’s Law plot. Absorption Spectroscopy The amount of light transmitted through a solution or suspension may be diminished by the absorption of light by colored compounds and/or the scattering of light rays by particulate matter. An instrument that measures the amount of light passing through a sample is called a spectrophotometer. Absorption spectroscopy has many uses; it can be used to study pigmented molecules, to monitor the density of bacteria in a culture, and to follow the progress of an enzymatic reaction. The main requirement is that light be absorbed or scattered by some substance in the sample under investigation. Apparatus : Spectrophotometer, beakers (30-, 50-, 100-, and 250-mL), 10-mL Mohr pipets, pipet pumps, 50-mL graduated cylinder, 10 test tubes (13 x 100 mm), test tube racks, stirring rods, wash bottle, Kimwipes Safety Equipment : aprons, goggles, fume hoods
Sharma3 Chemicals : Distilled water, 0.050 M CuSO 4 , 6 M NH 3 solutions, Unknown CuSO 4 Solutions PART 1: DETERMINING AN ABSORPTION SPECTRUM All colored compounds absorb certain wavelengths of visible light and transmit other wavelengths. This pattern of light absorption over a range of wavelengths is called an absorption spectrum. Each molecule has a characteristic absorption spectrum that depends upon its structure. 1. Obtain two square cuvettes that have been cleaned inside and out. One will serve as your blank (or reference ) and should be filled with distilled water (or whatever solvent is used for the colored compound). 2. Use this cuvette to adjust zero absorbance (100% transmittance). (See the instructions below on how to do this.) Start with a wavelength of 400 nm. It is important to use matched cuvettes; to check this, place at least 2.0 ml of water (or whatever liquid is in the reference cuvette) in the other cuvette (called the sample cuvette) and read the absorbance. 3. Is it identical to that of the first cuvette? The blank must be used to adjust the spectrophotometer every time the wavelength is changed. If you are working with a single wavelength, the blank should be placed in the spectrophotometer periodically to check for drift.

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