8-1Experiment 8 BEER’S LAW I. Learning Objectives… ±To introduce Beer’s Law, relating solution concentration to absorbance. ±To use the measurement of light absorption (colorimetry) to detect the colorimetric reaction of thiocyanate with iron. ±To determine the iron content in food samples. II. Background Information Chemists are often called upon to answer the question, "How much?". Many common substances are composed of a mixtureof chemicals. The challenge is to devise a test that will answer "How much?" of a particular chemical is present among the component chemicals in a mixture. One way to do this is to find a chemical that reacts with the substance (chemical) of interest (and nothing else) to produce a colored product. In this experiment the amount of iron in food samples prepared by pyrolysisis analyzed. The iron content is measured using the colorimetric reaction of thiocyanate ion, SCN-with iron III ion to produce a RED complex ion. Fe3+(aq) + SCN-(aq)²FeSCN2+(aq) color: yellow colorless dark red The intensity of the resulting dark red color can be measured to determine the concentration of the iron ion by a process known as colorimetry. Colorimetryis an instrumental method based on the measurement of light absorption by colored solutions and is widely used for performing chemical analyses. The colorimeterinstrument used in this experiment has four different wavelength settings, corresponding to the visible colors blue (460 nm), green (656 nm), orange (630 nm), and red (697 nm). The color of a solution is known to originate from transmittedlight, not the light color absorbed. Therefore, a red “colored” solution absorbs light in the violet or blue region of the visible spectrum. The colorimeter is used on the blue wavelength setting in this experiment.
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