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Exp8 - Experiment 8 BEERS LAW I Learning Objectives To...

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8-1 Experiment 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 mixture of 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 pyrolysis is analyzed. The iron content is measured using the colorimetric reaction of thiocyanate ion, SCN - with iron III ion to produce a RED complex ion. Fe 3+ (aq) + SCN - (aq) FeSCN 2+ (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 . Colorimetry is an instrumental method based on the measurement of light absorption by colored solutions and is widely used for performing chemical analyses. The colorimeter instrument 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 transmitted light, 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. A colorimeter measures the amount of light at a given wavelength that is transmitted through a liquid. The liquid sample is placed in a small plastic solution cell or cuvette that places the sample in the path of light. The light that passes through the solution strikes a
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8-2 photodiode . The intensity of the light (I) passing through the liquid is used to determine the concentration of the chemical in the liquid. A higher concentration of colored solution absorbs more light and thereby transmits less light to the photodiode. The colorimeter monitors the light received by the photodiode as either the absorbance or the percent transmittance. All of these components are housed in a colorimeter box. The signal collected by the colorimeter is transferred to the Pasco interface box that provides the link for the data feed to the computer. The mathematical relationship between concentration and absorbance is called Beer's Law. Beer’s Law is defined as A = ε b c These variables are defined as follows: A = absorbance = - log (I/I o ) Where, I = light intensity of sample and I o = light intensity for the blank solution.
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