Lab5-Spec - Chemistry 227 Fall 2010 Lab 5...

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Chemistry 227 –Fall 2010 Page 1 of 5 Lab 5: Spectrophotometric Analysis of a Four-Component Mixture Objectives and Overview: Spectroscopy is a powerful tool for both qualitative and quantitative analysis. In this experiment you will be introduced to one of the standard workhorse spectrochemical methods: spectrophotometry with ultraviolet and visible light (UV-VIS). You will apply the Beer-Lambert Law to the determination of the concentrations of four different species in a solution: Cr(III), Co(II), Ni(II), and Cu(II). Background The absorption spectrum of a sample can be used to identify and quantify its various components. This is useful for chemical analysis and to follow chemical changes. For example, spectra can be used to monitor the mechanism and kinetics of chemical reactions or to detect changes in the equilibrium form of a compound under varying conditions. Most often the spectrum of one species in solution will overlap that of another species, so that the concentration of neither species can be measured independently. When this occurs, measuring absorbance at two or more wavelengths and solving two or more simultaneous equations can still provide sufficient information to determine quantitatively the concentrations of those species present in the solution, as is explained below. If Beer’s Law is obeyed at wavelength j by each component i in a solution, then for each absorbing species, A ij = , ij bc i [ 1 ] where A ij is the absorbance, , ij is the molar absorptivity, b is the optical path length, and c i is the molar concentration. If the same cuvette is used for each sample, the factor b is constant, and we may combine the two constants , ij and b into a single absorptivity constant k ij , to obtain A ij = k ij c i [ 2 ] For a solution that contains n components whose absorbances are additive, the total absorbance of the solution at wavelength j is the sum of all the individual absorbances; that is, A= A ji j i=1 n [ 3 ] For a two-component solution, absorbances at only two different wavelengths are needed, giving us obtain two equations with two unknowns: A 1 = k 11 c 1 + k 21 c 2 [ 4 ] A 2 = k 12 c 1 + k 22 c 2 Similarly, for three absorbing species, we need absorbances at three different wavelengths for three equations with three unknowns, and so on. For n components, we need absorbances at n wavelengths. The general set of equations is A 1 = k 11 c 1 + k 21 c 2 + . .. + k n1 c n [ 5 ] A 2 = k 12 c 1 + k 22 c 2 + . .. + k n2 c n . . . A n = k 1n c 1 + k 2n c 2 + . .. + k nn c n
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2 We can then solve the simultaneous equations to determine the concentrations of the various species. The k ij values are determined from measurements on single component solutions. For this lab you will determine the spectra of four ions to get absorptivity, k, as a function of wavelength for each pure component and then determine the concentration of each ion in an unknown mixture.
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Lab5-Spec - Chemistry 227 Fall 2010 Lab 5...

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