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Topics: Spectroscopy, Electromagnetic radiation, Light Pages: 5 (1183 words) Published: March 10, 2014
Experiment 3
Spectrophotometric Analysis of Mixtures:
Simultaneous determination of two dyes in solution

Reading: Appendix J
Harris, Section 2.5, Chapter 18, and Section 19.1

Introduction
Spectrophotometric determinations based on the Beer-Lambert Law are among the most widely used analytical procedures. These methods involve the measurement of the fraction of incident electromagnetic radiation that is absorbed by a sample. To determine the concentration of a colored species in solution, a cuvet containing the solution is placed in the spectrometer, which consists of a spectrograph and a photometer. The spectrograph produces light of selected wavelengths and is calibrated to indicate the color of the light it produces in terms of wavelength, usually expressed in nanometers. The photometer measures the intensity of the light beam at a given wavelength. The amount of light that is absorbed by the sample can be used to determine the amount of the colored species in solution according to the Beer-Lambert Law.

The absorbance, A, of the solution containing a given metal ion is equal to the molar absorptivity  (in liters per mole-cm) times the path length b (in cm) times the concentration c (in moles per liter). This law is known as the Beer-Lambert Law, A = bc. Special sample cuvets are used which have a path length of 1.000 cm. The molar absorptivity depends on the wavelength of the light and is determined by measuring the absorbance of a series of standard solutions of known concentration. Once  and b are known, the concentration of an unknown solution can be determined by measuring the absorbance (see Appendix J) and using the Beer-Lambert Law given above.

You will determine the concentrations of both dyesthe Cr3+ and Co2+ simultaneously by a spectrophotometric determination. Since the total absorbance of the solution at a given wavelength is equal to the sum of the absorbances of the individual components, it is possible to analyze the individual components of a mixture even when their spectra overlap. Consider the spectra of M and N given in Figure 2.

There is obviously no wavelength at which the absorbance of this mixture is due simply to one of the components; thus an analysis for either M or N by a single measurement is not possible. However, the absorbances of the mixture at the wavelengths 1 and 2 may be expressed as follows:

At 1 :
At 2 :

The four molar absorptivities , , , and can be evaluated from standard solutions containing only M or N. Then, if the absorbances of the mixture are measured at 1 and 2, the concentration of the individual components can be calculated by solving the two equations given above simultaneously. The best accuracy in an analysis of this sort is obtained by choosing wavelengths at which the differences in molar absorptivities between the two ions is large. For your unknown, you will use the absorption spectra of yellow and red dyes 0.03 M Cr3+ and 0.1 M Co2+ to choose two wavelengths.

In this lab you will also gain experience in writing a formal laboratory report. Experimental
1.Obtain solutions of Yellow 5 (50 mg/L) and Red 40 (50 mg/L). You will need to create dilutions in order to measure the extinction coefficientPrepare 50 mL each of solutions containing 0.01, 0.02, 0.03, and 0.04 M Cr(III), and 0.02, 0.04, 0.06, and 0.1 M Co(II) by careful dilution of the master solutions provided [0.05 M Cr(III) and 0.2 M Co(II)]. You do not need a separate volumetric flask for each solution. Store solutions in Erlenmeyer flasks after they are prepared.

2.Obtain your unknowns from the Stockroomthe unknown orange solution in the requested glassware. It should be clean, dry, and labeled with your name and locker number.

3.Measure the absorbance vs. wavelength for the 0.1 M Co(II)yellow dye and red dye0.03 M Cr(III) solutions using the diode array spectrophotometer in Spectrum/Peaks mode. From these data, select the two wavelengths that...

References: (1 pt):
ACS style guidelines (http://library.williams.edu/citing/styles/acs.php)
General quality (3 pts):
Grammaer, formatting, quality of writing, etc.
Lab practice (2 pts)
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