Determination of the Composition of a Multi-Component Mixture by Spectrophotometric Analysis

Topics: Concentration, Absorbance, Analytical chemistry Pages: 5 (1931 words) Published: June 19, 2013
CHEM 3281 Experiment Seven Determination Of The Composition Of A Multi-Component Mixture By Spectrophotometric Analysis Objectives: 1.Solid sample handling 2.The composition of a three-component mixture will be assayed by dissolution of the soluble components in water and measurement of their individual concentrations by visible spectrophotometry. Text Reference: Handbook of Instrumental Techniques for Analytical Chemistry, Frank Settle, editor. Prentice Hall, , Upper Saddle River, NJ 1997, Ch 25. H.H. Willard, et. al. Instrumental Methods of Analysis, 7th edition, Wadsworth Publishing Co.:Belmont, CA 1988, p. 159-172. Skoog, Holler and Nieman, Principles of Instrumental Analysis, 5th edition, Saunders College Publishing, Fort Worth, TX 1998, Ch 13 & 14D. Introduction: The Beer-Lambert law states that the log of the ratio of the power of a beam of monochromatic light incident on the sample (Io) over the power passed through the sample (I) is defined as the sample absorbance (ABS). The absorbance of a solution containing only one chromophore will be proportional to the concentration of the chromophore, C, the absorptivity, a or ε, and the optical path length, b. ABS = log (Io/I) = εbC or abC at any one wavelength ε = molar absorptivity (L/mole-cm) a = absorptivity (L/g-cm) b = path length (cm) C = concentration (moles/L or g/L) (depends on whether you are using ε or a to represent absorptivity) The absorbance of a solution containing more than one chromophore will be equal to the sum of the absorbances of each of the components at any one wavelength. ABStotal = ABS1 + ABS2 + ... ABS n

where the subscripts refer to the individual chromophoric components 1, 2 ... n. For a solution containing n components, if the optical pathlength and the molar absorptivities for each component are known, the concentration of each chromophore can be determined by measuring the total absorbance of the solution at n wavelengths (selected where a1 = a2 = ... / / an) and solving the simultaneous equations. In this experiment, the optical path length is set by the physical dimensions of the sample compartment or cuvette. The width of the cuvette is 1.00 cm. The proportionality constants (molar or mass absorptivities) for each chromophore are determined by measuring the absorbance of solutions of known concentration and fitting the Beer-Lambert law to the data. The unknowns provided may contain an insoluble or non-absorbing component. If your particular unknown has an insolube component, dissolution in water will afford the separation of one component from the other two. Note that best results will be obtained when each aliquot prepared for analysis is representative of the sample. Procedure: 1. Turn on the tungsten power supply (switch on small blue box) and boot up the spectrometer software, OOIBase32. 2. Obtain the stock solutions and an unknown sample mixture from the teaching assistant. Decide on how to sample your solid reliably. Weigh (to the nearest 0.1 mg) a representative sample of approximately 0.50 g from your unknown mixture into a 25 ml volumetric flask and dilute to the mark with distilled, deionized water. Ask your T.A. how to prepare and transfer samples quantitatively so you will not lose any moles of sample if you do not know how. Prepare three sample solutions in this manner from the same unknown mixture. Observe whether or not your unknown contains insoluble components and record the unknown number. 3. For each stock solution provided, prepare a series of standard solutions of known concentration by precisely delivering (via analytical pipette) 5.00, 10.00, 15.00 and 20.00 ml aliquots of the standard to four separate and appropriately labeled 25.00 ml volumetric flasks. Dilute each to the mark with distilled, deionized water. Once again if you are unsure of how to pipette analytically so that no moles are lost in transfer, ask the T.A. Be sure you have the correct molecular weights and stock solution concentrations for each...
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