Experiment3

Experiment3 - CHEM 241L: Spectrophotometric Determination...

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CHEM 241L: Spectrophotometric Determination of the pKa of the Acid-Base Indicator Bromothymol Blue 69 3 Experiment 3 Spectrophotometric Determination of the pKa of the Acid-Base Indicator Bromothymol Blue Read Sections 9-2 & 9-3 (Chapter 9), Section 11-6 (Chapter 11), and Sections 18-2 & 18-3 (Chapter 18) in Harris, 7 th ed. Take the time to look over the Excel Graphing Tutorial presented in Experiment 0 of your lab manual. Note: You need to bring your laptop to lab for this experiment BACKGROUND & THEORY Beer’s Law & Visible Absorption Spectra The colors that we see are a manifestation of the absorption of light by molecules and materials. In chemistry we often use the absorption properties of molecules as a method of characterizing them. Absorbance ( A ) is defined by the following equation, = I I Log A O (1) In Equation 1, I 0 is the irradiance of the light measured in the absence of an absorbing solution, which in our case is just water, and I is the irradiance of the light measured after it passes through a solution of interest. Absorbance measurements are made using a spectrophotometer, such as the one illustrated in Figure 5. Absorbance measurements can be made at a single wavelength ( λ ) or they can be made at many wavelengths. A plot of absorbance versus wavelength is called a spectrum ( spectra , plural). A spectrum tells us what wavelengths or "colors" the sample absorbs, or by difference, what wavelengths or colors the sample transmits. The percent transmittance (% T) is simply defined as: Reading Assignment
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CHEM 241L: Spectrophotometric Determination of the pKa of the Acid-Base Indicator Bromothymol Blue 70 % T = I I 100 o ⎟ • (2) The transmittance can be converted to absorbance using the following expression: A = log I I = -log T 0 (3) Table 1 provides some typical values to illustrate how easily absorbance and transmittance values can be interconverted. In general, we measure absorbance between 0 and 2 for experimental reasons. I o /I Absorbance (A) Percent Transmittance (% T) 1 0 100 10 1 10 100 2 1 1000 3 0.1 Table 1. Relationship between absorbance and transmittance. A second useful relationship commonly employed when making absorbance measurements is called Beer's Law , which is mathematically expressed in Equation 4. A λ = ε λ b C (4) In Equation 4, A λ is the measured absorbance of the absorbing molecule at some wavelength ( λ ), ε λ is the molar absorptivity (units M -1 cm -1 ) of the absorbing molecule at the wavelength in which the absorbance measurement was made, b is the pathlength of the cell or cuvet (in cm), and C is the concentration (in molar units (M)) of the absorbing molecule. The molar absorptivity is a constant for every molecule and is basically an indicator for how much light a given molecule can absorb at a particular wavelength.
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This note was uploaded on 01/03/2009 for the course CHEM 241 taught by Professor Tiani during the Spring '08 term at UNC.

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Experiment3 - CHEM 241L: Spectrophotometric Determination...

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