|
CEE 772 |
Fall 2005 |
Homework #1
Based on Lab #1: UV/Vis
Spectrophotometry
a. Objective:
Determine molar absorptivity of 2,4 Dintrophenol (including uncertainty).
b. Procedure:
Prepare the following solutions by diluting the stock 10 mM solutions (prepared by Gladys)
- 1 mM
- 0.5 mM
- 0.2 mM
- 0.1 mM
- 0.05 mM
- 0.02 mM
Measure absorbance of each at a wavelength of 254 nm, and for the dinitrophenol only measure the absorbance at 360 nm too.
Proposed Assignments:
|
Spectrophotometer |
Location |
Group |
|
Lambda 3A |
308 |
Mercer, Sahu |
|
ThermoSpectronic Genesys
10 UV |
304 or 213 |
Briggs, Lui |
|
Hach DR/4000U |
304 |
Huang, Heymann |
|
Spec 21D |
Marston 24 |
|
|
Spec 20 Genesys |
Marston 24 |
|
|
Spectronic 20 (standard) |
308 |
Islam, Goodwell |
c. Report
Prepare on brief technical report per group (2 in a group) including the following elements
- Graphical and tabular absorbance vs concentration data for each spectrophotometer
- Assessment of molar absorptivity based on those data
- Assessment of uncertainty in that number
- Comparison of the estimated molar absorptivity with any published values
- What if any role does pH play in this exercise?
Solution
First the absorbance versus concentration data should be plotted (see figure below for the Genesys 10UV). This reveals that there is a departure from Beer’s law for the highest concentration sample. Therefore, a linear least squares regression was determined for all data except that last point. This is deemed to represent the most accurate absorptivity. The slope for this instrument was 1568 M-1cm-1.
An expansion of the lower concentration range shows that by excluding the 2 mM sample produces an absorbance standard curve that comes much closer to passing through the origin and shows very little signs of curvature.
Data from the Hach instrument is even clearer in this regard. Here a 10 mM standard was measured which is quite obviously beyond the linear range of this instrument. Excluding the two highest points gives a molar absorptivity of 1564 M-1cm-1, which is only about 0.3% less than the Genesys value.
The NIST website has data on phthalic acid’s molar absorptivity. The spectrum below shows this value for wavelengths within the UV region.
Below is an expanded graph showing the 254nm region. The Log ε is about 4.11, which corresponds to a molar absorptivity of 1290 M-1cm-1. This value is close to, but somewhat smaller than the measured value.
Reasons for the small discrepancy between the measured and reference values might be attributed to:
- Contamination of the phthalic acid reference compound by higher absorbing compounds
- Errors in preparing the primary phthalic acid solution
- Higher absorptivity of anionic forms over the fully protonated acid (i.e., phthalate versus phthalic acid)
Due: 21 Sept 2005