Absorption Spectroscopy
CHEM 251
Week of November 1st, 2010
Alexis Patanarut
CHEM 251 Laboratory
For the week of November 8th, 2010:
• Experiment: VSEPR experiment, pp.
99-105
• Due next week: Absorption
Spectroscopy lab report
• Due this week: Ideal Gas Law lab
report
Overview
• Today, a spectrometer will be used to determine the wavelengths of
visible light absorbed by substances in solution.
• After the wavelength of maximum absorption is found, a series of
standard solutions will be prepared.
– From the data obtained, a calibration curve will be plotted to relate
the intensity of absorbance and the concentration of a solution.
– From this curve, the concentration of an unknown can be
determined.
Class Data
1.000
y = 4.8793x - 0.0342
R2 = 0.9924
0.800
absorbance
0.600
y = 1.9526x + 0.0085
R2 = 0.9931
group 1
group 2
0.400
Linear (group 1)
Linear (group 2)
0.200
0.000
0.000
0.050
0.100
0.150
-0.200
concentration (M)
0.200
0.250
Absorbance = -log (% transmittance/100)
Procedure
Part I – Calibrating the instrument
1.
Turn on the spectrometer and set to 0% transmittance using
the left knob.
2.
Place a cuvette ½ filled with H2O in the sample holder and
set to 100% transmittance with the right knob.
3.
Remove the blank. The display should now read 0%
transmittance. If not, repeat steps 1-3.
Part II – Absorbance vs. Concentration
1.
Prepare two 10mL standard solutions from the sample
stock solution. Solution 1: ~3mL stock solution Solution 2:
~7mL stock solution
2.
Set the instrument to your lmax and calibrate the instrument.
3.
Read the absorbance of each of the four solutions (two
standard solutions, distilled water, and stock solution) at
lmax.
Solution lmax:
Cu2+ – 625nm; Co2+ – 515nm; Ni2+ – 395nm
Absorbance = -log (% transmittance/100)
Procedure
Part III – The Unknown
1.
Obtain a sample of unknown concentration and measure its
absorbance at lmax. Be sure to record the number of your
unknown.
• Do NOT empty the unknown test tubes.
2.
Once all of the wavelengths, volumes and absorbances have
been recorded, enter the appropriate values in the webbased data entry form. Come see me to get the
concentration for your unknown.
3.
Return all cuvettes and unknown test tubes to me when
you’re done.
For the lab report
•
You should have a total of two data tables and one
graph:
1.
2.
Your data table (no class data this week!)
1 X-Y Scatter plot showing the Calibration Curve
•
Show sample calculations for the molarity of your
standard solutions and the percent error.
•
Answer all of the discussion questions from the
handout in the Discussion/Conclusion section of
your lab report.
–
Ignore the question about the spectrum graph, since we’ve
already found the lmax for you