Thermal Lens Calorimetry

1
Thermal Lens Calorimetry

• Ashleigh Baber
• Dr. Seidman
• Karishma Rajani
• Summer 2004
• Randolph-Macon Womans College

2
Thermal Lens Phenomena
Higher energy level
photon
Laser Beam
Thermal energy
electron
Ground State
Sample
3
Divergence
4
(No Transcript)
5
Experimental Design
6
Experimental Design without Sample
7
Square Wave
ON
OFF
OFF
8
Experimental Design
9
tc
Ic
10
Cpheat capacity kthermal conductivity tctime
constant Mmolar mass wcconfocal beam
waist rdensity
11
Why Calibrate the Procedure?

• The appearance of the waveform is affected by
  factors such as
• Distances between components on the optical rail
• Laser power
• Frequency of the chopper
• Aperture diameter
• All factors except for the aperture diameter are
  kept constant.

12
Method for Calibrating the Procedure

• Prepare a set of standard solutions using
  indophenol blue dye and liquids with known heat
  capacities.
• Measure the bloom diameter and vary the aperture
  of the diaphragm until the correct time constant
  is obtained.
• Aperture diameter is plotted versus Bloom
  diameter and the result is a straight line. This
  graph may then be used as a calibration curve for
  finding the heat capacity for unknowns.

13
Total Calibration Curve
14
Determining the Cp of Unknowns

• A solution is made with indophenol blue dye and a
  liquid with an unknown heat capacity.
• The bloom diameter is measured and using the
  equation for the line, the aperture diameter is
  calculated.
• Several trials are conducted with the calculated
  aperture diameter to find an average time
  constant which is plugged into the equation to
  find the heat capacity.

15
(No Transcript)
16
Results

• A range of absorbances fit on one line, reducing
  the time necessary for sample preparation.
• Error in heat capacities are random, due to the
  precision of the oscilloscope.
• It seems as though this method could be as
  reliable as solution calorimetry and it has the
  potential to be faster.

17
Experimental Design
18
(No Transcript)
19
(No Transcript)
20
Proposed Procedure

• Prepare an unknown so that the absorbance falls
  within the range.
• Measure the sample to diaphragm distance.
• Calculate the slope and y-intercept from the
  calibration lines.
• Determine the bloom diameter.
• Calculate the aperture diameter and run the
  sample.
• A.D. slope B.D. y-intercept