Use of the Spectronic 20™ at a Fixed
Wavelength
A Description
Turning On The
Instrument
Adjusting the
Wavelength
Reading the Meter
Preparing a Cuvette
Setting Maximum
Absorbance
Setting Zero
Absorbance
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Main Menu
A Description
The Spectronic 20™ is an instrument that measures the amount of light
absorbed (or transmitted) by a solution.
Light from a lamp is dispersed by a grating into a continuous spectrum of colors.
Light of one wavelength (and color) is passed through a sample of solution and
than illuminates a phototube detector. The detector produces an electrical signal
that depends on the intensity of light. The size of the signal is displayed on a
meter. The wavelength is adjusted by rotating the grating so that a small selected
region of the visible spectrum passes through the exit slit into the sample.
This diagram is a representation of the inside of a Spectronic 20™
Turning on the Instrument
Turn the instrument on using the Power Switch/Zero Control knob, and allow
the Spectronic 20™ to warm up for at least 15 minutes. Why should it warm up
for 15 minutes?
Video: Turning the Spec 20™ On ( 1.37M )
Which of the following are likely to happen if the instrument has not
warmed up for fifteen minutes prior to use?
A. Recorded absorbance readings will be inaccurate.
B. The sample will be automatically ejected from the sample
compartment.
C. No light will pass through the sample.
Answer
Adjusting the Wavelength
After the instrument warms up, select the desired wavelength by turning the
wavelength control knob. Be sure to reset the maximum and zero absorbance
values every time you change the wavelength.
Video: Setting the Wavelength ( 1.07M )
Reading the Meter
The Spectronic 20™ meter displays both absorbance and transmittance.
What is the difference between absorbance and transmittance?
The top scale shows transmittance, and the bottom scale shows absorbance. Read
the meter so that the needle obscures its reflection in the mirror on the meter.
Correct and incorrect meter readings are shown below.
Correct reading, straight-on: Absorbance =
0.238
An incorrect reading from an angle - note the
reflection
Preparing a Cuvette and Inserting It into the Sample
Compartment
Use a clean cuvette that is free of chips and cracks.
Video: How to Prepare a Cuvette ( 3.33M ) Text description
Rinse the cuvette with a small amount of solution, then fill the cuvette at least
half full. Check your sample for air bubbles. WHY?
Remember to wipe the cuvette with a laboratory tissue and to align the mark on
the cuvette with the mark on the Spectronic 20™. WHY?
Click on the sample below which is appropriate for use in the
Spectronic 20™...
A
B
C
D
E
Setting Maximum Absorbance
The maximum absorbance is measured when no light is transmitted to the
phototube detector. It is set using the power switch/zero control knob.
Video: How to set the maximum absorbance ( 1.01M ) Text description
Remember, the maximum absorbance is set with the sample compartment
empty. WHY?
The Spec 20™ in the video is being set for maximum
absorbance. Would any subsequent measurements be
Video ( 914K accurate? Why or why not? Answer
)
Setting Zero Absorbance
A reference solution is used to calibrate the scale of absorbance readings. It
should be the same as the solution to be analyzed except that it must have zero
concentration of the absorbing species. The reference with no absorbing species
is used to set zero absorbance.
Place the reference solution in the sample compartment and use the
transmittance/absorbance control knob to set the meter to zero absorbance
(which is equivalent to 100% transmittance).
Video: How to set the zero absorbance ( 2.05M )
Click on the knob used to adjust the absorbance/transmittance
within the picture of the Spectronic 20™ shown below.
The maximum and zero absorbance values must be set each time the wavelength
is changed. WHY?
It is a good idea to recheck the maximum absorbance after setting the zero
absorbance to verify that the instrument is working properly.
The instrument is now ready for the sample. Place the cuvette containing the
sample into the sample compartment and record the absorbance.
A student is measuring the absorbance of a solution of iodine in
hexane. Suggest an appropriate reference sample. Answer
Miscellaneous exercises
Do you expect the absorbance measured by the student to
be higher or lower than the actual absorbance of the
Video ( 2.66M solution? Why? Answer
)
Related module: Spectronic 20™ at Variable Wavelength
Copyright © 1995-1996 NT Curriculum Project, UW-Madison