3B SCIENTIFIC® PHYSICS
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U14390 Polarization Demonstration Device
Operating instructions
3/03 ALF
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4
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®
1Base plate
2Angular scale
3Plug
4Rotary grip with a pointer
5Analyzer
The U14390 polarization device is designed for use on
a daylight projector as part of qualitative and quantitative experiments conducted before large audiences
at schools and universities, for example in order to
demonstrate optical activity as well as determinations
of specific angles of rotation, or concentrations if these
angles are already known.
1. Safety notes
• Do not clean the polarization demonstration device with aggressive agents.
• Do not fill the cuvette with liquids which attack
plexiglass.
• Ensure that the filters do not get scratched.
2. Description, technical data
A yellow filter and a polarizer are set in the middle of a
black, plastic base plate. A cuvette marked at 50 mm
and 100 mm and containing a solution of the substance
to be examined is inserted into the inner holder and
subsequently covered by an analyzer mounted on an
outer holder equipped with a rotary knob and a pointer.
Turning the analyzer allows the angle of rotation to be
read on a transparent scale ranging from + 40° to
–40° and having divisions of 1°.
Dimensions: 370 mm x 330 mm x 190 mm
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Illustration 1
2. Operating principle
Light (visible electromagnetic radiation) emitted by the
daylight projector is made to pass through a yellow filter, as yellow light by definition increases the measurement accuracy.
This light oscillates in a number of planes:
z
Licht
Light
x
Illustration 1:
1Yellow filter 5Holder with rotary knob
z
2Polarizer and pointer
3Holder for the cuvette 6100-mm mark
450-mm mark 7Cuvette
4
x
The first polarization filter, or polarizer, preferentially
α
[]
20
D
allows the passage of one of the oscillation planes, thus
polarizing the light. If a second polarization filter (analyzer) rotated by 90° with respect to the first one is connected in series, the polarized light is largely absorbed,
because the lattices formed by these mutually perpendicular filters are transverse with respect to the oscillation planes. The result is maximum extinction.
y
z
Polarizer
AnalysatorPolarisator
Analyzer
If the light path is made to pass through a substance
(comprising the solution in the cuvette) which rotates
the oscillation plane of the polarized light either to
the left or the right, i.e. an optically active substance,
the analyzer needs to be turned accordingly in order
to maximize extinction again.
The angle (in degrees) between maximum absorption
with and without the cuvette solution, or between pure
solvent and solution, is determined by turning the analyzer; this angle is a decisive parameter, in addition to
the concentration of the solution and the filling level
of the cuvette.
4. Operation
• Place the polarization demonstration device on the
daylight projector and focus the image of the scale.
• Set the pointer to zero. Rotate the analyzer so that
the extinction is maximized. No light spot from the
light path should be visible on the projection area.
• Fill the cuvette with the pure solvent and insert it
into its holder.
• Turn the pointer to the left and the right until a
light spot just becomes visible again on both these
sides of the scale. The value located exactly between
these two measurement results serves as the zeropoint or reference point for further measurements.
Ideally, it coincides with the zero mark on the scale.
Example: Measurement limits of –6° and +4° result in a reference value of –1°.
• After that place the cuvette with a solution of the
optically active substance in the light path, and note
the filling level for future calculations.
• As in the case of the pure solvent, establish the
points on either side at which maximum absorption occurs, i.e. at which the light spot just appears
again. This will allow you to determine the angle
of rotation.
For instance, limits of –21° and –11° would result
in a reference value of –16° here. If the reference
value of the pure solvent was –1°, the measured
angle of rotation a is –15°.
5. Polarimetry
Compounds which carry four different substituents or
ligands at a center (activity center) and which can be
reflected on a mirror plane are termed optically active
(chiral).
Mirror plane
Spiegelebene
AA
BB
DDD
C
C
zz
Such compounds behave like objects and their mirror
images, and are not superimposable (enantiomeric
forms). Optically active substances rotate the oscillation plane of light. If 50% of each form is present in
the mixture (racemate), rotation is cancelled. If one of
the two forms predominates, the oscillation plane is
rotated as a whole. The angle of rotation α is a material constant which depends on the following conditions, in addition to the nature of the particles:
• Wavelength of the light: As the general convention
is to use the sodium-D line of the emitted light (Na
vapor discharge lamps) for exact measurements, the
bottom of the device is fitted with a yellow filter to
approximate this spectral range.
• Temperature: 20°C are usually specified for measurements.
• The number of rotating particles: Dependence on
the concentration of the dissolved substance and
the layer thickness of the solution (= filling level of
the cuvette); proportional relationship.
• Solvent.
Rotation expressed with respect to a particular quantity of optically active substance (right-handed = +, lefthanded = -; angle of rotation) is a material constant
termed specific rotation (specific angle of rotation).
α
±⋅
α
[]
20
100
=
⋅Dcd
= Spec. angle of rot. for the Na-D line at 20°C
α
=Measured angle of rotation (scale reading)
c =Concentration in grams/100 ml (g/0.1 dm3)
of solution
d =Layer thickness (filling level) in dm.
5.1 Examples
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Examples of specific angles of rotation
tation) in degrees:
α
[]
(End ro-
D
D-glucose:+52.7; D-fructose: –92.4; D-mannose:+14.6;
D-galactose:+80.2; D-xylulose:+33.1; D-ribose:–23.7;
Saccharose:+66.5; Maltose+130.4; Lactose+52.5
(values provided by Aebi, Einführung in die praktische
Biochemie, Karger 1982)
α
-D-glucose 113.0 (crystallized from water); α-D-glucose +19.0 (crystallized from pyridine); α-Hydroxybutyric acid -24.8; Protein -52.8
(values provided by Rapoport/ Raderecht,
Physiologisch-chemisches Praktikum, VEB Verlag Volk
u. Gesundheit, 1972).
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6. Experiment examples
α
[]
20
D
6.1 Specific angle of rotation of saccharose
Initial weight: Dissolve 50 g of saccharose in water in a
volumetric flask and fill up to the 100 ml mark. The
resulting solution is poured into the cuvette to a height
of 10 cm (1 dm). The following angle of rotation is
measured: 32°, right-handed.
Specific angle of rotation:
[]
=
D
50 1
=+
⋅
64
+⋅
32 100
20
α
The determined specific angle of rotation is thus of the
same order of magnitude as the bibliographic value.
Note: Even high-precision polarimeters are not always
able to achieve bibliographic values. Due to tautomerism or mutarotation (α- or β- form), it may be necessary for a certain amount of time to elapse before equilibrium is reached. Solutions of mutarotating sugar
should be left standing for extended periods (overnight)
following their preparation.
Watch out for yeast and bacteria after long periods of
storage! When weighing sugars (for instance, glucose),
carefully read the label on the chemical bottle. Any
crystal water (monohydrate) must be indicated on the
label, and either compensated by means of an additional calculated dose or subtracted during the calculation later (g/100 ml).
6.2 Measurement of concentration
The specific angle of rotation of a substance is measured first. After that, a solution with an unknown concentration of this substance (or known only to the
trainer) is prepared. Filling level d = 1 dm,
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α
c = ?
α
(measured) = + 14°
[]
D
=+64°
The concentration c in g/0.1 dm3 is calculated as follows:
α
100
⋅
c
20
α
[]
D
⋅
d
+⋅
=
14 100
64 1
+⋅
21 9 100,/
=
gml=
6.3 Inversion of saccharose
Acid can be used to split the disaccharide saccharose
into D-glucose and D-fructose. The solution of these
fission products – also optically active – has a different
angle of rotation compared with saccharose (inversion).
A glucose-fructose mixture with a molar ratio of 1:1 is
therefore termed invert sugar (for instance, in artificial honey). At room temperature, the specific angle of
rotation changes over a period ranging between several hours and several days, depending on the acid
concentration. Higher temperatures notably accelerate the inversion process (to a matter of hours). The
specific angle of rotation changes from +66° to roughly
–22° (saccharose: +66°; glucose („equilibrium glucose“): +52°; D-fructose: -92.4°).
Recommendation: Dissolve up to 50 g of saccharose in
a little water, and top the solution up to 100 ml with
more water and 5 - 20 ml of dilute hydrochloric acid.
At room temperature, perform measurements initially
at 10-minute intervals, then at hourly intervals; convert the read angles of rotation into specific angles of
rotation, and plot these values in a diagram.
20
α
[]
D
50
0
+ (Tage)
2
(days)
64
If inversion is to be performed at higher temperatures,
it is advisable to use a thermostatted solution (water
bath) of a higher volume (1-2 l). Before performing the
measurements, draw samples, allow them to cool
quickly, and pour them into the measuring cuvette.
6.4 Wine
Wine exhibiting right-handed rotation may have been
mixed with glucose before or after fermentation, or with
saccharose after fermentation. Wine exhibiting lefthanded rotation is natural (according to Dr. Steeg &
Reuter).
6.5 Mutarotation in the case of anomeric C-atoms
Mutarotation occurs when a solution of an optically
active substance changes its angle of rotation, gradually leading to a state of equilibrium.
D-glucose is weighed and dissolved quickly by shaking. The angle of rotation is determined at regular time
intervals, converted immediately into the specific angle
of rotation, and plotted in a diagram.
of α-D-glucose: 112-113°; after equilibrium has
been reached (several hours): +52°
A mixture of α - and β -D-glucose is now present. Mutarotation in the case of fructose takes place much more
quickly.
[α]
100% a-D-glucose
100% α-D-glucose
Approximately 40% a-D-glucose
Several
hours
5.Care
The Perspex cuvette is only suitable for liquids which
do not attack it. However, the focus of interest here in
any case is aqueous solutions. Before inserting the cuvette, always ensure that it is clean and dry! The cover
must on all accounts be closed if the measurement will
take long or if the cuvette is to remain inside the device (as in the case of mutarotation, refer to 4.5). Cleaning should be performed with a soft, dust-free cloth.
Do not scratch the filters! It is advisable to store the
device under dust-free conditions (in an anti-dust
jacket).
3B Scientific GmbH • Rudorffweg 8 • 21031 Hamburg • Deutschland • www.3bscientific.com • Technische Änderungen vorbehalten
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