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Verfügbare Sprachen
Verfügbare Sprachen
The first polarization filter, or polarizer, preferentially
allows the passage of one of the oscillation planes, thus
polarizing the light. If a second polarization filter (ana-
lyzer) rotated by 90° with respect to the first one is con-
nected in series, the polarized light is largely absorbed,
because the lattices formed by these mutually perpen-
dicular filters are transverse with respect to the oscil-
lation planes. The result is maximum extinction.
y
z
Polarizer
Polarisator
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 ana-
lyzer; 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 zero-
point or reference point for further measurements.
Ideally, it coincides with the zero mark on the scale.
Example: Measurement limits of –6° and +4° re-
sult 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 absorp-
tion 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).
D
D
Such compounds behave like objects and their mirror
images, and are not superimposable (enantiomeric
forms). Optically active substances rotate the oscilla-
Analysator
Analyzer
tion 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 mate-
rial constant which depends on the following condi-
tions, in addition to the nature of the particles:
• Wavelength of the light: As the general convention
• Temperature: 20°C are usually specified for mea-
• The number of rotating particles: Dependence on
• Solvent.
Rotation expressed with respect to a particular quan-
tity of optically active substance (right-handed = +, left-
handed = -; angle of rotation) is a material constant
termed specific rotation (specific angle of rotation).
α [ ]
α
c
d
5.1 Examples
Examples of specific angles of rotation
tation) in degrees:
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-glu-
cose +19.0 (crystallized from pyridine); α -Hydroxybu-
tyric acid -24.8; Protein -52.8
(values provided by Rapoport/ Raderecht,
Physiologisch-chemisches Praktikum, VEB Verlag Volk
u. Gesundheit, 1972).
5
Mirror plane
Spiegelebene
A
z
B
C
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.
surements.
the concentration of the dissolved substance and
the layer thickness of the solution (= filling level of
the cuvette); proportional relationship.
± ⋅
α
20
α
=
[ ]
D
20
= Spec. angle of rot. for the Na-D line at 20°C
D
= Measured angle of rotation (scale reading)
= Concentration in grams/100 ml (g/0.1 dm3)
of solution
= Layer thickness (filling level) in dm.
A
z
B
C
D
100
⋅
c d
20
α [ ]
(End ro-
D
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