3B SCIENTIFIC PHYSICS U14390 Mode D'emploi page 6

6. Experiment examples
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:
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 tautomer-
ism or mutarotation ( α - or β - form), it may be neces-
sary for a certain amount of time to elapse before equi-
librium 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 addi-
tional calculated dose or subtracted during the calcu-
lation later (g/100 ml).
6.2 Measurement of concentration
The specific angle of rotation of a substance is mea-
sured first. After that, a solution with an unknown con-
centration of this substance (or known only to the
trainer) is prepared. Filling level d = 1 dm,
20
α [ ]
c = ?
D
α (measured) = + 14°
The concentration c in g/0.1 dm3 is calculated as follows:
α
⋅ + ⋅
100 14 100
= =
c
+ ⋅
20
α
[ ] ⋅
d
D
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 artifi-
cial honey). At room temperature, the specific angle of
rotation changes over a period ranging between sev-
eral hours and several days, depending on the acid
concentration. Higher temperatures notably acceler-
ate the inversion process (to a matter of hours). The
specific angle of rotation changes from +66° to roughly
–22° (saccharose: +66°; glucose („equilibrium glu-
cose"): +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
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+ ⋅
32 100
20
α [ ] = = +
64
D
⋅
50 1
=+64°
=
21 9 100 , g / ml
64 1
more water and 5 - 20 ml of dilute hydrochloric acid.
At room temperature, perform measurements initially
at 10-minute intervals, then at hourly intervals; con-
vert the read angles of rotation into specific angles of
rotation, and plot these values in a diagram.
20
α [ ]
D
50
0
2
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 left-
handed 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, gradu-
ally leading to a state of equilibrium.
D-glucose is weighed and dissolved quickly by shak-
ing. The angle of rotation is determined at regular time
intervals, converted immediately into the specific angle
of rotation, and plotted in a diagram.
20
α [ ]
of α -D-glucose: 112-113°; after equilibrium has
D
been reached (several hours): +52°
A mixture of α - and β -D-glucose is now present. Mu-
tarotation in the case of fructose takes place much more
quickly.
100% a-D-glucose
[α]
100% α -D-glucose
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 cu-
vette, 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 de-
vice (as in the case of mutarotation, refer to 4.5). Clean-
ing 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).
6
(days)
+ (Tage)
4 6
Approximately 40% a-D-glucose
Several
hours
5.Care