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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5986_Библиотеки_им_академика_М_И_Перельмана.pdf
187
Properties of Liquids
Experiment 6 Study the effect of temperature on viscosity.
Requirements
Sucrose solution, distilled water, Ostwald viscometer, stopwatch, thermostat,
specific gravity bottle, 10 mL pipette
Procedure
1. Prepare a saturated solution of sucrose in purified water and filter it.
2. Arrange the assembly as in Experiment 5.
3. Determine the time taken to flow for distilled water and sucrose solution under
the same conditions at different temperature (RT, 35, 45, 55, 75 and 85°C).
4. Determine the density of water and sucrose solution at the same tempera-
tures with the help of pychnometer.
5. Plot a graph between temperature and viscosity.
Observation and calculation
1. Room temperature _______°C
2. Viscosity of water at room temperature _______
3. Density of water at room temperature ( _______°C)
a. Weight of the empty pychnometer w
1
g
b. Weight of the pychnometer water w
2
g
Density of water at room temperature ( _______°C) w
2
w
1
/v
Calculate density of water at different temperatures.
Similarly determine the density of the sucrose solution (10 mL) at different
temperature ( _______°C) using pychnometer.
Calculate viscosity of the sucrose solution at different temperatures by
equation:
Ș
1
Ș
2
(d
1
t
1
/d
2
t
2
)
Observation
Liquid Temperature
(°C)
Density
(g cm
3
)
Time taken to
flow (s)
Viscosity
Sucrose
solution
RT
35
45
55
65
75
85
188
Practical Pharmaceutical Analytical Chemistry
Liquid Temperature
(°C)
Density
(g cm
3
)
Time taken to
flow (s)
Viscosity
Water RT
35
45
55
65
75
85
Result The effect of temperature on viscosity was observed and it was seen
that viscosity _________ with the increase in temperature.
Experiment 7 Study the effect of concentration on viscosity.
Requirements
Sucrose solution, distilled water, Ostwald viscometer, beaker, stopwatch,
thermostat, specific gravity bottle, 10 mL pipette
Procedure
1. Prepare a sucrose solution of different concentrations (4, 16, 24 and 40%) in
purified water by dissolving 1, 4, 6 and 10 g of sucrose in 25 mL of distilled
water, respectively.
2. Filter all the solutions.
3. Arrange the assembly as in Experiment 5.
4. Determine the time taken to flow for distilled water and sucrose solution
of different concentrations under the same conditions at room temperature
5. Determine the density of water and sucrose solution at the room tempera-
ture with the help of pychnometer.
6. Plot a graph between concentrations and viscosity.
Observation and calculation
1. Room temperature _______°C
2. Viscosity of water at room temperature ______
3. Density of water at room temperature ( ______°C)
a. Weight of the empty pychnometer w
1
gm
b. Weight of the pychnometer water w
2
gm
Density of water at room temperature ( ______°C) w
2
w
1
/v
189
Properties of Liquids
Similarly determine the density of sucrose solution of various concentrations (10 mL) at room
temperature ( ________°C) using pychnometer.
Calculate viscosity of the sucrose solution at different temperatures by equation:
Ș
1
Ș
2
(d
1
t
1
/d
2
t
2
)
Observation
Liquid Concentration Density (g cm
3
) Time taken to flow (s) Viscosity
Sucrose
solution
4
16
24
40
Water
Result The effect of concentration on viscosity was observed and it was seen
that viscosity _________ with the increase in concentration.
Experiment 8 Determine the composition of unknown mixture of glycerin
and water by using Ostwald viscometer.
Requirements
Glycerin, distilled water, Ostwald viscometer, beaker, stopwatch, thermostat,
specific gravity bottle, 10 mL pipette
Procedure
1. Prepare different compositions of glycerin and distilled water (20:80, 30:70,
40:60, 50:50, 60:40, 70:30, 80:20) at room temperature by mixing different
volumes of glycerine and distilled water as mentioned in the table.
2. Determine viscosity of different compositions under the same conditions at
different room temperature as in Experiment 5.
3. Determine the density of different compositions at the room temperature
with the help of pychnometer.
4. Plot a graph between compositions and viscosity and read the compositions
of unknown from the graph by intrapolation.
Observation and calculation
1. Room temperature ______°C
2. Density of water at room temperature ( ______°C)
a. Weight of the empty pychnometer w
1
gm
b. Weight of the pychnometer water w
2
gm
190
Practical Pharmaceutical Analytical Chemistry
Observation
Compositions Glycerin
(mL)
Distilled
water
(mL)
Density
(g/cm
3
)
Time taken
to flow (s)
Viscosity
(Poise)
20:80 5.0 20
30:70 7.5 17.5
40:60 10.0 15
50:50 12.5 12.5
60:40 15 10
70:30 17.5
7.5
80:20 20 5
Unknown – –
Density of water at room temperature ( ______°C) w
2
w
1
/v
Similarly, determine the density of the composition (10 mL) at room temperature
( ______°C) using pychnometer
Result The composition of unknown mixture was found to _____% glyc-
erine and _____% water.
191
Properties of Liquids
Judge yourself
1. Define the surface tension.
2. How does temperature effects surface tension?
3. How is surface tension measured in a laboratory?
4. Explain the mechanism of falling sphere stalagmometer.
5. What is the importance of surface tension measurements?
6. With the increase in temperature surface tension decreases, why?
7. Define interface with an example.
8. What are the factors that affect surface tension measurements?
9. What is the surface tension of water at 20°C?
10. How is surface tension and chemical constitution of substance related?
11. Define relative and absolute viscosities.
12. How is viscosity of liquids determined?
13. Name various viscometers used to determine viscosity of liquids.
14. Describe the factors that affect the viscosity.
15. Describe the principle of Ostwald viscometer.
16. What are applications of viscosity measurement?
17. Measurement of viscosity is based on two laws. State them.
18. Define rheochor.
19. What are the units and dimensions of viscosity?
20. What do you understand by molecular viscosity?
21. What precautions should be taken while carrying out viscosity mea-
surement exercises?
22. With increase in temperature viscosity of a liquid decreases, why?
23. With increase in concentration viscosity of a liquid increases, why?
24. Define the molar surface area.
7KLVSDJHLQWHQWLRQDOO\OHIWEODQN
Chapter
7
Chapter
Properties and
Molecular Constitution
of Substances
7.1 Polarimetry
One of the physical properties of molecules that have asymmetric carbon atoms,
i.e. a carbon atom with four different substitutions, is that the molecules will
rotate in the direction in which the plane-polarized light is vibrating as it passes
through them. This property is known as optical activity. Optical activity is the
ability of a chiral molecule to rotate the plane of plane-polarized light. It is
measured using polarimeter.
Normal white light vibrates in all directions, so when it rotates it is still
vibrating in all directions. This property becomes apparent only when polarized
light is used. A polarized light is light obtained by passing normal white light
through a polarizer or a polarizing filter such that it allows light to pass through
it but the vibration is parallel to the alignment of the filter.
The chirality of molecules was first reported in 1874 by Jacobus van’t Hoff, and
Joseph Le Bel. They recognized that a compound with four different substituents
could exist in two forms that were mirror images of each other. For example,
consider the following CHFClBr molecule.
H
F
Cl
C
C
H
F
Cl
Br
Br
Fig. 7.1
Scheme of CHFClBr
molecule
194
Practical Pharmaceutical Analytical Chemistry
If the molecular structure on the left is rotated on the right by 180° around the
C-H bond, the structure shown on the right is obtained. These structures cannot
be superimposed on each other. Such pair of stereoisomers that are mirror images
of each other are called enantiomers and are optically active. A racemic mixture (a
50:50 mixture of two enantiomers) will not rotate the plane polarized light and is
optically inactive. Stereoisomers that are not mirror images of each other are called
diastereomers. Mesomers are compounds which have more than one chiral centres
and possess symmetry; these types of compounds are also optically inactive due to
compensation of optical rotation due to one chiral centre by another chiral centre.
In 1813, Jean Baptiste Biot observed that plane-polarized light was rotated either
to the right or to the left when it passes through single crystals of quartz or aqueous
solutions of tartaric acid or sugar. Substances that can rotate plane-polarized light
are said to be optically active. Those that rotate the plane clockwise (to the right) are
said to be dextrorotatory and that rotate the plane counter clockwise (to the left)
are called levorotatory. Optical activity of a substance is one of the macroscopic
properties of the substance that arises from the way they interact with the light.
Dextrorotary is designated as (), clockwise rotation (Latin: dexter, right).
Levorotary is designated as (), anticlockwise rotation (Latin: laevous, left).
If only one enantiomer is present in a sample, then it is considered to be optically pure.
The polarimeter, which is an instrument with which optically active
compounds are studied, is shown in Figure 7.2.
The following factors affect the optical activity of a substance:
1. the wavelength of the light;
2. the length of the cell through which the light passes;
3. the concentration of the optically active compound in the solution through
which the light passes;
4. nature of the solvent;
5. temperature;
6. the specific rotation of the compound, which reflects the relative ability of
the compound to rotate plane-polarized light.
The magnitude of the rotations observed for a pair of enantiomers is always the
same. The only difference between these compounds is the direction in which
they rotate the plane-polarized light.
lr
l
lr r
r
l
llrl
lr r
Fig. 7.2
Schematic
representation of
a polarimeter
b
a
195
Properties and Molecular Constitution of Substances
Mutarotation: It is a phenomenon of showing change in optical rotation value
by some substances. It is a characteristics feature of carbohydrates. Mutarotation
is defined as the change in the optical rotation showed by compound as a result
of epimerization (i.e. change of one epimer to another). The classic example is
that of D-
D Glucose, freshly prepared aqueous solution of which has [D]
D
20°C
of
113° but gradually changed to a final values of 52°.
α l
C α
β l
C
β
CH H
H
H
H
CH
H
H
H
H
l
CH H
H
H
H
H
H
H
H
H
CH H
H
H
H
H
H
H
H
H
H
H
H
7.1.1 Measuring Optical Activity
When rotation is measured by using a polarimeter, it is known as an observed
rotation. Since observed rotation depends on a number of factors as mentioned
earlier, the specific rotation, [a] is used to eliminate the effect of the factors involved.
The specific rotation, [a] 100 a/cl (1)
where the concentration is expressed as 1 g sample/100 mL solution.
The specific rotation [a] is defined as the observed angle of rotation a when
plane-polarized light is passed through a sample of path with length of 1 dm and
concentration of 1 g/1 mL. The specific rotation of a pure material at a given
wavelength and temperature is an intrinsic property. The values of a specific rotation
are accompanied by the temperature at which the specific rotation is measured
and the solvent in which the material was dissolved. When the temperature is not
specified it is always assumed to be measured at room temperature.
The unit for specific rotation values is deg dm
1
cm
3
g
1
or just degree. A
negative value of specific rotation means levorotatory, i.e. anticlockwise rotation
and a positive value means dextrotatory, i.e. clockwise rotation. Some examples
are listed in Table 7.1:
Compound Specific
rotation (°)
Compound Specific
rotation (°)
Compound Specific
rotation (°)
Sucrose
66.47
Camphor
44.26
Glucose
52.7
Lactose
52.3
Penicillin V
223.0
Fructose
Fig. 7.3
Mutarotation of
D-glucose
Table 7.1
196
Practical Pharmaceutical Analytical Chemistry
Equation for calculation of specific rotation of pure liquids:
α
α
λ
[]
=
×
5
ld
(2)
Equation for calculation of specific rotation of solutions
α
α
λ
[]
=
t
100
lc
(3)
Molecular rotation: Molecular rotation is a constitutive property and is obtained by
multiplying specific rotation with molecular mass of the substance:
Μ
Μ
[]
=
[]
λλ
α
tt
100
(4)
Intrinsic rotation: The intrinsic rotation is the limiting value of specific rotation
at zero concentration. Since specific rotation also depends on certain factors
like the concentration of solution and the length of tube, to eliminate the effect
of these factors intrinsic rotation was used as a kind of physical quantity of
substances. It can be obtained by plotting a graph between specific rotation and
concentration and extrapolating the plot to zero concentration. The point at
which it intersects the Y-axis is the value of intrinsic rotation (see Fig. 7.4).
Experiment 1 Determine the specific and molecular rotations of sucrose at a
number of concentrations and obtain the value of intrinsic rotation for sucrose.
Requirement
Polarimeter, sugar solutions, 100 mL flasks, distilled water, cotton, filter paper.
Procedure
Calibration of polarimeter
1. Switch on the polarimeter before the start of the experiment so that the
sodium lamp heats up enough to obtain proper illumination.
2. Take polarimeter tube and clean it properly.
Fig. 7.4
Measuring intrinsic
rotation
Y
X
r
r
r
Cr rl