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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5708_Библиотеки_им_академика_М_И_Перельмана.pdf
197
Properties and Molecular Constitution of Substances
3. Fill the tube with distilled water carefully so as to avoid the entry of air
bubbles into the tube.
4. Close the tube and wipe off any water droplets on the outside of the glass
plate windows of the polarimeter tube by filter paper.
5. Place the tube between polarizer and analyzer, look through the eye piece
on the analyzer and focus it on the light shade boundary in the field of view.
6. Determine the zero point of the polarimeter by rotating the analyzer until
the two halves of the field of view are equally dark.
7. The reading on the scale is read through the scale’s eye pieces.
8. If the reading is not exactly zero an adjustment may be made in the
instrument, if provided in the instrument, or the reading is recorded and
used for correcting the subsequent readings.
9. After performing the calibration, the tube is rinsed and filled with solution
or liquids whose optical activity is to be measured and the measurements
are carried out in the same manner as in the calibration step.
10. Reading is repeated five times and the average value is taken as optical rotation.
11. Prepare solution of 5, 7.5, 10, 15 and 20% by dissolving 1.5, 2.25, 3, 4.5 and 6 g of
sugar in 30 mL of distilled water and record the optical rotations for each solution.
12. Calculate the specific rotation. Plot a graph between rotation and
concentration of solutions.
Precautions
Wipe off any water droplets on the outside of the glass plate windows of
the polarimeter tube by a filter paper.
Use distilled water for making sugar solutions.
After finishing the experiment the solution or liquid used is removed from the
polarimeter tube and wash the tube thoroughly with distilled water and dry
before being stored. The caps should be unscrewed, or loosed before storing.
Observations and calculations
Length of polarimeter tube ____dm
Temperature ____˚C
Zero correction ____˚
Observations
Entry Concentration of
solution (%)
Average
observed
rotation
Corrected
average
rotation (D)
Specific rotation
Molecular
rotation
1 20.0
2 15.0
3 10.0
4 7.5
5 5.0
198
Practical Pharmaceutical Analytical Chemistry
Calculate the value of specific and molecular rotations by using Equations (5)
and (6), respectively.
α
α
λ
[]
=
t
100
lc
(5)
Μ
Μ
[]
=
[]
λλ
α
tt
100
(6)
Obtain intrinsic rotation by extrapolation of concentration from the graph
plotted between specific rotation and concentration to 0 (zero).
Result The specific, molecular and intrinsic rotation of sugar solution was
found to be _________, _________ and _________.
Experiment 2 Find the percentage of D-sugar and D-tartaric acid in a freshly
prepared mixed solution using the polarimeter.
Requirement
Polarimeter, pure D-sugar and D-tartaric acid, 100 mL flasks, distilled water,
cotton, filter paper.
Procedure
1. Set the polarimeter as described in the previous experiment and note the
optical rotation of mixture, pure
D-sugar and D-tartaric acid as in Experiment 1.
2. Calculate the specific rotation for pure
D-sugar and D-tartaric acid. Optical
rotation of mixture is calculated as
ααα
αα
[]
=
[]
+
[]
=
[]
+−
()
[]
mix
CC
CC
(7)
(8
)
C 's are weight fractions of the solutes in their dry mixture and [D]s are
specific rotation of separate components of the mixture.
αααα
αα α
α
[]
=
[]
+
[]
−
[]
=
[]
−
[]
{}
+
[]
=
[]
mix
m
CC
C
C
(9)
(10)
iix
−
[]
[]
−
[]
α
αα
(11)
CC
mix
21
12 2
12
1=− =
[]
−
[]
{}
−
[]
−
[]
{}
[]
−
[]
αα α α
αα
(12)
C
mix
1
1
12
=
[]
−
[]
[]
−
[]
αα
αα
(13)
199
Properties and Molecular Constitution of Substances
C
C
mix
mix
2
1
1
2
=
[]
−
[]
[]
−
[]
αα
αα
(14)
These ratio equations will be valid for every wavelength used in the experiment.
3. Prepare mixtures of
D-tartaric acid and D-sugar of different concentrations,
say 2%, 4%, 8%, 10%, 15%, 20% of
D-sugar by adding 2,4,8,10,15 and 20 mg
of
D-sugar in 10 ml of distilled water and add 2 mg D-tartaric acid in each
solution to obtain mixture.
4. Record the optical rotation for all the mixtures prepared and sample solution
as done in Experiment 1.
5. Plot a graph between observed rotation and concentration of
D-sugar and seed
concentration of mixture from the graph.
6. The concentration of
D-sugar and D-tartaric acid can be calculated by using
Equations (13) and (15).
Result The percentage of D-sugar and D-tartaric acid present in mixture
was found to be _________ and _______.
Experiment 3 Study the kinetics of the inversion of cane sugar in presence of
hydrogen ions as catalyst.
Requirement
Polarimeter, sugar solutions, 100 mL flasks, distilled water, cotton, filter paper.
Theory
The inversion of sugar in presence of acid may be represented as
CHO HOH CHO
Sucrose
acid
D
12 22 12 6 12 6
()
()
+
Glucose D
++
()
CH O
Fructose L
6126
The rate of reaction is determined by the concentration of sugar alone when the
concentration of acid is kept constant. The rate may be represented as
dx/dt D>sugar]
or dx/dt k [sugar]
The progress of the reaction can be followed by noting the changes in the optical
activity of the solution with the help of a polarimeter. Both cane sugar and glucose
are dextrorotatory whereas fructose is strongly levorotatory. The levo-rotatory
power of fructose is much higher than the glucose at equimolar concentrations.
Hence, as the reaction proceeds sucrose will break down into glucose and fructose,
therefore the rotation of the solution will change gradually from positive to
200
Practical Pharmaceutical Analytical Chemistry
negative through zero. As sucrose is used up and the glucose–fructose mixture is
formed, the angle of the rotation to the right becomes less gradually, and finally the
light is rotated to the left. Due to the inversion of the sign of the optical rotation
of the solution the reaction is termed as inversion of sugarcane.
Procedure
1. Prepare 50 mL of 20% sugar solution by dissolving 10 mL sucrose in 50 mL
of distilled water and 1 N HCl in two separate conical flasks.
2. Place them in the thermostat for half an hour.
3. Set up the polarimeter as discussed in Experiment 1.
4. Add quickly 50 mL of acid solution to sugar solution stating the stop clock
immediately.
5. The concentration of sugar is reduced to half.
6. Rinse the polarimeter tube with sugar solution and then fill the tube with
solution and record the angle of rotation at t
0
. Place the solution again in
the thermostat.
7. Record the angle of rotation after every 5 min as above.
8. Take out 25 mL of reaction mixture and place in the thermostat or
water bath at 60°C for 1 h.
9. After 1 h of placing reaction mixture in the thermostat or water bath
at 60°C, fill the polarimeter tube with this solution record the angle of
rotation. This will be the reading for t
a
.
10. Record all the readings in table as mentioned below and calculate the
velocity constant.
Observations and Calculations
Room temperature ____°C
=
−
−
−t
a
ax
rr
rr
t
α
Observations
S. no. Time (min) Observed angle of
rotation
Rate Constant (k)
1 t
0
r
0
k
0
2 t
10
r
10
k
10
3 t
20
r
20
k
20
4 t
30
r
30
k
30
5 t
40
r
40
k
40
6 t
50
r
50
k
50
7 t
120
r
120
k
120
8
t
c
r
c
201
Properties and Molecular Constitution of Substances
a initial concentration
a
k
=−
()
−
()
=−
=
−
∞
∞
rr
xrr
t
rr
t
0
10
0
2 303.
log
α
rrr
t
−
∞
Result The inversion of sugarcane was studied under room temperature and
it was observed that the inversion of sugar follows first order rate equation.
7.2 Refractive Indices
Refractive index is a physical property of a substance and is highly significant
for the pharmaceutical chemistry. Refraction is a phenomenon that occurs as
monochromatic light passes from one medium to another and only when there
is a difference in the index of refraction between the two materials. The effects
of refraction are responsible for a variety of familiar phenomena, such as the
apparent bending of an object that is partially submerged in the water and the
mirages observed on a dry, sandy desert. The refraction of visible light is also an
important characteristic of lenses that enables them to focus a beam of light onto
a single point. If a beam of light passes from a rarer medium, e.g. air to a denser
medium, e.g. liquid or solid, it suffers refraction, i.e. change in the direction and
bends towards normal and vice versa.
The refractive index (or index of refraction) of a medium is a measure of how
much the velocity of a wave is reduced inside that medium compared to the first
one. The refractive index of a transparent substance or material is defined as the
relative speed at which light moves through the material with respect to its speed
in a vacuum. By convention, the refractive index of a vacuum is defined as having
a value of 1.0, which serves as a universally accepted reference point. The index of
refraction of other transparent materials is denoted by h, and is defined as
I (Re ) /fractive Index = cv
where c is the speed of light in a vacuum and v is the velocity of light in the
material. Because the refractive index of a vacuum is defined as 1.0, and light
attains its maximum speed in a vacuum (which is devoid of any material), the
refractive index of all other transparent materials exceeds the value of 1.0, and
can be measured by a number of techniques. For most practical purposes, the
refractive index of air (1.0003) is so close to that of a vacuum that it can be
employed to calculate refractive indices of unknown materials. Materials with
higher refractive indices slow the speed of light to a greater degree than those
202
Practical Pharmaceutical Analytical Chemistry
with lower refractive indices. In effect, these materials are said to be more
refractive, and they exhibit a larger angle of refraction for incoming light rays
passing through an air interface (see Table 7.2).
Snell’s law of refraction (or Descartes’ law): Snells’ law is named after the Dutch
mathematician Willebrord Snellius. According to this law “the ratio of the sines of the
angle of incidence, i and that of the angle of the refraction r is constant for the liquid
or solid considered the given colour of light”. This constant is termed as refractive index.
I
Sin
Sin
i
r
(15)
when the angle of incidence, i.e. sin i becomes equal to right angle (90°)
then,
sin i sin 90 1
.
Equation (5) becomes
I
1
Sin r
(16)
Table 7.2 Refractive indices of various substances at 20°C
Substance Refractive index Substance Refractive
index
Vacuum 1.00 Air 1.0002
Benzene 1.501 Carbon tetrachloride 1.461
Ethyl alcohol 1.361 Carbon disulphide 1.628
Water 1.333 Diamond 2.419
Methylene iodide 1.90 Acetone 1.360
Glycerol 1.4729 Butenol 1.399
Cyclohexane 1.4264 Cinnamaldehyde 1.6209
Methanol 1.3312
r
rl
rr
r
q
q
r
rr
Fig. 7.5
203
Properties and Molecular Constitution of Substances
Also expressed as
Sin
Sin
θ
θ
η
η
1
2
1
2
2
1
v
v
(17)
Refraction of light at the interface between two media of different refractive
indices, with h
2
! h
1
. Since the velocity is lower in the second medium (v
2
v
1
),
the angle of refraction q
2
is less than the angle of incidence q
1
; that is, the ray in
the higher index medium is closer to the normal.
The critical angle: When the angle of incidence reaches a certain critical value that
the refracted ray lies along the boundary, having an angle of refraction of 90
o
. This
angle of incidence is known as the critical angle; it is the largest angle of incidence for
which refraction can still occur. For any angle of incidence greater than the critical
angle, light will undergo total internal reflection. So the critical angle is defined as
the angle of incidence which provides an angle of refraction of 90
o
or the angle of
incidence for which the angle of reflection is 90
o
. It must be noted that the critical
angle is an angle of incidence value. When the angle of incidence equals the critical
angle, the angle of refraction is 90°. The angle r in Equation (17) is the critical angle.
The following factors affect the refractive index:
1. temperature,
2. viscosity,
3. wave length.
For a given wavelength, refractive index decreases with increase in temperature
at a rate about 4 u 10
4
per degree increase in temperature and decreases with
increasing wavelength. Therefore, a term specific refractivity was introduced by
Lorentz and is defined as
R =×
η
ηρ
2
2
1
2
1
(18)
where R is the specific refractive index, n is the refractive index observed, r
is the density of liquid. Equation (18) is independents of factors affecting the
refractive index.
Molar refractivity (R
M
) of a compound is a product of specific refractivity and
molecular mass of the substance, i.e.
R =
−
−
×
η
η
Μ
ρ
2
2
1
2
(19)
where M is molecular mass of the substance. Molar refractivity of a compound
depends only on the wavelength and remains constant with temperature. In
Equation (18 ) h is dimensionless; thus molar refractivity is proportional to M/r
204
Practical Pharmaceutical Analytical Chemistry
(the molar volume), and hence the dimensions of molar volume is cm
3
/mol. The
experimental determination of molar refractivity or molar refraction is important in
elucidating the molecular structure. Molar refractivity is an additive and constitutive
property.
Refractivity of solutions: Molar refraction of a homogenous mixture
(solution) is the sum of contributions of its constituents, i.e.
RXRXR
mix
=+
11 22
(20)
where X
1
and X
2
are mole fractions and R
1
and R
2
the molar refractions of
the two components of the solution.
Molar refraction of a solution of A and B can be calculated as
R
soln
=
−
−
×
+η
η
ΝΜ ΝΜ
ρ
2
2
1
2
ΑΑ ΒΒ
(21)
where R
soln
is the molar refraction of solution, h is the observed refractive index,
N
A
, N
B
and M
A
, M
B
are the mole fraction and molar mass of A and B, respectively,
and r
soln
is the density of the solutions.
Refractometric measurements are also useful in the quantitative analysis of
binary solutions. Equation (22) can be used to relate the specific refractivity and
the composition of a solution to those of separate components.
RR
p
R
31 2
100
100
100
=+
−
()
S
(22)
where R
1
, R
2
and R
3
stands for specific refractivities of the components 1, 2
and the mixture respectively, and p is the percentage of component 1.
Table 7.3 Atomic and molar refraction values for the d-line of sodium
CH
2
4.647 cm
3
C O
4.601
H 1.028 –OH 2.546
C 2.591 –COOH 7.226
Cl 5.844
!C C
1.575
–NO
2
6.713
–C∫C–
1.977
6-carbon ring
0.15
4-carbon ring 0.317
5-carbon ring
0.19
3-carbon ring 0.48
205
Properties and Molecular Constitution of Substances
7.2.1 Measurement of Refractive Index
The principle of refractive index measurement is based on the critical angle.
Generally, three instruments have been used in the laboratory to measure the
refractive index, namely
1. Abbe refractometer,
2. Pulfrich refractometer,
3. Immersion re
fractometer.
Abbe refractometer is the most commonly used refractometer as it is economic,
easy to handle, requires small quantity of sample, normal light is used and covers
a wide range for the determination of refractive index.
l
B
r
Cross section of the
instrument
Abbes refractometer
Experiment 4 Determine the refractive indices of given liquid samples
(A and B) by using Abbe’s refractometer and calculate their specific refractions.
Requirement
Sample liquids, Abbe’s refractometer, dropper, specific gravity bottle, cotton,
alcohol, acetone, filter paper.
Procedure
1. Place the instrument in front of the proper light source.
2. Open the prism box (B) and clean the surfaces of prisms using clean swabs
of soft cotton previously impregnated with alcohol and then acetone.
3. Allow the prism to dry and place a drop of water and adjust the instrument.
206
Practical Pharmaceutical Analytical Chemistry
4. Focus the through eye piece on the cross-section of the instrument and
rotate the level (L
1
) until a coloured band or fringe is seen through the
telescope.
5. Record the refractive index to the third decimal, take three readings.
6. Place 2-4 drops of sample liquid A with the help of a dropper on the top of
prism and then press the prisms tightly together.
7. Make sure that the liquid spreads evenly on the surface of prism.
8. Allow the natural light to fall on the mirror (M) and set the mirror to
reflect maximum light into the prism box (B).
9. Circulate water at room temperature through the hose (H) to maintain
the temperature of prism and of the liquid film held between then
constant.
10. Rotate the prism box (B) by moving the lever (L
2
) until the boundary
between the shaded and the bright parts appears in the field of view.
11. If the light shade boundary has a band of colours, makes it sharp by rotating
the compensator knob.
12. Then adjust the lever (L
2
) by its micrometer screw so that the light’ shade
boundary passes exactly through the centre of the cross wires.
13. Read the refractive index of the liquids directly from the scale given on
side reading lenses of the eye piece (E).
14. Take at least three readings from bright to dark and another three readings
from dark to bright fields.
15. Record all the readings and find the average value of all the readings.
16. Determine the density of the liquid at room temperature, using a specific
gravity bottle.
17. Repeat step 6–16 for sample liquid B and determine the density and
specific refractive index.
Precaution
1. The two surfaces of prisms must be cleaned properly.
2. The prism should be allowed to dry before applying the liquid on the
surface of prism.
3. The light should be reflected from the upper surface of the upper prism
and enter the viewer telescope.
Observations and calculations
Liquid used ____
Room temperature ____˚C
Refractive index Sample A 1____ 2____ 3____
Sample B 1____ 2____ 3____
Average value ____