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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5986_Библиотеки_им_академика_М_И_Перельмана.pdf
177
Properties of Liquids
Take out the stalagmometer from the bottle. Fill it by immersing the lower end
into distilled water and sucking the water above mark A. Close the pinch cock and
insert the stalagmometer in the weighing bottle. Now clamp the stalagmometer
in a thermostat. Allow the stalagmometer to acquire the temperature of the bath.
Count the number of drops obtained from a volume of water between the marks
A and B. Record it and repeat the procedure three times and see that the drop
numbers do not vary by more than 0.3 – 0.5 of a drop.
Remove the stalagmometer from the weighing bottle, and thoroughly dry
it. Fill it with alcohol and reset the stalagmometer in the weighing bottle as
for water. Replace the whole assembly in the thermostat. Count the number
of drops for the volume of alcohol between the marks A and B. Repeat the
procedure three times and record the observations. Now, determine the density
of alcohol.
Note
For the measurement of surface tension at room temperature, use of a thermostat
is not essential.
Precautions
1. Washing of stalagmometer should be done thoroughly, so that there may
not remain any trace of grease in the capillary, particularly on its end.
2. After washing and drying, great care must be taken to ensure that
the tip of the stalagmometer does not come in contact with hands or
working table. Keep it covered with a clean dry test tube. Even a slight
trace of grease at the tip will very much alter the size of the drops.
3. The rate of dropping should be less than 15 drops per minute. Drops
should be allowed to fall only under the weight force and should not
be pushed away. It is best to have one drop per minute but this rate
is too slow to be experimentally convenient.
4. While counting the drops, the apparatus should not be disturbed;
disturbance may cause the drops to fall, before they develop to their
full size.
5. Errors may arise due to incomplete contact with the ground surface
of the capillary tip. This can be easily avoided by attaching below the
capillary tip a glass or metal nipple with sharp edge.
6. For determination of surface tension of a volatile liquid drop forma-
tion should be near the liquid surface in the receiving vessel and ves-
sel should have narrow mouth to guard against air currents to avoid
the evaporation from the surface of liquid drop.
178
Practical Pharmaceutical Analytical Chemistry
Observation
Temperature of measurement
t°C
Density of water at t°C
d
w
(from tables)
Surface tension of water at t°C
g
w
(from tables)
Mass of empty specific gravity bottle m
1
g
Mass of specific gravity bottle alcohol m
2
g
Mass of specific gravity bottle water m
3
g
Calculations
Density of alcohol d
a
{(m
2
m
1
)/(m
3
m
1
)} u d
w
Relative surface tension of alcohol g
a
/g
w
(d
a
/d
w
) u (n
w
/n
a
)
Absolute surface tension of alcohol g
a
g
w
[(d
a
/d
w
) u (n
w
/n
a
)]
Result The surface tension of methanol wat found to be ______ (absolute ST)
and ______ (relative ST).
Experiment 2 Determine the percentage of methanol in a solution of it in
water by surface tension measurement.
Liquid No. of drops from a fixed volume
Water
1
2
3
Average n
w
Alcohol
1
2
3
Average n
a
Requirements
Traube’s stalagmometer, a wide mouth weighing bottle, rubber stopper, a small
piece of rubber tube, a screw pinch cock.
Procedure
Methanol and water are miscible in all proportions. A series of known mixtures
of methanol and water are prepared by mixing in the following proportions:
179
Properties of Liquids
Methanol (mL) 25 20 15 10 5 0
Water (mL) 0 5 10 15 20 25
Measure the number of drops formed in each of these mixtures and of
unknown mixture. Calculate surface tension and plot a graph between surface
tension on Y- axis and percent composition on X-axis, the composition of
unknown mixture is read off from the graph by intrapolation. The result can
also be obtained by plotting graph between the number of drops on Y- axis and
percent composition on X- axis.
Result The sample of methanol in water was found to contain _____% of
methanol by surface tension method.
Experiment 3 Determine molar surface energy and the association factor
for ethyl alcohol.
Requirements
Traube’s stalagmometer, a wide mouth weighing bottle, rubber stopper, a small
piece of rubber tube, a screw pinch cock.
Procedure
Determine the surface tension and density of ethyl alcohol at 25˚C and 45˚C
using a stalagmometer as in Experiment 1 and record the observations as given
below.
Theory
The molar surface energy at a given temperature is defined by g (M/d)
2/3
where
g is the surface tension, M is the molar mass of the liquid and d is the density of
the liquid at a given temperature. The molar surface energy of any liquid at a
given temperature may thus be obtained by determining the surface tension and
the density and using the above formula.
The association factor x, for any associated liquid is given by the expression
x (2.12/k)
3/2
(24)
where k is a constant value of which is given by the expression
k {g
1
(M/d
1
)
2/3
g
2
(M/d
2
)
2/3
}/{T
2
T
1
} (25)
From this equation it is concluded that the association factor of a liquid
requires the determination of its surface tension and density at two different
temperatures.
180
Practical Pharmaceutical Analytical Chemistry
Observation
Temperature Surface tension Density
25˚C
g
1
d
1
45˚C
g
2
d
2
Experiment 4 Determine the surface tension of methyl acetate, ethyl acetate,
n-heptane, chloroform, and ethylene bromide and calculate the atomic parachor
of C, H, Cl, Br.
Requirements
Methyl acetate, ethyl acetate, n-heptane, chloroform, ethylene bromide and
equipments as in Experiment 1.
Theory
Parachor of a compound is additive as well as constitutive property. The
parachor value can be calculated with the help of known values as given in
Table 6.1.
P(CH
3
COOHCH
2
CH
3
) P(CH
3
COOHCH
3
) P(CH
2
)
P(CH
3
(CH
2
)
5
CH
3
) 7P(CH
2
) 2P(H)
P(CH
2
) 2P(H) P(C)
P (CHCl
3
) P(C) P(H) 3P(Cl)
P (C
2
H
4
Br
2
) 2P(C) 2P(H) 2P(Br)
Procedure
1. Determine the surface tension of each of the liquid at room temperature.
2. Determine density of each liquid at the same temperature using
pyknometer.
3. Calculate parachor value of the liquids, and compute the parachor equiva-
lents of various atoms as discussed above.
Calculations
From the above observations calculate the molar surface energies and the value
of K and then by using Equation (11), evaluate the value of x.
Result The molar surface energy of ethyl alcohol was found to be _____ and
the association factor was __________.
181
Properties of Liquids
Observation
Liquid Surface tension Density
Methyl acetate
Ethyl acetate
n-heptane
Chloroform
Ethylene bromide
Result The surface tension of methyl acetate, ethyl acetate, n-heptane, chlo-
roform and ethylene bromide was found to be _______, _______, _______,
_______ and _______ and parachor value for C, H, Cl and Br was found to be
_______, _______, _______, _______ and _______ respectively.
6.2 Viscosity
Viscosity is a representation of nature the fluids (gases and liquid), i.e. of the
resistance of fluid to flow. Viscosity measures the resistance to flow offered by a
layer of liquid when another adjacent and parallel fluid layer tends to flow past
the first layer. Higher the viscosity greater is the resistance.
Units and dimensions of viscosity: Units in CGS are poise and centipoise
(10
2
poise), millipoise (10
3
poise). Dimensions are M/L/T.
Coefficient of viscosity: Coefficient of viscosity of the liquid at a given
temperature is the force required per unit area of the layer to maintain a unit
difference in viscosity between two parallel planes of the liquid one unit dis-
tance apart. Commonly coefficient of viscosity is known as viscosity and is rep-
resented as h.
Molecular viscosity: Molecular viscosity is the product of viscosity and the
molar surface and is independent of the temperature.
Molecular viscosity viscosity u molar surface h u (M/d)
2/3
(26)
where M is the molecular weight and d is the density of liquid.
Measurement of viscosity of the liquid is based on
1. Poisuilles equation (for laminar flow in a horizontal capillary tube).
2. Stoke’s law equation (for movement of small spherical particle like ball, a
liquid drop or gas bubble through a liquid column).
182
Practical Pharmaceutical Analytical Chemistry
Poisuilles equation
V S/8 ptr
4
/lh
where V is the volume of liquid flowing out if the capillary in time t, p is the
constant vertical pressure for maintaining constant flow, r is the radius of the
capillary, l is the length of the capillary, and h is the viscosity.
Stoke’s law equation
u f/6phr (28)
where u is the constant of velocity of ball or rise of spherical through the column,
f is the force responsible for the motion of the sphere, r is the radius of the sphere,
and h is the viscosity.
The following factors affect the viscosity:
1. Molecular weight: If the molecular weight is increased using similar cat-
egory of the substance viscosity increases.
2. Branching of organic compounds: Most of the branched chain organic com-
pounds are more viscous then straight chain compounds.
3. Impurities: Impure substances exhibit more viscosity due to presence of
solids, colloidal or other substances.
4. Polarity: Polar compounds have less viscosity than nonpolar compounds.
5. Temperature: Arrhenius equation is used to relate the temperature and
viscosity by following equation:
h Ae
E/RT
(29)
or In
h In A E/RT (30)
or 2.303log
h 2.303log A E/RT (31)
or log
h log A E/2.303RT (32)
where A is the constant and E is the activation energy for viscous flow or energy
required by a mole of the liquid to allow its molecule to overcome forces resisting
the flow of the liquid.
According to the above equation temperature is inversely proportional to the
viscosity, i.e. when temperature increases viscosity decreases.
6.2.1 Determination of Viscosity
A variety of viscometers are available commercially to determine the viscosity.
Therefore depending upon the nature of a modified solution, a viscometer
should be selected. Viscometers are based on the following three basic principles:
1. Measurement of rate of flow of liquid through an orifice or a duct of simple
geometry.
2. Resistance to rotation of a metallic body in contact with or immersed
within the liquid.
183
Properties of Liquids
3. Viscosity of a metal sphere rolling or falling through the liquid under the
effect of gravity or of an air bubble rising because of the limited use.
6.2.2 Types of Viscometers
1. Capillary viscometer
2. Falling sphere viscometer
3. Cup and bob viscometer
4. Cone and plate viscometer
5. Brookefield synchrolectric
6. Stromer viscometer
7. Coaxial cylinder viscometer
8. Mac Michael viscometer
9. Couette viscometer
10. Haake Rotovisco viscometer
Capillary viscometer: The glass capillary Ostwald, Cannon–Fenske,
Ubbelohde viscometer are the most commonly used apparatuses based on
the first principle of the flow of liquid. Ostwald viscometer is the most com-
monly used for comparing viscosities of different liquids. Ostwald viscom-
Fig. 6.4
Ostwald viscometer
E
c
A
1
2
B
D
F
G
eter consists of a cylindrical capillary tube and the driving force causing the
liquid to flow through it is weight.
It is based on the Poisuelles equation, i.e.
v/t = Sr
4
p/8lh (33)
where v/t is the rate of flow of liquid, p is the given by height of column h, density
of liquid d and gravity g.
184
Practical Pharmaceutical Analytical Chemistry
So p = hdg (34)
Therefore, v/t = pr
2
hdg/8lh (35)
or
h = Sr
4
hdgt/8lv (36)
if the instrument is constant k Sr
4
hg/8lv (37)
Then h ktd (38)
To determine the viscosities it is not necessary to evaluate k. If two liquids A
and B of viscosities h
1
and h
2
and the density d
1
and d
2
are allowed to flow
through the capillary viscometer, the time taken by them to flow between two
fixed two pints is t
1
and t
2
, respectively then their viscosities can be expressed
by equation
h
1
kt
1
d
1
(39)
h
2
kt
2
d
2
(40)
By dividing Equation (39) with Equation (40) we get
h
1
/h
2
t
1
d
1
/t
2
d
2
(41)
To calculate viscosity of any liquid, Equation (41) can be used as given below
Ș
1
h
2
¥ (t
1
d
1
/t
2
d
2
) (42)
Ostwald viscometer (Fig. 6.4) consists of fine capillary tube AB of about 10 cm
long and 0.4 mm in diameter. A bulb C attached to it at its upper end and a
U tube and a bulb E at the lower end. There are two marks 1 and 2 above and
below bulb C.
Precautions
1. The viscometer should be completely cleaned using chromic acid solution
and then with any organic solvent like methanol or acetone.
2. For accurate results readings must be obtained in triplicates.
Rheochor
Newton Friend showed that the product of molar volume and eighth root of
coefficient of viscosity is constant and independent of the temperature and is
termed as rheochor. Rheochor is both additive and constitutive property, i.e. it
changes with molecular structure.
Rheochor M/d u h
1/8
(43)
185
Properties of Liquids
Viscosities of various liquids at various temperatures
Liquid Temperature
(°C)
Viscosity
Ș (mPa·s)
Liquid Temperature
(°C)
Viscosity
Ș (mPa·s)
Aniline 20 4.40 Ethylene glycol 100 1.98
Benzene 20 6.49 Glycerin 20 1420.00
Chloroform 20 5.64 Glycerin 40 280.00
Ether 20 2.34 Mercury 15 1.55
Ethylalcohol 20 1.10 Water 0 1.79
Isopropyl alcohol 20 2.40 Water 20 1.00
Methyl alcohol 20 0.59 Water 40 0.65
Ethylene glycol 25 16.10 Water 100 0.28
Experiment 5 Determine relative and absolute viscosities of sample liquid.
Requirements
Ostwald viscometer, stopwatch, thermostat, specific gravity bottle, 10-ml pipette,
methanol and distilled water
Procedure
1. Clean the viscometer with chromic acid (potassium dichromate in conc.
H
2
SO
4
) and then with distilled, methanol or acetone.
2. Dry the viscometer by setting it vertically in the thermostat. Fit the vis-
cometer on the burette to stand vertically.
3. Attach a peace of clean rubber tubing to the top of F.
4. Introduce a definite volume of water (5 mL) through G (Fig. 6.3) into the
large bulb E. Suck the liquid into the bulb C with the aid of rubber tube
attached at F orifice till the liquid reach above the mark 1. Care must be
taken that the U bend at the bottom of the liquid is filled with the liquid.
5. Start the stopwatch as the liquid reaches the mark 1. Allow the liquid to
flow freely under gravity and note the time taken to flow between points
1 and 2.
6. Repeat the determination until the two readings obtained differ by
0.2
–0.5 s on 100 s high deviation indicates that solution is not clear and/or
viscometer is not clean.
7. Empty the viscometer, rinse it with acetone and dry it.
8. Clamp again the viscometer in the vertical plane in the thermostat
bath.
9. Fill the viscometer with exactly equal volume of test liquid (methanol) and
determine the flow time as before.
186
Practical Pharmaceutical Analytical Chemistry
10. Determine the specific gravity of the liquid at the same temperature with
the help of pychnometer.
11. Record the temperature of the bath.
Observation and calculation
1. Temperature of the thermostat at room temperature _______°C
2. Viscosity of water at room temperature _______
3. Density of water at ____°C
a. Weight of the empty pychnometer w
1
g
b. Weight of the pychnometer water w
2
g
Density of water at ________ °C w
2
w
1
/v
4. Similarly determine the density of the test sample (10 mL) using pych-
nometer.
Observation
Liquid Time of flow (in s) Average
Water
Methanol
Relative viscosity of methanol:
Ș
1
/Ș
2
d
1
t
1
/d
2
t
2
(44)
Absolute viscosity of methanol:
Ș
1
Ș
2
(d
1
t
1
/d
2
t
2
) (45)
where
Ș
1
is the viscosity of the unknown sample, Ș
2
is the viscosity of water, d
1
is the density of the unknown sample, d
2
is the density of water, t
1
is the time of
flow of the unknown sample between two fixed points, t
2
is the time of flow of
the water between two fixed points.
Result The absolute and relative viscosity of methanol against water as refer-
ence was found to be _________ and ______ respectively.