PHM2213 practical
533576098
PHM 2213 Physical Pharmacy 2
Measurements of Viscosity of Fluids by Using a Capillary Viscometer
NOTE
Students will be divided into groups. Although experimental work will be carried out
in a group, reports shall be submitted individually.
Introduction
The internal friction of liquids, due to intermolecular attractions, is known as
viscosity. In a flowing liquid each layer of molecules exerts a drag on the next, and to
cause the liquid to flow, work must be done to push the layers past one another.
Newton showed that the applied force, F was proportional to A and to du/dx ; the
proportionality constant being the coefficient of viscosity, η, hence,
or
F/A= η du/dx
where F/A is the shearing stress (force/area), du/dx is the strain (rate of shear, velocity
gradient).
Liquid which obey this equation when flowing are said to give stream-lined or
Newtonian flow. Very rapid flow can cause Newtonian to become turbulent flow, in
which the energy causing the liquid to flow is no longer used exclusively to slide to
planes of molecules over one another, but part is dissipated as eddies and turbulence.
Units of Viscosity
The normal c.g.s. unit of viscosity is the poise (g.cm -l sec-l) e.g. water at 20ο C =
0.01002 poise = 1.002 centipose. Kinematic viscosity, v, is absolute viscosity divided
by density, and the units are stokes and centistokes. For water at 20° v = 0.01004 =
1.004 centistokes. Another term sometimes used is the fluidity, which is the reciprocal
of the absolute viscosity.
Source: Dr. Tanveer Ahmad Khan
Prepared by: Kausar Ahmad
Date updated: 23-Dec-2005
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PHM2213 practical
533576098
Measurement by a Capillary Viscometer
Poiseuille's law can be written
…………………………………………… (1)
Where V = total volume of liquid (ml) of viscosity η (poise), flowing time ‘t’ (second) under pressure
difference ‘P’, down a capillary tube at length ‘l’ (cm) and radius ‘r’ (cm).
For a liquid flowing under its own head, we may express the pressure in term of density,
ρ, acceleration due to gravity ‘g’ and height ‘h’; remembering ‘V’ was defined per unit
time, equation (1) can be rewritten as:
P = h ρg
…………………………………………………(2)
The Ostwald’s viscometer has the form of U-tube with bulbs at A (B) and C, a capillary
tube and marks at E and F. Liquid is forced up bulb C to above mark E, and the time
taken for the meniscus to fall from E to F noted.
Source: Dr. Tanveer Ahmad Khan
Prepared by: Kausar Ahmad
Date updated: 23-Dec-2005
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PHM2213 practical
533576098
If a liquid of known viscosity is compared with another (2) of unknown viscosity, in the
same apparatus under identical experimental conditions, then:Known liquid :
η1 = K ρ1 t1
Unknown liquid :
η2 = K ρ2 t2
η2 =
η1 ρ2 t2 ……………………………(3)
ρ1 t1
The constant for a particular viscometer need not to be determined. To obtain readily
measured flow times, viscometers with narrow capillary tubes are used for liquids of
low viscosity, while wider capillary tubes are used in viscometers for high viscosity
liquids.
Materials
1.
2.
3.
4.
Size-C-Ostwald’s viscometer
60% w/w sucrose solution
Glycerin
Water bath w i t h a thermostat
Method
Clean and dry a size-C-Ostwald’s viscometer. Set the viscometer upright; using a plumb
line in a thermostat set at 37.8ο. Fill the viscometer through tube ‘V ’ with 60% w/w
sucrose solution to mark F. After the apparatus has come to temperature, adjust the
liquid level exactly to this mark. Slip a short length of clean rubber tubing on to W,
and suck until the liquid rises above mark at E. Place the finger on the end of the tube
to prevent the liquid level falling while removing the rubber tubing. Allow the liquid
level to fall, timing between marks E and F. Repeat until consistent results are
obtained. Remove the viscometer from the bath, wash it out, dry it, and repeat the
procedure using the sample of glycerin to be tested.
Treatment of Results
1. Using the viscosities and densities of 60% w/w sucrose solution given, plot a
graph a log viscosity against 1/T (absolute) and determine the viscosity at
37.8οC. Similarly determine the density at this temperature from a graph at ρ
against temperature.
2. Calculate the viscosity of the glycerin in centipoises from equation (3). Take
ρ237.8 of glycerin solution as 1.142 g / m l .
3. Calculate the kinematic viscosity of the sucrose solution, and the kinematic
viscosity of the glycerin solution.
Source: Dr. Tanveer Ahmad Khan
Prepared by: Kausar Ahmad
Date updated: 23-Dec-2005
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PHM2213 practical
533576098
Include the following in your report:
1. Why it is essential to have a suitable viscosity in a suspension preparation (refer
your answer to the stability and flow ability of the preparation).
2. List four suspensions from B.P. or B.P.C. and name the thickening agent(s) used
in the preparations to increase the viscosity of the preparations.
3. Compare the accuracy of the above methods with theoretical viscosity of glycerin
at 37.8οC.
4. Comment on the merit and limitation of each method.
References
1. E.L. Parrot and W. Saski, Experimental Pharmaceutics, 4th Ed., p. 2055, Burgess
Publishing Co, (1977).
2. A.H. Beckett and J.B. Stenlake; Practical Pharmaceutical Chemistry, 2nd Ed. Part
Two. p. 24, Athlone Press (1970).
3. Martin, A.N. Physical pharmacy: Physical chemical principles in the
pharmaceutical sciences, 4th Ed. 1993. Lippincott Williams & Wilkins.
4. Rabek, J.F. Experimental Methods in Polymer Chemistry, Physical Principles and
Applications. P.123-141, 1980. New York: John Wiley and Sons.
Source: Dr. Tanveer Ahmad Khan
Prepared by: Kausar Ahmad
Date updated: 23-Dec-2005
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