To determine the relative viscosity of given liquid using Ostwald’s viscometer *Presentation Overview* Aim, Apparatus and Chemicals, Theory, Procedure, Observations, Calculations, Result

1. To determine the relative viscosity of given liquid
using Ostwald’s viscometer
Dr. N. G. Telkapalliwar
Associate Professor
Department of Chemistry
Dr. Ambedkar College, Nagpur
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2. Presentation Overview
Aim: To determine the relative viscosity of given liquid using Ostwald’s viscometer
1) Aim
2) Apparatus and Chemicals
3) Theory
4) Procedure
5) Observations
6) Calculations
7) Result
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3. Aim: To determine the relative viscosity of given liquid using Ostwald’s viscometer
Theory
Viscosity is the one of property of liquid. A liquid may be considered to be consisting of
molecular layers arranged one over the other. When a shearing force is applied to a liquid,
it flows. However, the forces of friction between the layers offer resistance to this flow.
Viscosity of a liquid is a measure of its frictional resistance.
Let us examine a liquid flowing on a glass surface. The molecular layer in
contact with the stationary surface has zero velocity. The successive layers above it move
with increasingly higher velocities in the direction of the flow.
Now consider two adjacent moving layers of a liquid. Let these be separated by
a distance dx and have a velocity difference dv.
The force of friction (F) resisting the relative motion of the two layers is
directly proportional to the area A and the velocity difference dv, while it is inversely
proportional to the distance dx between the layers.
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4. Aim: To determine the relative viscosity of given liquid using Ostwald’s viscometer
That is,
Where, η (Greek letter eta) is the proportionality
constant. It is known as the Coefficient of Viscosity or
simply viscosity of a liquid.
η has a specific value for a given liquid at the same
temperature.
Coefficient of Viscosity is defined as the force of
resistance per unit area which will maintain unit
velocity difference between two layers of a liquid at a
unit distance from each other.
η = F
When,
A=1, dv=1, & dx=1
Units of Viscosity
CGS unit of η is g cm–1 s–1. It is called poise (P).
SI unit is kg m–1 s–1.
 One poise is equal to one-tenth of the SI unit
i.e. 1 poise = 1 g cm–1 s–1 = 0.1 kg m–1 s–1
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5. Aim: To determine the relative viscosity of given liquid using Ostwald’s viscometer
Ostwald Viscometer
There are intermolecular forces in liquid layers which are responsible for viscosity.
Viscosity of a liquid can be determined on the basis of flow time. In fact, viscosity is
directly proportional to the flow time. This principle has been used in Ostawald’s
viscometer. It consist of two bulbs joined by a U-tube containing a capillary tube.
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6. Aim: To determine the relative viscosity of given liquid using Ostwald’s viscometer
Apparatus Required
Beaker Ostwald’s viscometer Pipette Stop Watch
The relative viscosity coefficient is calculated from the expression:
Where,
η1 and η2 are the viscosity coefficients of water and given liquid.
t1 and t2 are the flow time of water and given liquid
d1 and d2 are the densities of water and given liquid.
Knowing the value of the viscosity coefficient of water (η1) at the temperature of the
experiment, the absolute viscosity coefficient (η2) of the given liquid can be found.
Formula
Density bottle
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7. Aim: To determine the relative viscosity of given liquid using Ostwald’s viscometer
Procedure
Part-I : Determination of densities of water and given liquid (ethanol)
1. Clean the density bottle with chromic acid solution and then with distilled
water. Rinse with water and ether and then dry.
2. Weigh an empty density bottle (W1).
3. Fill the density bottle with distilled water completely, insert the stopper and
weigh again (W2).
4. Take out the distilled water and rinse the density bottle 2-3 times with given
experimental liquid (ethanol).
5. Fill the given experimental liquid (ethanol) into the density bottle and record
the weight again (W3).
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8. Aim: To determine the relative viscosity of given liquid using Ostwald’s viscometer
Procedure
Part-II : Determination of viscosity
1) Clean the viscometer with chromic acid solution and then with distilled water.
Rinse with water and ether and then dry.
2) Attach the viscometer to the burette stand in exactly vertical position.
3) Fill the distilled water in the lower bulb of the viscometer till the bulb fills
completely.
4) Suck the distilled water from other end
through rubber tube above mark (A).
5) Allow water to flow freely. Start the
stopwatch as soon as the upper layer of
water crosses mark A and stop the
stopwatch as soon as it crosses mark (B).
6) Repeat the recording of flow time three
times for distilled water as well as for the
given experimental liquid (ethanol).
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9. Aim: To determine the relative viscosity of given liquid using Ostwald’s viscometer
Observations and Calculations
Part-I : Determination of densities of water and given liquid (ethanol)
1) Room temperature = …………… OC
2) Weight of empty density bottle= W1= …………. g.
3) Weight of density bottle with water= W2= …………. g.
4) Weight of density bottle with given liquid (ethanol) = W3= …………. g.
5) Volume of density bottle =V= ………. ml
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10. Aim: To determine the relative viscosity of given liquid using Ostwald’s viscometer
Observations and Calculations
Part-II : Determination relative viscosity and absolute viscosity of ethanol
Sr.
No.
Flow Time (Sec) Relative Viscosity Absolute Viscosity
For
Water
Mean
(t1)
For
Ethanol
Mean
(t1)
1 --
--
--
--
2 -- -- -- --
3 -- --
Result: 1) Relative viscosity of given liquid (ethanol) relative to water was found to be ……….
2) Absolute viscosity of given liquid (ethanol) was found to be ………… Poise
Viscosity of water at room temperature = η1 = ……… Poise
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11. References
• Essentials of Physical Chemistry, S. Chand Publication by Arun Bahl, B.S. Bahl, G. D. Tuli.
• University Chemistry, Volume I, Alliance & Co., by R. M. Jugade.
• A Text Book of Chemistry, B.sc. SEM- I, Himalaya Publishing House.
The contents, materials, images, etc. used in this video/presentation are
taken from open sources (Internet and Books) for educational purposes
only and not meant for any commercial distribution.
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