Experiment or Lab Report

1. 1
Lab Report No: 02
VISCOMETRY
Applied Chemistry ІІ (Lab)
Submitted by: Muhammad Zakaria.
Registration No: 16PWCHE1111.
Submitted to: Dr. Najeeb Ullah.
Date of Submission: 28, February, 2017
DEPARTMENT OF CHEMICAL ENGINEERING
UNIVERSITY OF ENGINEERING TECHNOLOGY, PESHAWAR

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VISCOSITY:
The resistance of a fluid to its flow is known as viscosity.
Equation for viscous force due to flow of liquid is given as;
Fv = Ƞ
𝐴𝑣
𝑑
Where Ƞ is called coefficient of viscosity. It is given as;
Ƞ =
𝐹𝑑
𝐴𝑣
=
𝑁𝑚𝑠
𝑚2 𝑚
Units:
Unit of viscosity is Nm-2
s or Pa.s, other unit for viscosity is poise and;
10 Poise = 1 Pa.s
Coefficient of viscosity of water at 0℃ is 1.8 mPa.s or 1.8 cP and that of blood
is 3-4 mPa.s or cP.
EFFECT OF PRESSURE AND TEMPERATURE ON VISCOSITY:
Viscosity of fluids varies with pressure and temperature. For most fluids
the viscosity is rather sensitive to changes in temperature, but relatively
insensitive to pressure until rather high pressures have been attained. The
viscosity of liquids usually rises with pressure at constant temperature. Water is
an exception to this rule, its viscosity decreases with increasing pressure at
constant temperature. For most cases of practical interest, however, the effect of
pressure on the viscosity of liquids can be ignored. Temperature has different
effects on viscosity of liquids and gases. A decrease in temperature causes the
viscosity of a liquid to rise. Effect of molecular weight on the viscosity of
liquids is that the liquid viscosity increases with increasing molecular weight.
NEWTONIAN FLUID:
The fluid whose coefficient of viscosity does depends upon the speed
with which it is flowing is known as Newtonian fluid.
NON-NEWTONIAN FLUID:
The fluid whose coefficient of viscosity does not depends upon the
speed with which it is flowing is known as non-Newtonian fluid.
POISEUILLE’S LAW:
Consider a fluid flowing through a stationary tube. Then according to
Pioseuille’s law, volumetric flow rate of the fluid is given as;

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𝑉
𝑡
=
∆𝑃𝜋𝑅4
8ƞ𝐿
Where ∆P = P1 – P2, R = radius of the tube, Ƞ = viscosity of the fluid, L = length
of the tube.
(Note: This law can be used only for Newtonian fluid.)
TYPES OF VISCOMETERS:
 Redwood viscometer.
 Engler viscometer.
 Saybolt viscometer.
 Ostwald viscometer.
 Falling ball viscometer.
OSTWALD VISCOMETRY:
It is simple method for the measurement of viscosity in which viscosity of
a liquid is measured by comparing the viscosity of an unknown liquid with that
of the known liquid. In this method, viscosity of a liquid is measured by
comparing the flow times of two liquids of equal volume using Ostwald
viscometer. Usually the reference liquid is water.
When vertical viscometer is used then pressure will be applied on a liquid
due to gravitational force i.e. P = 𝜌𝑔ℎ
As two liquids are used in viscometer. So for unknown liquid, the
coefficient of viscosity is given as;
Ƞl =
ℎ𝑔𝜋𝑟4
8𝑉𝑙
× dltl
The second liquid is usually water, whose coefficient of viscosity is given
as;
Ƞw =
ℎ𝑔𝜋𝑟4
8𝑉𝑙
× dwtw
Ƞ𝑙
Ƞw
=
ℎ𝑔𝜋𝑟4
8𝑉𝑙
× 𝑑𝑙𝑡𝑙
ℎ𝑔𝜋𝑟4
8𝑉𝑙
× dwtw
Ƞ𝑙
Ƞw
=
𝑑 𝑙 𝑡 𝑙
𝑑 𝑤 𝑡 𝑤
Where Ƞw = viscosity of water, tw = time taken by water, dw = density of water,
Ƞl = viscosity of liquid, tl = time taken by liquid, dl = density of liquid.

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EXPERIMENT:
TO DETERMINE THE VICOSITY OF A LIQUID USING
OSTWALD VISCOMETER.
APPARATUS:
Ostwald viscometer, 10ml Pipette, Stop watch, Analytical balance.
PROCEDORE:
 First of all, wash and dry viscometer and other apparatus.
 Take empty gravity bottle and weight it.
 Fill the gravity bottle with water and then with ethanol.
 Weight the filled gravity bottle and calculate densities of water and
ethanol.
 Take 20ml water and put it in viscometer, suck water up to the upper
mark and then note time (tw) taken by water to reach the lower mark of
viscometer.
 Take 20ml of ethanol in viscometer and note the time (tl) taken as for
water.
 Use the following formula to calculate the viscosity of ethanol.
Ƞl =
𝑑 𝑙 𝑡 𝑙
𝑑 𝑤 𝑡 𝑤
× Ƞw
Fig: Ostwald viscometer.

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CALCULATIONS:
For density of ethanol (dl):
Mass of empty gravity bottle (m1) = 17.9g
Mass of filled gravity bottle (m2) = 60.08g
Mass of ethanol (m) = m2-m1 = 42.18g
Volume of ethanol (v) = 50ml
Density of ethanol (dl) = m/v = 0.8404g/ml
For density of water (dw):
Mass of empty gravity bottle (m1) = 18.25g
Mass of filled gravity bottle (m2) = 69.85g
Mass of water (m) = m2-m1 = 51.6g
Volume of water (v) = 50ml
Density of water (dw) = m/v = 1.032g/ml
Time taken by ethanol (tl) = 131.55s Time taken by water (tw) = 57.5s
Ƞw (20℃) = 1.003 N.s/m2
Now viscosity of ethanol:
Ƞl =
𝑑 𝑙 𝑡 𝑙
𝑑 𝑤 𝑡 𝑤
× Ƞw
Ƞl =
0.8404×131.5×1.003
1.032×57.5
Ƞl = 1.867 N.s/m2
PRECATIONS:
 All readings must be taken carefully.
 Same viscometer must be used for both liquids.
 Temperature must be constant during experiment.