Post on 22-Dec-2015
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Physical Pharmacy
VISCOSITYVISCOSITYBy
Abdul-Wahhab KhedrDemonstrator of Pharmaceutics and Industrial
Pharmacy
Definition:Definition:• Viscosity (ƞ): Viscosity (ƞ): is the resistance of a fluid to flow.• A fluid with high viscosity flows slowly while a fluid
with low viscosity flows quickly.
• Units of Viscosity: Units of Viscosity: 1 Poise = 100 centipoise (cps)
Determination of Viscosity of liquidsDetermination of Viscosity of liquids• Using Ostwald Viscometer (Capillary-type
viscometer): - It is U-shaped glass apparatus with a wide arm and
a narrow one with a capillary tube in the narrow part.
- There are two marks; above and below the bulb in the narrow part.
- In this method the viscosity of a liquid (ƞ1) can be determined in relation to another liquid of known viscosity (ƞ2).
Determination of Viscosity of liquidsDetermination of Viscosity of liquids• Theory:Theory:• The viscosity of the liquid under test (ƞ1) can be
found by measuring the time (t1) required for this liquid to pass between the two marks as it flows by gravity through the vertical capillary tube.
• This time is compared with the time (t2) required for a liquid of known viscosity (ƞ2) [usually water] to pass between the two marks.
• If the two liquids have the same or close densities, they can be neglected.
22
11
2
1
.
.
t
t
2
1
2
1
t
t
Procedure:Procedure: 00:0100:0100:0300:0300:0700:0700:0600:0600:0400:0400:0500:0500:0200:0200:0800:0800:1000:1000:0900:09
tt11= 10 sec= 10 sec
(1) Pour water into the wide arm(2) Suck water from the narrow arm till rise to the upper mark
(3) Close the tip of narrow arm with your finger and invert the viscometer to empty
the wide one
(4) Record the time taken by water to move from the upper to
the lower mark
(5) Repeat the last step twice again and find t2 & t3
Then calculate Mean time (t)
00:0000:00
(6) Repeat the previous steps for the liquid to be measured
and fill the table of results
ResultsResults
Samplet1t2t3Mean tȠ (cps)
Water1
Sample 1
Sample 2
Sample 3
water
sample
water
sample
t
t