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