Physical%20Pharmacy%20VISCOSITY

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Physical PharmacyVISCOSITY

• By
• Abdul-Wahhab Khedr
• Demonstrator of Pharmaceutics and Industrial
  Pharmacy

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Definition

• Viscosity (?) 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
• 1 Poise 100 centipoise (cps)

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Determination 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 (?1)
  can be determined in relation to another liquid
  of known viscosity (?2).

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Determination of Viscosity of liquids

• Theory
• The viscosity of the liquid under test (?1) 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 (?2) usually
  water to pass between the two marks.
• If the two liquids have the same or close
  densities, they can be neglected.

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Procedure
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(1) Pour water into the wide arm
(2) Suck water from the narrow arm till rise to
the upper mark
(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)
(6) Repeat the previous steps for the liquid to
be measured and fill the table of results
(3) Close the tip of narrow arm with your finger
and invert the viscometer to empty the wide one
t1 10 sec
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Results
? (cps) Mean t t3 t2 t1 Sample
1 Water
Sample 1
Sample 2
Sample 3
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