1. Physical Pharmacy SURFACE TENSIONBy
Abdul-Wahhab Khedr
Demonstrator of Pharmaceutics and Industrial Pharmacy

2. Surface tension of liquids
Why the free drops of water form spherical droplets?

3. Surface Tension:
Equals the work (W) or energy necessary to increase the surface area (A) by 1 cm2
Units:
Is defined as “ The force in (dynes) acting along the surface of a liquid at right angle to any line 1 cm in length”.

4. Experimental Determination of Surface Tension
Drop weight and Drop number method:
(Using Stalagmometer)
In this method the surface tension of a liquid (γ1) can be determined in relation to another liquid of known surface tension (γ2). a b

5. Experiment Procedure:
By using Stalagmometer, determine the relative surface tension of the given liquids with reference to distilled water (S.T=72.8 dyne/cm at 20⁰C)
Procedure:
Wash the stalagmometer several times with water before use.
Introduce the flat tip into small beaker half-filled with water, suck water till its level becomes above the upper mark (avoid entrance of air bubbles).
Adjust the level of water to the upper mark and remove any suspended drop.
Count the number of drops falling until the level of water reaches the lower mark (n1).
Repeat the procedure twice again and determine (n2) & (n3).
Repeat the procedure for the liquid to be measured after washing the apparatus.

6. Surface tension (dyne/cm)
Results
Surface tension (dyne/cm)
Number of Drops
Sample
Mean (n) n3 n2 n1
72.8
Water
Sample 1
Sample 2
Sample 3

7. Comment
The drop of a mass (m) gets released when its weight G=mg is equal or greater than the surface force at the end of tube

8. Comment
There are two line marks on the stalagmometer: top line above the wide part and bottom line bellow it. The volume between these two lines is (V), and liquid with density (ρ) contained in this volume has a mass (M)
Such a volume V corresponds to n drops, which are released from the stalgmometer upon the decrease of liquid level from top to bottom line mark.
Here, the average mass (m) of one drop is