MEASUREMENT OF SURFACE AND INTERFACIAL TENSION Capillary Rise Method, Drop Count and Weight Method. Wilhelmy Plate Methods ,The DuNouy Ring Method. Capillary Rise Method: Upward force due to surface tension: Drop count and Weight method Downward Force: Drop weight method: Drop count method

1. IIIrd Semesester B. pharmacy
Physical Pharmaceutics-I
Unit-III
Surface and Interfacial tension [Part-3(a)]
(Measurement of Surface and Interfacial tension: 1.
Capillary Rise Method 2. Drop count and Weight method )
Miss. Pooja D. Bhandare
(Assistant professor)
Kandhar college of pharmacy

2.  MEASUREMENT OF SURFACE AND
INTERFACIAL TENSION
1. Capillary Rise Method.
2. Drop Count and Weight Method.
3. Wilhelmy Plate Methods
4. The DuNouy Ring Method.

3. 1. Capillary Rise Method
• When a capillary tube is placed in a liquid contained in a beaker, the liquid
generally rises up the tube to a certain distance.
• Because the force of adhesion between the liquid molecule and the capillary
wall is greater than the cohesion between the liquid molecules, the liquid is said
to wet the capillary wall, spreading over it and rising in the tube.
• By measuring this rise in a capillary, it is possible to determine the surface
tension of the liquid.
• It is not possible, however to obtain interfacial tension using the capillary rise
method.

4. • Upward force due to surface tension:
Surface tension at any point of
circumference of capillary tube = γcosθ
• Total upward force = 2пr γ cosθ
Where, θ = Contact angle between the
surface of the liquid and the capillary wall
(degree)
r = inside radius of capillary
For liquid completely wets the capillary, θ=0
thus, cosθ = 1

5. • Downward Force: countering force due to weight of the liquid column.
• Downward force = mass X acceleration = volume X density X acceleration
= cross sectional area X height X density X acceleration
= п𝑟2hpg
• At equilibrium,
Upward force = Downward Force
2пr γ = п𝑟2hpg
γ =
𝒓𝒉𝒑𝒈
𝟐

6. Drop count and Weight method
Principle:
• Surface tension measures the strength of the cohesive forces of liquids.
• The lower the surface tension of the liquid, the smaller the size of drops formed.
• Then more number of drops is formed for the given volume of the liquid when compared
to water.
• Therefore simply counting the number of drops for unknow liquid and water is sufficient
to calculate the surface tension.
• This is applicable when the density of liquid are same, as the falling of drops method can
be used.

7. • Apparatus: Stalagmometer is used.
• It consist of a glass tube with a bulb
blown approximately in the middle of
the tube.
• It is clamped vertically and sample
liquid is to be sucked into it up to the
mark A.
• Liquid is then allowed to drop slowly
from the tip of the pipette.

8. Drop weight method:
• 20-30 drops are collected into a clean, tarred vessel for sample liquid as
well as for water.
• Weight of one drop is determined (𝑊1).
• Surface tension of liquid is then given by: W = 2пrγ
• Similarly water is taken in pipette and weight of one drop water (𝑊2) is
obtained.
• The ratio of the weight of a drop of the liquid (𝑊1) to the reference
substance (𝑊2) falling the same capillary orifice is equal to the ratio of their
surface tension

9. • If γ1 and γ2 are the surface tension of experimental liquid and reference standard
respectively then
• The weight of the drop in mg of a test liquid 𝑊1 = 2пr γ1
• The weight of the drop in mg of a reference liquid 𝑊2 = 2пr γ2
• As the same apparatus is used for both the liquids the correction factor is same
assume that the drop volumes are not different.
• Hence
𝑊1
𝑊2
=
2пr γ1
2пr γ2
Rearranging the equation
𝜸 𝟏
𝜸 𝟐
=
𝑾 𝟏
𝑾 𝟐

10. Drop count method
• Similar to drop weight method except that number of drops formed when the
liquid level falls from mark A to B is counted instead of weighing.
𝜸 𝟏
𝜸 𝟐
=
𝑾 𝟏
𝑾 𝟐
• Since, weight of drops is w = mg and mass = volume X density i.e m= v.d
• Then, weight of one drop of liquid 𝑤1= v 𝑑1g/ 𝑛1
• Weight of one drop of reference liquid 𝑤2 = v 𝑑2g/ 𝑛2
• Hence, Where,
•
𝜸 𝟏
𝜸 𝟐
=
𝒅 𝟏 𝒏 𝟏
𝒅 𝟐 𝒏 𝟐
𝑑1= Density of experimental liquid ,
𝑑2= Density of water
γ2 = Surface tension of water 72 dyne/cm

11. Thank you!