Surface and Interfacial tension [Part-3(a)]�(Measurement of Surface and Interfacial tension: 1. Capillary Rise Method 2. Drop count and Weight method )�

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 Capillary Rise Method. Drop Count and Weight Method. Wilhelmy Plate Methods 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 = γ cos θ Total upward force = 2 п r γ cos θ 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, cos θ = 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 = п hpg At equilibrium, Upward force = Downward Force 2 п r γ = п hpg γ =  

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 ( ). Surface tension of liquid is then given by: W = 2пr γ Similarly water is taken in pipette and weight of one drop water ( ) is obtained. The ratio of the weight of a drop of the liquid ( ) to the reference substance ( ) falling the same capillary orifice is equal to the ratio of their surface tension  

9. If and are the surface tension of experimental liquid and reference standard respectively then The weight of the drop in mg of a test liquid = 2 п r The weight of the drop in mg of a reference liquid = 2 п r As the same apparatus is used for both the liquids the correction factor is same assume that the drop volumes are not different. Hence = Rearranging the equation =  

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. = Since, weight of drops is w = mg and mass = volume X density i.e m= v.d Then, weight of one drop of liquid = v g/ Weight of one drop of reference liquid = v g/ Hence, Where, = , 72 dyne/cm  

11. Thank you!