Determination of Surface Tension by Drop Weight Method

Aim: To determine the surface tension of a liquid (glycerin) using a stalagmometer by drop weight method.

Principle of Surface tension

The Surface tension (ST) measurement involves counting the number of drops formed when a definite volume of liquid is allowed to flow slowly out of a capillary orifice (stalagmometer). The ratio of weight (wI) of a drop of the liquid to that of a reference substance (w2 ) falling from the same capillary orifice is equal to the ratio of their surface tension.

The weight of the drop in mg of a test liquid, W₁=2πrϒ₁

Correction factor: The correction factor depends on the radius of the tip, and the cube root of the volume of the drop.

The weight of a drop of the reference liquid, W₂=2πrϒ₂ correction factor (from the same capillary).

As the same apparatus is used for both liquids, the correction factor is the same assuming that the drop volumes are not different.

ϒ₁ and ϒ₂ are the surface tension of the test liquid and reference liquid respectively.

The value of the ST of the liquid can be calculated by taking two observations: the weight of one drop of liquid and that of a reference liquid if the ST of the reference substance is known. Water is usually used as a reference liquid.

Apparatus and materials required: Stalagmometer, glycerin, water, balance, weighing bottle.

Process:

1. The cleaned stalagmometer is fixed with a clamp and filled with water by sucking.
2. Around 20-30 drops of water, falling from the stalagmometer are collected in a dry and tared (preweighed) weighing bottle, and weighed.
3. The stalagmometer is then dried and the experiment is repeated with the liquid whose ST is to be determined by a stalagmometer by drop weight method.

Care should be taken during an experiment that the number of drops does not exceed 20 per minute.

Observation and Calculation:

Room temperature: ____ °C.

1. Weight of the clean and dry weighing bottle = X₁ g.
2. Weight of the dry weighing bottle + 20 drops liquid = Y₁ g.
3. Weight of the clean and dry weighing bottle = X₁ g.
4. Weight of the dry weighing bottle + 20 drops of water = Y₂ g.

Therefore, weight of 20 drops of liquid = (Y₁– X₁) g

Similarly,

 Then,

ϒ₂ where ϒ₂ is the surface tension of water which is taken as 72mN/m.

Report: The surface tension of the liquid (glycerin) is ____ mN/m at____________ °C.

Notes: ST of few common liquids at 20°C in mN/m

Water	72
Glycerin	63.4
Liquid paraffin	33.1
Benzene	35.0

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