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.
![Drop Weight Method](https://pharmacyscope.com/wp-content/uploads/2023/01/Screenshot-2023-01-27-154847-1024x256.png)
ϒ1 and ϒ2 are the surface tension of the test liquid and reference liquid respectively.
![Surface Tension](https://pharmacyscope.com/wp-content/uploads/2023/01/Screenshot-2023-01-27-155212.png)
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:
- The cleaned stalagmometer is fixed with a clamp and filled with water by sucking.
- Around 20-30 drops of water, falling from the stalagmometer are collected in a dry and tared (preweighed) weighing bottle, and weighed.
- 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.
- Weight of the clean and dry weighing bottle = X1 g.
- Weight of the dry weighing bottle + 20 drops liquid = Y1 g.
- Weight of the clean and dry weighing bottle = X1 g.
- Weight of the dry weighing bottle + 20 drops of water = Y2 g.
Therefore, weight of 20 drops of liquid = (Y1– X1) g
![Weight of 1 drop of liquid](https://pharmacyscope.com/wp-content/uploads/2023/01/Screenshot-2023-01-27-155650-1024x227.png)
Similarly,
![Weight of 1 drop of water](https://pharmacyscope.com/wp-content/uploads/2023/01/Screenshot-2023-01-27-155931-1024x205.png)
Then,
![Surface Tension](https://pharmacyscope.com/wp-content/uploads/2023/01/Screenshot-2023-01-27-155212.png)
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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