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Drop Weight Method (Stalagmo-meter):
- The stalagmometer consist of a glass bulb about 10 cm in capacity with narrow capillary tube joined above and below.
- The straight parts of the tube above and below are graduated. The liquid whose surface tension is to be determined is sucked into the tube to fill it above the upper fixed mark.
- By adjusting pressure of pinchcock the liquid is allowed to flow out slowly in such a manner that each drop is fully formed to a maximum size at the tip before breaking off.
- A drop will cling to the end of the tube as long as its mass does not exceed surface tension force.
- In this way the number of drops flowing out as liquid runs down between mark A and B is determined. Usually, volume of liquid (v) between marks A and B would not confined to an integral number of drops of liquid but it would be a whole number plus a fraction of a drop.
This fraction is calculated by noting a number of divisions remaining filled above or below B as last drop flow out and by separately determining the number of divisions equivalent to one drop of liquid.The principle of this method consists of equating weight (w) of fully developed one drop just before it breaks off from the tip with force of surface tension (i.e., 2pir) where 'r' is the radius of exit tip and y is surface tension of liquid.
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