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Stalagmometer is the device for measuring the surface tension of
surfactant solutions (fig. 9.2). The surface tension of surfactant
solutions by means of stalagmometer is determined by the method
of measuring the volume of drops forming on the borders:

“water solution of surfactant – nonpolar liquid” or “water solution of
surfactant – air”.
The principle of determining the surface tension of water

solutions of surfactant by means of stalagmometer consists in
pressing out 10 – 20 drops of kerosene into water solution of
surfactant or distilled water.
The main part of the device is micrometer 1 which provides the

exact determination of the volume of the drop which is pressed out.
The micrometer by means of clamp 14 is attached to sleeve 13 which
is freely moved on stand 11. The location (position) of the micrometer

is locked by screw 12. In this sleeve the body of syringe 4 is also
attached. The upper end of syringe piston 3 is attached to spring 2 and
due to this the uncontrolled syringe piston’s movement is avoided.

The principle of stalagmometer operation consists in following: at
rotating the micrometer the spring presses the moving piston rod
which forces the liquid in the form of the drop from the syringe by

the capillary.

9.4 Procedure and calculations

9.4.1 Determine the capillary constant (the stalagmometer
constant) by pressing out 10 drops of kerosene into the distilled water

and determine the average volume of one drop V .
ker water
Calculate the magnitude of stalagmometer constant K by using

the following equation:

47 5 .
K  , (9.3)
V ρ (   ρ )
ker water water ker

2
where 47.5 (mJ/m ) is a surface tension of water on the border

“kerosene–distilled water”; V is a volume of 1 drop of
ker water
3
kerosene in distilled water, m .

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