What is Drop Weight and Drop Count Methods

Drop Weight and Drop Count Methods

If liquid is allowed to fall slowly through a capillary tube, the liquid first forms a drop at the tip of the tube which gradually increases in size and finally detaches from the tip when the weight of the drop equals the total surface tension at the circumference of the tube. Expressing mathematically,

w=2πr γ

where
w is the weight of one drop of the liquid
r is the radius of the capillary
γ is the surface tension of the liquid.

Surface tension of a liquid can be determined using the above principle by either of the following two methods using a drop pipette (Stalagmometer).

1. Drop weight method 

2. Drop count method

1. Drop Weight method

The drop pipette or stalagmometer consists of a glass tube with a bulb blown approximately in the middle of the tube . There are two markings A and B on the tube, one above the bulb and the other below it. There is a capillary bore at the tip of the stalagmometer.

The stalagmometer is clamped vertically and the given liquid, whose surface tension is to be determined, is sucked into it upto the mark A. The liquid is then allowed to drop slowly from the tip of the pipette. Twenty to thirty drops are collected from the pipette into a clean tarred vessel and the weight of one drop of the liquid is determined (w). The surface tension of the liquid is then given by

w = 2πr γ

or

γ =w/2πr

It is a general practise to determine and report the surface tension of a liquid with respect to water. For this, the liquid is first taken in the pipette. The weight of one drop of the liquid (w1) is determined as described above. Similarly, water is taken up in the pipette and the weight of one drop of water (w2) is obtained.
The relative surface tension of the liquid

=Surface tension of the liquid/Surface tension of water.

=W1/2πr/W2/2πr.

=W1/W2.

2. Drop Count method
This method is similar to the drop weight method except that the number of drops of the liquid formed are counted instead of weighing.
In this method, the given liquid is sucked into the stalagmometer upto the mark A. Keeping the pipette vertically, the number of drops formed when the liquid level falls from the mark A to B is counted.

We know that,

γ = weight of one drop of liquid/2πr.

= m g/2πr  n

where m is the mass of 1 drop,g is the gravitational force and n is the number of drops.

γ = vdg/2πr n

Where vd = volume × density  = mass(m)

For determination of relative surface tension of a liquid, the number of drops of the liquid (n1) as well as of water (n2) formed for the same volume are determined.

The relative surface tension of the liquid is given by.... 

=Surface tension of the liquid/ Surface tension of water