APPARATUS REQUIRED:
Stalagmometer, Pycnometer or specific gravity bottle, Beaker,Pipette
CHEMICALS REQUIRED:
Distilled water, liquid paraffin
THEORY:
Surface tension is the contracting force which attracts the surface molecules of a liquid towards the interior of the liquid. The surface tension of a liquid is mainly a force that mainly acts to reduce the surface area of a liquid. The SI unit of surface tension is Newton / meter and CGs unit is Dyne / centimeter.
Surface tension (γ) = Force / Length
There are a number of methods for determining surface tension of liquids among which drop fall method is one of them.
Drop fall method:
Drop number method:
This is the method which is generally used to determine the surface tension of liquids in the laboratory. The method is based on the principle that the weight of a liquid falling from a capillary tube held vertically is approximately proportional to the surface tension of liquid. Hence, if the surface tension of two liquids be γ1 and γ2 and weight w1 and w2 that drops falling from the capillary tube then,
γ1/γ2 = w1/w2
The drops fall out when its weight becomes equal to
2𝝅rγ where γ= radius of tube.
The apparatus used in there determination consists of a bulb A fused with capillary tube B and is called a stalagmometer, It is more convenient to count the drops formed by a given volume of liquid.
let, the number of drops of two liquids be n1 and n2 for the same volume V of the liquids then,
W1 = v1/n1 x d1 and w2 = v/n2 x d2
where d1 and d2 are the respective densities of two liquids Hence,
γ1/γ2 = w1/w2 = vd1/n1 / vd2/n2
therefore, γ1/γ2 = n2d1/n1d2
PROCEDURE
A clean and dry stalagmometer was taken and a piece of a clean rubber tube with a screws pinch cock was attached to the upper end of the stalagmometer that was used to regulate the flow of liquid by limiting the influx of air The stalagmometer was filled with water (reference liquid) by dipping it in a beaker containing water and water was sucked till the water rises above the mark A. The rubber tube was removed and was adjusted to maintain the rate of flow of drops. Then, counting of drops was started when the meniscus passed the upper mark A and was stopped when it just crossed the lower mark B. The process was repeated for 8 times and drop count was recorded. Then, the stalagmometer was rinsed with liquid paraffin and the liquid was filled on it. The number of drops for the flow of liquid from mark A to B was counted. That process was repeated three times and drop count was recorded. A clean and dry pycnometer was weighed and its value was noted. Then, the bottle was completely filled with water and weighed and its value was noted.
Similar process was repeated for liquid paraffin to determine its density.
OBSERVATIONS:
Room temperature = 25°C
weight of empty pycnometer (w1) = 18.32 gm
weight of pycnometer + water (w2) = 42.48 gm
weight of pycnometer + liquid paraffin (w3) = 38.34 gm
For water,
No. of drops (n1)= 33
No. of drops (n2) = 35
No. of drops (n3) = 32
Average (N1) = 33+35+32 / 3
= 33.33
For Liquid paraffin,
No. of drops (n1) = 78
No. of drops (n2) = 81
No. of drops (n3) = 79
Average (N2) = 78+81 +79 / 3
= 79.33
CALCULATION:
Density of paraffin (d2) / Density Of water (d1) = weight of paraffin / weight of water
= w3-w1 / w2-w1
= 38.34 – 18.32 / 42.480 – 18.32
= 20.02 / 24.16
we know,
No. of water drops (N1) = 33.33
No. of liquid paraffin drops (N2) = 79.33
Surface tension of water at 25°C (γ1) = 72.8 dyne/cm
Surface tension of liquid paraffin (γ2) = ?
now,
γ2/γ1 = d2/d1 x n1/n2
or, γ2 = d2/d1 x n1/n2 x γ
= 20.02/24.16 x 33.33/79.33 x 72.8
= 48577.00848 / 1916.6128
= 25.35 dyne/cm
RESULT:
Hence, the surface tension of given liquid paraffin at 25°c room temperature was found to be 25.35 dyne/cm
PRECAUTIONS:
i) The stalagmometer should be held vertical.
ii) The stalagmometer should be absolutely clean from any greasy matter.
iii) The drops should be allowed to fall off from the stalagmometer tip under their own weight and should not be pushed away by the kinetic flow,