PRACTICAL PHYSICAL PHARMACEUTICS – DETERMINATION OF SURFACE TENSION

Object – To determine the surface tension of given liquid by –

  1. Drop number method
  2. Drop count method

Reference – Bahl B.S. etal, Essentials of Physical chemistry, S.Chand & Company Limited, New Delhi,  24th  edition, 1997, 348-352

Materials required –

Chemicals – Benzene or Toluene or ethyl alcohol or methyl alcohol

Glasswares & Apparatus – Measuring cylinder, Weighing balance, beaker, stalagmometer, weighing bottle

Theory – Surface tension arises on the surface of the liquid due to the intermolecular forces among the molecules of liquid. Molecules in the interior of the liquid is pulled in all directions equally but the molecules in the surface of the liquid is pulled in three direction only i.e. sidewards and downwards. This downward force pulls the molecules of the liquid to come inside which is called surface tension and this is the reason why a drop of water takes spherical shape. Hence, on this concept surface tension is defined as the force in dynes acting on the surface of liquid at right angles to a line 1 centimeter in length. In CGS system the unit of surface tension is dyne/cm and in SI system the unit of surface tension is Newton/meter.  Stalagmometer is the apparatus which is used for the determination of surface tension.

Surface tension of some liquids at 20C are –

LIQUIDS SURFACE TENSION
Water 72.75
Ethyl alcohol 22.27
Methyl alcohol 22.6
Acetone 23.7
Toluene 28.43
Benzene 28.9

 

Procedure –

  1. Drop number method

For sample liquid (1)

 

  1. A cleaned and dried stalagmometer was taken and it was filled with the sample liquid say toluene or benzene.
  2. 20 drops of the sample liquid from the stalagmometer was taken in weighing bottle.
  • Now the weighing bottle was weighed.
  1. From the obtained weight of 20 drops of sample liquid , weight of one drop of sample liquid (m1) was calculated.

For reference liquid (2)

  1. A cleaned and dried stalagmometer was taken and it was filled with the sample liquid say toluene or benzene.
  2. 20 drops of the sample liquid from the stalagmometer was taken in weighing bottle.
  • Now the weighing bottle was weighed.
  1. From the obtained weight of 20 drops of sample liquid , weight of one drop of sample liquid (m2) was calculated.

Observation table  for sample liquid –

Weight of empty weighing bottle (a) Weight of weighing bottle with the 20 drops of sample liquid (b) Weight of 20 drops of sample liquid (c)= b-a Weight of one drop of sample liquid (m1)= c/20
 

 

 

Observation table  for reference liquid –

Weight of empty weighing bottle (a) Weight of weighing bottle with the 20 drops of reference liquid (b) Weight of 20 drops of reference liquid (c)= b-a Weight of one drop of reference liquid (m2) = c/20
 

 

 

 

Formula used to calculate surface tension-

                                           γ1                                              m1

                                                       ———————-              =        ———————–         OR      γ1   = m1 γ2  / m2

                                           γ2                                              m2         

       where           γ= surface tension of sample liquid, γ= surface tension of reference liquid, m= weight of one drop of sample liquid and m= weight of one drop of reference liquid     

  1. Drop Count Method –

Procedure –

  1. First of all, a cleaned and dried stalagmometer was taken and it was filled with the sample liquid.
  2. Now the number of drops of sample liquid were counted from mark A to B.
  3. The same above steps were repeated for the reference liquid (water).
  4. Density of the sample liquid was determined by using the weighing bottle.

Observation table  for density of sample liquid –

Weight of empty weighing bottle (a) Weight of weighing bottle with sample liquid (b) Weight of sample liquid (M)= b-a Volume of sample liquid (V)
 

 

 

Observation table –

No. of drops of sample liquid (n1)  

 

 

No. of drops of reference liquid (n2)

 

 

Calculation –

Density (d1) = Mass / Volume = M / V =…………………..g/ml

Surface tension of sample liquid (γ1) is calculated by using the formula –

γ1  / γ2 = n2d1 / n1d2

Where γ1 = Surface tension of sample liquid, γ2 = Surface tension of reference liquid (water = 72.75 dyne / cm), n1 = Number of drops of sample liquid, n2 = Number of drops of reference liquid, d1 = Density of sample liquid, d2 = Density of reference liquid (water = 1.0 g/ml)

Result – The surface tension of given sample liquid was carried out by drop weight method and drop count method and found ……………………. dyne /cm and ………………… ..dyne /cm respectively.

One Response to PRACTICAL PHYSICAL PHARMACEUTICS – DETERMINATION OF SURFACE TENSION

  1. TKM-editor says:

    CORRECTION –
    FORMULA USED TO CALCULATE SURFACE TENSION –
    γ1 / γ2 = m1 / m2 OR γ1 = m1 γ2 / m2

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