Aim of Experiment: To compare the cleaning power of different samples of soaps and detergents by surface tension measurements.

Theory: Detergent action or cleaning property of soaps detergents depend mainly upon the extent of reduction in the surface tension of caused by the detergents or soaps . The greater is the reduction, more will be the cleaning power of the detergents. Thus cleaning power of the detergents can simply be compared by measuring the surface tension their solutions containing equal weights in a fixed volume of water.

Procedure:

1) Equal amounts of four samples of detergents and soaps are weighed and 100 cc. of preheated water is added to them separately in beaker. The contents are stirred very gently to make the solutions taking care that no foam is formed.

2) The solutions are allowed to stand so that insoluble impurities ,if any settle down and the temperature falls to a constant value. Alternatively the solutions can be filtered while to remove insoluble impurities.]

3) The no. of drops formed from a fixed volume of four solutions are determined along with water using stalagmometer. The stalagmometer must be fitted with solutions by very gentle suction so that foam formation does not takes place.

Calculation:

Let us consider n, n1, n2, n3, and n4 are the no. of drops from a fixed volume of water ,solution 1,solution 2,solution 3 and solution 4 respectively. Suppose d, d1,d2,d3 and d4 be their respective densities.

Then,

                r1/ r = nd1/ n1d

             or    r1 =  (nd1 /  n1d) r

Similarly        r2 =  (nd2 / n2 d) r

                     r3 = (nd3 / n3 d) r

                     r4 = (nd4 / n4 d) r

Where  r1, r2, r3  and r4  are the respective surface tension of water, solutions 1,2,3 and 4 .

Since the solutions are very dilute ,their densities will almost be same as that of water. Hence

                      r1 =  (n / n1) r  or   r-r1  =  (1- n / n1) r

                      r2 =  (n / n2) r  or   r-r2  = (1- n / n2) r

                      r3 =  (n / n3) r   or  r- r3 = (1- n / n3) r

                      r4 =  (n / n4 ) r   or  r- r4 =(1- n / n4) r

TABLE 1:  Calculation of no. of drops from a fixed volume

 

   Sl  no.    Name of sample (0.1g) No. of drops of liquids
   1    Water    30
   2    Solution1 (Surf Excel)    32
   3    Solution2  (Tide)             44
   4    Solution3  (Nirma)    72
   5    Solution4   (Lux)    85

   Calculation of density of sample: 

                Density of liquid = (density of water). (weight of liquid)/(weight of water)