Category Archives: Physical Pharmaceutics – I (Practical)

PRACTICAL – DETERMINATION OF HLB NUMBER (HYDROPHILLIC-LYPOPHILLIC BALANCE)

Object – To determine the HLB number (Hydrophillic-Lipophillic Balance number) of a surfactant by saponification method.

Reference – Mohanta G.P. & Manna P.K., Physical pharmacy – Practical Text, Pharma Book Syndicate, Hyderabad, 2006, 30-33

Materials required –

Chemicals – polyoxyethylene sorbitan monolaurate (Tween 20), alcoholic potassium hydroxide (2.82%), phenolphthalein, lauric acid, standard N/2 hydrochloric acid, standard N/10 Potassium hydroxide, alcohol, ether.

Glasswares & Apparatus – round bottom flask, condenser, waterbath, burette, burette stand, pipette, beaker, measuring cylinder, conical flask

Theory – The hydrophilic-lipophillic balance (HLB) of a surfactant is a measure of its polarity. The HLB value of nonionic surfactant can be calculated using the formula –

HLB = 20 (1-S/A)

Where S= Saponification number of ester and A = Acid number of fatty acid.

Saponification number is defined as number of milligram of potassium hydroxide required to neutralize the acid present in one gram of substance.

Acid number is defined as the number of milligram of potassium hydroxide required to neutralize the free acid present in one gram of substance.

The normal values of tween 20 (polyethelene sorbitan monolaurate) are Saponification number (S) = 45.5, Acid number (A) = 276 and Hydrophillic-Lypophillic balance number  (HLB) = 16.7

Procedure – Determination of saponiofication number – First of all, 1 gram of sample (Tween 20) was accurately weighed and taken in a round bottom flask. Then 30 ml of alcoholic potassium hydroxide (2.82%) was added and refluxed on boiling water bath for 1 hour. A blank experiment was performed in the same way but without sample. The reaction mixtures were cooled down to room temperature and titrated against standard N/2 hydrochloric acid using phenolphthalein as indicator taking color change from pink to colorless or slightly yellow as end point.

Determination of acid number – First of all, 500 milligram of lauric acid was accurately weighed and taken in a conical. Then 10 ml of alcohol and 10 ml ether were added. It was heated slightly to dissolve the lauric acid. The above mixture was titrated against standard N/10 potassium hydroxide using phenolphthalein as indicator.

Observation & Calculation – Saponification number-

Weight of sample taken for saponification number determination = ……………. gram

Let the titre value of sample = V1 ml

Titre value of blank = V2 ml

(V2-V1) ml of N/2 hydrochloric acid equivalent potassium hydroxide was required to neutralize the sample taken

(V2-V1) ml of N/2 HCl = (V2-V1) ml of N/2 KOH

1000 ml N KOH = 56000 x N/2 x V2-V1/1000 mg KOH

Saponification number = 56000 x N/2 x V2-V1/1000 whole divided by amount of sample in gram.

Acid number-

Let the titre value of N/10 KOH = V3 ml

V3 ml of N/10 KOH was required to neutralize the free acid in taken sample

Acid number (A) =

V3 X  Eq. weight of KOH / weight of sample taken in gram x normality of KOH / Weight of sample in gram

HLB = 20 X (1-S/A)

Result – The HLB value of polyoxyethelene sorbitan monolaurate (Tween 20) was ……………..

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.