When an ointment base contains several solid ingredients such as white beeswax, cetyl alcohol, stearyl alcohol, stearic acid, hard paraffin, etc. as base components, it is required to melt them.
The melting can be done in two methods:
Method-I: The
components are melted in the decreasing order of their melting point i.e. the
higher m.p. substance should be melted first, the substances with the next
melting point, and so on. The medicament is added slowly to the melted
ingredients and stirred thoroughly until the mass cools down and a homogeneous
product is formed.
Advantage:
This will avoid overheating of substances having low melting points.
Method II: All
the components are taken in a subdivided state and melted together.
Advantage: The
maximum temperature reached is lower than Method I, and less time was taken
possibly due to the solvent action of the lower melting point substances on the
rest of the ingredients.
Cautions:
(i) Melting time is shortened by grating waxy components
(i.e. bees wax, wool alcohols,
hard-paraffin, higher fatty alcohols, and emulsifying waxes) by stirring
during melting and by lowering the dish as far as possible into the water bath
so that the maximum surface area is heated.
(ii) The surface of some ingredients discolors due to
oxidation e.g. wool fats and wool alcohols and these discolored layers should
be removed before use.
(iii) After melting, the ingredients should be stirred until
the ointment is cool, taking care not to cause localized cooling, e.g. by using
a cold spatula or stirrer, placing the dish on a cold surface (e.g. a plastic
bench top) or transferring to a cold container before the ointment has fully
set. If these precautions are ignored, hard lumps may separate.
(iv) Vigorous-stirring, after the ointment has begun to
thicken, causes excessive aeration and should be avoided.
(v) Because of their greasy nature, many constituents of
ointment bases pick up dirt during storage, which can be seen after melting.
This is removed from the melt by allowing it to sediment and decanting the
supernatant, or by passage through muslin supported by a warm strainer. In both
instances, the clarified liquid is collected in another hot basin.
(vi) If the product is granular after cooling, due to the separation
of high M.P. constituents, it should be remelted, using the minimum of heat,
and again stirred and cooled.
Example
(i) Simple ointment B.P.
- contains Wool fat - 50 g
- Hard paraffin - 50 g
- Cetostearyl alcohol - 50 g
- White soft paraffin - 850 g
Type of preparation: Absorption ointment base.
Procedure:
Hard paraffin and cetostearyl alcohol in a water bath. Wool
fat and white soft paraffin are mixed and stirred until all the ingredients are
melted. If required decanted or strained and stirred until cold and packed in a
suitable container.
(i) Paraffin ointment base
- Type of preparation: Hydrocarbon ointment base.
(ii) Wool alcohol ointment B.P.
- Type of preparation: Absorption base.
(iii) Emulsifying ointment B.P.
- Type of preparation: Water-miscible ointment base.
(iv) Macrogol ointment B.P.C
- Type of preparation: Water soluble ointment base.
Formula:
- Macrogol 4000
- Liquid Macrogol 300
Method: Macrogol 4000 is melted and previously warmed liquid macrogol 300 is added. Stir until cool.