For the preparation of ointments: An ointment should be:
(a) Uniform throughout i.e. it contains no lumps of separated high melting point ingredients of the base, there is no tendency for liquid constituents to separate insoluble powders are evenly dispersed.
(b) Free from grittiness, i.e. insoluble powders are finely subdivided and large lumps of particles are absent. Methods of preparation must satisfy these criteria.
Two mixing techniques are frequently used in making ointments:
1. Fusion, in which ingredients are melted together and stirred to ensure homogeneity.
2. Trituration, in which finely subdivided insoluble medicaments are evenly distributed by grinding with a small amount of the base or one of its ingredients followed by dilution with gradually increasing amounts of the base.
Preparation of Ointments by Fusion Method:
When an ointment base contains several solid ingredients such as white beeswax, cetyl alcohol, stearyl alcohol, stearic acid, hard paraffin, etc. as components of the base, it is required to melt them.
The melting can be done in two methods:
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 in the melted ingredients and stirred thoroughly until the mass cools down and a homogeneous product is formed.
This will avoid over-heating of substances having a low melting point.
All the components are taken in a subdivided state and melted together.
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.
(i) Melting time is shortened by grating waxy components (i.e. beeswax, 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 discolours due to oxidation e.g. wool fats and wool alcohols and these discoloured 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 benchtop) 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 separation of high M.P. constituents, it should be remelted, using the minimum of heat, and again stirred and cooled.
(i) Simple ointment B.P. contains
Wool fat -50g
Hard paraffin -50g
Cetostearyl alcohol -50g
White soft paraffin -850g
Type of preparation: Absorption ointment base.
Hard paraffin and Cetostearyl alcohol on 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.
(ii) Paraffin ointment base
Type of preparation: Hydrocarbon ointment base.
(iii) Wool alcohols ointment B.P.
Type of preparation: Absorption base.
(iv) Emulsifying ointment B.P.
Type of preparation: Water-miscible ointment base.
(v) 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. Stirred until cool.
Preparation of Ointments by Trituration:
This method is applicable in the base of a liquid present in a small amount.
(i) Solids are finely powdered are passed through a sieve (# 250, # 180, #125).
(ii) The powder is taken on an ointment-slab and triturated with a small amount of the base. A steel spatula with a long, broad blade is used. To this additional quantities of the base are incorporated and triturated until the medicament is mixed with the base.
(iii) Finally, liquid ingredients are incorporated. To avoid loss from splashing, a small volume of liquid is poured into a depression in the ointment and thoroughly. Incorporated before more is added in the same way. Splashing is more easily controlled in a mortar than on a tile.
(i) Whitfield ointment (Compound benzoic acid ointment B.P.C.)
Benzoic acid, in fine powder – 6 gm
Salicylic acid, in fine powder – 3gm
Emulsifying ointment – 91gm
Method: Benzoic acid and salicylic acid are sieved through No. 180 sieves. They are mixed on the tile with a small amount of base and levigated until smooth and dilute gradually.
(ii) Sulphur ointment I.P.
Sublimed sulphur – 10 g
Simple ointment – 90 g
Prepare an ointment.
Method: Sublimed sulphur is sieved through no. 180 sieves. Then sublimed sulphur is triturated with a small amount of simple ointment. Then the remaining amount of simple ointment is added and the mixture is levigated until a smooth and homogenous mass is obtained.
Preparation of Ointments by Chemical Reaction:
Chemical reactions were involved in the preparation of several famous ointments of the past, e.g. Strong Mercuric Nitrate Ointment of the 1959 B.P.C.
An ointment containing free iodine
Iodine is only slightly soluble in most fats and oils. Lodine is readily soluble in a concentrated solution of potassium iodide due to the formation of molecular complexes KI-I2, KI-2I2, KI-3I2 etc.
These solutions may be incorporated in absorption-type ointment bases.
For example, Strong Iodine Ointment (British Veterinary Pharmacopoeia) is used to treat ringworm in cattle. It contains free iodine. At one time this type of ointments was used as counter-irritants in the treatment of human rheumatic diseases but they were not popular because they stain the skin a deep red colour. Due to improper storage, the water dries up and the iodine crystals irritate the skin, hence glycerol is sometimes added to dissolve the iodine-potassium iodide complex instead of water.
Example: Strong Iodine Ointment.
Iodine – 4 g
Yellow soft paraffin – 76 g
Potassium iodide – 4 g
Water – 12 g
(i) KI is dissolved in water. I2 is dissolved in it.
(ii) Wool fat and yellow soft paraffin are melted together over a water bath. The melted mass is cooled to about 40°C.
(iii) I₂ solution is added to the melted mass in small quantities at a time with continuous stirring until a uniform mass is obtained.
(iv) It is cooled to room temperature and packed.
Use: Ringworm in cattle.
An ointment containing combined iodine
Fixed oils and many vegetables and animal fats absorb iodine which combines with the double bonds of the unsaturated constituents, e.g.
CH3 (CH2) 2CH = CH(CH2) → COOH + I2CH3(CH2) 2CHI CHI (CH2)-COOH
Oleic acid Di-iodostearic acid
Example: Non-staining Iodine Ointment B.P.C.
Iodine – 5 g
Arachis oil-15 ml
Yellow soft paraffin q.s. to 100 g
(a) Iodine is finely powdered in a glass mortar and the required amount is added to the oil in a glass-stoppered conical flask and stirred well.
(b) The oil is heated at 50°C in a water bath and stirred continually. Heating is continued until the brown colour is changed to greenish-black; this may take several hours.
(c) From 0.1 g of the preparation the amount of iodine is determined by the B.P.C. method and the amount of soft paraffin base is calculated to give the product the required strength.
(d) Soft paraffin is warmed to 40°C. The iodized oil is added and mixed well. No more heat is applied because this causes the deposition of a resinous substance.
(e) The preparation is packed in a warm, wide-mouthed, amber colour, glass bottle. It is allowed to cool without further stirring.
Preparation of Ointments/Cream by Emulsification:
An emulsion system contains an oil phase, an aqueous phase and an emulsifying agent. For o/w emulsion systems the following emulsifying agents are used:
(i) Water-soluble soap
(ii) Cetyl alcohol
(iii) Glyceryl monostearate
(iv) Combination of emulsifiers: triethanolamine stearate + cetyl alcohol
(v) Non-ionic emulsifiers: glyceryl monostearate, glyceryl monooleate, propylene glycol stearate
For w/o emulsion creams the following emulsifiers are used:
- Polyvalent ions e.g magnesium, calcium and aluminium are used.
(ii) Combination of emulsifiers: bees wax+ divalent calcium ion
The viscosity of this type of creams prevent coalescence of the emulsified phases and helps in stabilizing the emulsion.
Example: Cold cream
(i) Water immiscible components e.g. oils, fats, waxes are melted together over the water bath (70°C).
(ii) Aqueous solution of all heat-stable, water-soluble components are heated (70°C).
(iii) Aqueous solution is slowly added to the melted bases with continuous stirring until the product cools down and a semi-solid mass is obtained.
Note: The aqueous phase is heated otherwise high melting point fats and waxes will immediately solidify on the addition of cold aqueous solution.
Make sure you also check our other amazing Article on : Semi-Solid Dosage Forms