* Non-volatile, miscible liquids may be mixed with
the molten cream in the evaporating basin. Alternatively, if a pre-prepared base is used, then incorporate as for volatile or immiscible liquids.
* Volatile or immiscible liquids (e.g. coal tar
solutions) should be triturated with the cream on the ointment tile. A very small amount of the cream should be placed on the tile and a ‘well’ made in the centre. Traditionally, small quantities of liquid should be gently folded in to avoid splashing. An alternative method is to spread a small amount of the cream on the tile and then ‘score’ it with a spatula. Then add small quantities of the liquid and fold into the base gently. If using coal tar or other volatile ingredients, these should not be weighed until immediately before use and the beaker in which it has been weighed should be covered with a watch glass to prevent
evaporation.
See Ointments video for a demonstration of the incorporation of a liquid into a base.
Example 9.1 Preparation of 20 g Cetrimide Cream BP
Example label: we have assumed that the name and address of the pharmacy and the words 'Keep out of the reach and sight of children' are pre-printed on the label):
Cetrimide Cream BP 20 g
Product directions. For External Use Only Do not use after (4 weeks)
Patient name Date of dispensing
Product formula (British Pharmacopoeia 1988, page 653):
1000 g 100 g 10 g 30 g Cetrimide BP 5 g 500 mg 50 mg 150 mg Cetostearyl Alcohol BP 50 g 5 g 500 mg 1.5 g Liquid Paraffin BP 500 g 50 g 5 g 15 g Freshly boiled and cooled purified water 445 g 44.5 g 4.45 g 13.35 g
Point of clarity – Product formula
The exact quantity cannot be prepared for a cream as losses will be encountered on transfer. Therefore, a suitable overage must be produced in order to dispense the required final amount. Note that all liquid ingredients (including the Liquid Paraffin BP) must be weighed.
Melting points and solubilities of the ingredients: Melting point:
Cetostearyl Alcohol BP 49–56C
Solubilities:
Cetostearyl Alcohol BP Practically insoluble in water. When melted, miscible with liquid paraffin Cetrimide BP Soluble in 2 parts water
Liquid Paraffin BP Practically insoluble in water. Slightly soluble in alcohol. Miscible with hydrocarbons
Method to prepare 30 g of Cetrimide Cream BP from the formula above: 1 Weigh 1.5 g Cetostearyl Alcohol BP on a Class II or electronic balance. 2 Weigh 15 g of Liquid Paraffin BP on a Class II or electronic balance.
3 Weigh 13.35 g of freshly boiled and cooled purified water on a Class II or electronic balance. 4 Weigh 150 mg of Cetrimide BP on a Class II or electronic balance.
5 Melt the Cetostearyl Alcohol BP in an evaporating basin over a water bath to a temperature no higher than 60C.
6 Add the Liquid Paraffin BP to the molten Cetostearyl Alcohol BP and remove from the heat. 7 Stir to form the OILY phase.
8 Transfer the freshly boiled and cooled purified water to a beaker and heat to 60C.
Worked examples
9 Add the Cetrimide BP to the freshly boiled and cooled purified water and remove from the heat. 10 Stir to form the AQUEOUS phase.
11 When the oily phase and the aqueous phase are both at about 60C, add the aqueous phase to the oily phase with constant, not too vigorous stirring.
Point of clarity – Step 11
Stirring is constant and not too vigorous to ensure that there are no ‘cold spots’ within the cream as these would hasten cooling in discrete areas and result in a lumpy cream.
12 Stir until cool enough to pack.
13 Weigh 20 g of the product and pack into an amber glass jar; label and dispense.
See Creams video for a demonstration of the preparation of a cream from first principles.
Example 9.2 Preparation of 20 g of Salicylic Acid and Sulphur Cream BP
Example label (we have assumed that the name and address of the pharmacy and the words 'Keep out of the reach and sight of children' are pre-printed on the label):
Salicylic Acid and Sulphur Cream BP 20 g
Product directions. For External Use Only Do not use after (4 weeks)
Date of dispensing Patient name
Product formula (British Pharmacopoeia 1980, page 548):
1000 g 100 g 10 g 30 g Salicylic Acid BP 20 g 2 g 200 mg 600 mg Precipitated Sulphur BP 20 g 2 g 200 mg 600 mg Aqueous Cream BP 960 g 96 g 9.6 g 28.8 g
Method to prepare 30 g of Salicylic Acid and Sulphur Cream BP from the formula above: 1 Weigh 600 mg Salicylic Acid BP on a Class II or electronic balance.
2 Transfer to a glass mortar and grind with a pestle.
3 Weigh 600 mg Precipitated Sulphur BP on a Class II or electronic balance.
4 Add to the Salicylic Acid BP in the glass mortar and continue mixing with a pestle until a smooth, well mixed powder is formed.
5 Transfer the powder to a glass tile.
6 Weigh 28.8 g Aqueous Cream BP on a Class II or electronic balance.
7 Transfer the Aqueous Cream BP to the glass tile and triturate with the powders to produce a smooth product.
Point of clarity – Step 7
Note: a vulcanite spatula would be the spatula of choice as traditional stainless steel spatulas may react with acids, tannins, iodine and mercury salts, etc. Vulcanite (also called ebonite) is a hard, usually black, rubber, produced by vulcanising natural rubber with sulphur. Such spatulas are used for making ointments containing corrosive substances or substances that react with steel.
Example 9.3 Preparation of 50 g of Dermovate Cream 25%
Example label (we have assumed that the name and address of the pharmacy and the words 'Keep out of the reach and sight of children' are pre-printed on the label):
Dermovate Cream 25% w/w 50 g
Product directions. For External Use Only Do not use after (2 weeks)
The product contains:
Dermovate Cream 25% Cetomacrogol Cream (Formula A) BPC 75%
Date of dispensing Patient name
Product formula (i.e. the diluent to use) must be decided by the compounder. Refer to a diluent directory, which in this case states that although the dilution of the product is not recommended by the manufac- turer, in cases where it is insisted upon, the following may be used:
* Aqueous Cream BP – Only stable if less than 50% of the resultant cream. Therefore unsuitable for this formulation.
* Buffered Cream BP – May be used but can raise the pH of the resulting cream.
* Cetomacrogol Cream (Formula A) BPC – No problems with dilution recorded.
Therefore in this instance the diluent of choice would be Cetomacrogol Cream (Formula A) BPC. Point of clarity – sources of information
Suitable sources to provide information on the dilution of creams and ointments would be:
* NPA Diluent Directory
* Product data sheet (Summary of Product Characteristics – SPC)
* Reports in the pharmaceutical literature
* Personal contact with product manufacturer. Product formula:
Master formula 100 g 10 g 60 g Dermovate Cream 25% 25 g 2.5 g 15 g Cetomacrogol Cream (Formula A) BPC 75% 75 g 7.5 g 45 g
Method to prepare 60 g of Dermovate Cream 25% from the formula above: 1 Weigh 15 g Dermovate Cream on Class II or electronic balance. 2 Transfer to a glass tile.
3 Weigh 45 g Cetomacrogol Cream (Formula A) BPC on a Class II or electronic balance. 4 Transfer to the glass tile.