Displacement value in suppositories is important parameter to consider before its formulation. The volume of a suppository from a particular mould is same or uniform but its weight will differ because of the differences in densities of base and medicament.
The displacement value of a drug is the number of parts by weight of drug which displaces 1 part by weight of the base.
Displacement value of drug added in to suppository base is to be find out the, displacement of equivalent amount of base with amount of drug, so as to adjust the drug in same volume of mould, which has been calibrated for base alone.
Calculation of Displacement value
When displacement value of a drug is not known it can be calculated by the following method:
1. Prepare six suppositories using the base alone. Let = A mg
2. Prepare six suppositories containing a known percentage of a drug. Let = B mg
3. Calculate the amount of base present in the medicated suppositories. Let = C mg
4. Calculate the amount of medicament present in the suppositories. Let = D mg
5. Therefore, (A-C) will be the weight of the base displaced by the medicament.
6. Displacement value of the medicament for a particular base shall be:
Displacement value= D/(A-C)
Example:
Ø Weight of six unmedicated suppositories = 6 g.
Ø Weight of six suppositories containing 40% drug = 8.8 g.
Ø Weight of base is then = 60% = 60/100 x 8.8 = 5.28 g.
Ø Weight of drug in suppositories = 40% = 40/100 x 8.8 = 3.52.
Ø Weight of base displaced by drug = 6 - 5.28 = 0.72 g.
Ø If 0.72 g of base is displaced by 3.52 g of drug, then
1g of base will be displaced by 3.52/0.72g= 4.88 g.
Ø Therefore displacement value of drug = 4.88
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