This article throws light upon the three main ways for expressing percentage concentration in a drug. The ways are: 1. Weight in Weight (W/W) 2. Weight in Volume (W/V) 3. Volume in Volume (V/V).
Way # 1. Weight in Weight (W/W):
This expression is used when solids are mixed with solids e.g. in powders, paste, ointments etc. A 5% w/w preparation means that 5 parts of solid ingredient by weight is to be mixed with 95 parts of other solid ingredient (s) by weight which will make a total of 100 parts of the formulation.
Way # 2. Weight in Volume (W/V):
This expression is used when solids are mixed with liquids e.g. in lotions, mixtures etc. A 5% w/v preparation means that 5 grams of the solid is mixed with liquid ingredient (s) to make a total of 100 millilitres of the final product.
Way # 3. Volume in Volume (V/V):
This expression is used when liquids are mixed with liquids e.g. in spirit, emulsion etc. A 5% v/v of a solution means 5 part by volume of liquid is mixed with 95 parts by volume of another liquid (solvent or base) to produce 100 parts of volume of the solution. The quantity of a substance required to prepare a solution of particular strength or percentage is given in Table. 5.2.
