Percentage rate per hundred is a convenient means of expression of the concentration of a solute in solution or amount of a potent drug in a dosage form or the quantity of an ingredient in a mixture. The common types of percent solutions or mixtures in Pharmacy are classified as follows-

1. Percentage of solid in liquid usually expressed as % w/v

2. Percentage of solid in solid usually expressed as % w/v

3. Percentage of liquid in solid usually expressed as % v/w

4. Percentage of liquid in liquid usually expressed as % v/w

5. Miscellaneous mixtures

Percentage of solid in liquid

The true percentage of solid in liquid is expressed as weight in weight e.g., 10% solution of sugar in water means 10 g of sugar dissolved in sufficient water to make up 100 g of solution or 10 grains of solution dissolved in sufficient water to make up 100 grains of solution.

In this type where the percentage strength is expressed as weight by weight (W/W) the solutions are to be done by weighing solids and liquids. It is not easy to weigh the liquids. It is also necessary to take into consideration the specific gravity of the solvent and the final solution when finding the amount of a substance in equal volume of two solutions of the same substance.

Useful Tips

1 fl oz of 1% w/v solution requires 4.375 grains of solute

1 pint of 1% w/v solution requires 87.5 grains of solute

1 quart of 1% w/v solution requires 175 grains of solute

1 gallon of 1% w/v solution requires 700 grains of solute.

1 pound = 16 ounce

1 fl oz = 8 fl drachm