ABSTRACT
Expressions of concentration The vast majority of the pharmaceutical preparations used in the UK contain an active ingredient (drug) dissolved or dispersed in a solvent or diluent. Various expressions may be used to defi ne the concentration of a drug in a preparation and a knowledge of these is essential in the practice of pharmacy. Additionally, understanding expressions of concentration is also important when examining clinical laboratory test results, as biochemical results may be given in a variety of ways. In this chapter, we will consider four different ways of expressing concentrations:
quantity per volume ■ percentage concentrations ■ parts ■ ratios. ■
Quantity per volume Quantity per volume expressions are used to give the concentration of drugs in solution and also for clinical laboratory test results. A quantity per volume expression gives the amount or weight of drug (in terms of moles or grams, respectively) in a volume of solution. For example, a 9 g/L solution of sodium chloride means that 9 g of sodium chloride are dissolved in 1 litre of solution. A 1 mmol/L solution of sodium chloride contains 1 mmol (equivalent to 0.058 g) of the compound dissolved in 1 litre of solution.
