Chemistry Reference
In-Depth Information
Concentration of active ingredients
Although, in chemistry, all concentrations are expressed in molarity,
pharmacists and pharmaceutical analysts have to contend with the medical
profession, which tends to prescribe drugs not in molarities but in units of
mass per volume or weight per millilitre. The most common way to express
the concentration of active drug in a medicine is in terms of mass or volume
of active ingredient per 100 grams or millilitres of medicine. This can be
expressed in four ways, of which the first is the most common.
'Percentage weight in volume' (% w/v) is the number of grams of
drug in 100 mL of final product. This term is used for the concentrations of
solutions, suspensions, etc. where the active ingredient is a solid; for
example, 5% dextrose infusion is 5 g of dextrose in 100 mL of final solution.
•
'Percentage volume in volume' (% v/v) is the number of millilitres of drug in
100 mL of final product. This version is found in medicines where the active drug
and the final product are both liquids. This terminology should be familiar to
students since the strength of alcoholic drinks is usually expressed in this way. A
single malt whisky is 40% by volume alcohol. This means that for every 100 mL
of 'Glen Fusel' you drink you consume 40 mL of ethanol. Most beers are approx-
imately 5% by volume alcohol. Thus, for every 100 mL of beer consumed, the
drinker has taken in 5 mL of ethanol. (A pint is approximately 568 mL.)
•
'Percentage weight in weight' (% w/w) is the number of grams of drug in 100 g of
final product. This term is encountered most often in solid dosage preparations
such as powders, and semi-solid preparations such as creams and ointments, e.g.
1% salicylic acid ointment.
•
'Percentage volume in weight' (% v/w) is the number of millilitres of drug in 100 g
of final product. This usage is quite rare and is only encountered in ointments and
creams where the active ingredient is a liquid, e.g. 1% glycerol ointment.
Design of an assay
Before a substance is analysed, or assayed, the experiment must be designed
and planned. Initially, students will be told what to do in the analysis
laboratory, but they must quickly begin to plan assays and experiments
for themselves. The procedures to be followed when designing an assay are
outlined below.
Identify functional groups on the molecule that can react rapidly and quantita-
tively (i.e. the reaction should proceed almost 100% to the products; to put it
another way, the chosen reaction should have a high equilibrium constant,
K
).
1.
Work out the stoichiometric ratio, i.e. the number of moles of each compound
reacting.
2.
