Definition
A concentration (C) indicates the content of one substance in another as a quotient. By definition, it refers to the amount of a substance present in a given volume. However, concentrations can also refer to masses. In pharmacy, concentration is often used in connection with liquid and semisolid dosage forms. For solid dosage forms such as tablets or capsules, it is more common to refer to the mass of the active ingredients.
Mass concentration
For example, an oral oxycodone solution contains 10 milligrams (mg) of anhydrous oxycodone hydrochloride per milliliter (ml): 10 mg/ml. This example is what is known as mass concentration, with the unit grams per liter (g/L, or m/V).
A physiological saline solution contains 9 g of sodium chloride (mass) to 1 liter of water (volume). Its concentration is usually expressed as a percentage: 0.9%. It is 9 g/L or 9 mg/ml. For the preparation, 9 g of sodium chloride are weighed and added to 1000 ml (not grams!). For example, a volumetric flask can be used to determine the volume (see under Glassware in Chemistry).
Mass percent
Two masses can also be related to each other. For example, 100 g of a diclofenac gel contains 1 g of diclofenac sodium. The concentration is 1% or 10 mg/g (m/m). If such a gel is prepared, it is not necessary to work with volume. The gel consists of 1 g of active ingredient and 99 g of base. The components can be weighed with a balance. Note the difference from the example above.
Volume concentration
In volume concentration, two volumes are related to each other. The unit is L/L. Percentages of ethanol often refer to volume. 100 ml of ethanol 20% (V/V) means that there are 20 ml of pure (anhydrous) alcohol in this preparation.
- C (volume concentration) = V (volume) / V (volume).
Caution: volumes with different densities cannot be added! See under dilutions.
Substance volume concentration
In the substance amount concentration (molarity), the amount of the substance is given in the substance amount (mol), i.e. the number of particles contained. The unit is mol per liter (mol/L), officially according to SI: mol/m3. A 1-molar solution contains 1 mole of the substance in 1 liter of water. This is also referred to as 1 M. One mole of a substance is equal to 6.022 140 76 × 1023 particles (= Avogadro number).
- C (substance amount concentration) = n (substance amount) / V (volume).
Active substance salts
Active ingredients are contained in many drugs in the form of salts ( active ingredient salts). Since these have a different (higher) molecular mass than the active ingredient, the concentrations differ in relation to the active ingredient and its salt. In the above example with oxycodone, the concentration of the pure oxycodone base is only 9 mg/ml instead of 10 mg/ml (salt). This may play a role in dosing. See also under active ingredient salts.
Concentration changes
If the substance contained or the volume is changed, for example increased or reduced, the concentration changes. So, for example, if 1 g of sodium chloride is contained in 1000 ml of water, the concentration is 1 g/L. If water is added to 2 liters, the concentration is only 0.5 g/L. For calculations related to dilutions, there is the following formula, called the mixture cross:
- C1 (concentration 1) x V1 (volume 1) = C2 (concentration 2) x V2 (volume 2).
C: percent or substance volume concentration For detailed information, see the article Dilutions.
Calculating with concentrations
Example: an ibuprofen suspension contains 20 mg/ml of ibuprofen. You want to give a five-year-old child a single dose of 100 mg of ibuprofen for fever. How much suspension do you need? Solution: 5 ml