Page 18 - Pharmaceutical analytical chemistry |

Percentage weight/weight is frequently employed to express the
concentration of commercial aqueous reagents. Example HCl is sold as
37% solution which means that the reagent contains 37 g of HCl per 100
g of solution. It is a common measure to express the concentration of
active ingredient in a formulation such as a cream or to express the
content of a minor impurity in a drug substance.

b) Percentage volume/volume,

Percentage volume/volume is used to specify the concentration of a
solution that was prepared by diluting a pure liquid compound with
another liquid, e.g. 5% aqueous solution of methanol is prepared by
diluting 5 mL of pure methanol with enough water to give 100 mL of
solution. It is most often encountered in relation to the composition of
mobile phases used in HPLC.

c) Percentage weight/volume,

Percentage weight/volume is often employed to indicate the composition
of dilute aqueous solutions of solid reagents, e.g. 5% silver nitrate
solution. It is normally used to express the content of active ingredient in
liquid formulations such as injections, infusions, and eye drops.

4. Parts per million
For very dilute solutions, parts per million (ppm) is a very convenient
way to express concentration: 1 ppm =1 mg L-1.

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