a. When an emulsion separates into its separate ingredients (when the oil and
water are clearly separated and will not recombine) the emulsion is said to have
"cracked" or "broken." Do not confuse a "cracked" emulsion with one that has creamed!
Creaming is a natural occurrence with most emulsions and simple shaking will restore
the uniformity of the preparation. However, no amount of shaking will restore the
cracked emulsion to its original state.
b. The most common reason that an emulsion cracks is the addition of too much
or too concentrated alcohol or electrolyte solution. Freezing will also cause an emulsion
DISPENSING AN EMULSION
a. Container. Emulsions of thick consistency should be dispensed in a
wide-mouthed bottle or jar. The very thin emulsions may be dispensed in ordinary
prescription bottles, if they will not become thicker as the emulsifier hydrates on
standing. Thick emulsions cannot be conveniently poured from narrow-mouthed
bottles. Clear bottles are generally preferred to amber ones, unless their contents are
sensitive to light.
b. Special Instructions. All emulsions are damaged by excessive heat or cold.
Patients should be instructed to store emulsions away from either extreme. Emulsions
for internal use should be stored in the refrigerator, but they should be protected from
freezing. All emulsions should bear the "Shake Well Before Using" label. Those for
external use should be so labeled.
Section II. SUSPENSIONS
Both emulsions and suspensions are types of dispersions. Suspensions are
liquid preparations that contain a finely divided dispersed solid. They are quite closely
related to emulsions except that their internal phase is a solid rather than another liquid.
If the particle size is less than 0.1 micrometer, where a micron is a millionth of a meter,
we say that we have a colloid, not a suspension. Suspensions are usually intended for
external use, oral administration, or injection. The tendency of suspensions to settle out
makes them slightly less desirable as a dosage form than solutions; the solid particles
agglomerate and settle to the bottom, leaving the liquid phase supernatant. It is quite
possible to prepare a suspension that will not settle appreciably for several months. In
the pharmacy, this should be the goal for every suspension prepared: to lengthen the
settling time as much as possible. The absolute minimum requirement of settling time is
a rate slow enough to allow the medication to be shaken and a dose to be removed
from the container that will be uniform in content each time.