c When Heat is Inadvisable. Heating aids solution and may be desirable
when preparing solutions. But heat is inadvisable when:
The solvent will deteriorate from heat.
(2) A volatile or aromatic liquid must be added to the newly formed solution
which, if hot, would result in loss of the volatile substance.
(3) The solute may be totally or partially decomposed by the heat, as is the
case with chloral hydrate or sodium bicarbonate.
(4) It is possible for a supersaturated solution to occur by accident.
Supersaturated solutions are not suitable for dispensing.
d. Mixtures of Liquids.
(1) Potent liquids. When small quantities of potent liquids are prescribed in
solution, part of the solvent should be placed in a graduate that is capable of measuring
the final volume of the intended preparation. The powerful liquid is then measured
carefully in a smaller graduate and added to the solvent in the larger graduate. The
graduate used to measure the potent liquid is then rinsed with successive portions of
the solvent into the larger graduate until the proper volume is attained. In the same
manner, viscid liquids, such as glycerin, should be rinsed from their measuring vessel
into the final solution with portions of the solvent. Using this method, you can be
reasonably sure that the final preparation contains the desired amount of the active
(2) Shrinkage. When two or more liquids are mixed, the resulting volume
may be less than the sum of the two portions. The "shrinkage" is more noticeable with
some liquids than others. Alcohol and water shrink about five percent when mixed.
e. Solvents. The universal solvent, water, is the first solvent we generally think
of, but many other solvents are used in pharmaceuticals, such as glycerin, alcohol,
alcohol-water mixture, propylene glycol, and oils. Other solvents less commonly used
are carbon tetrachloride, ether, acetone, carbon disulfide, and benzene. Many of the
solvents used in internal preparations are complex solutions in themselves, such as
syrups, elixirs, and aromatic waters.
In a strict pharmaceutical sense, a compound solution is one in which one
substance has been made soluble by the prior solution of another substance in the
solvent. For example, iodine is not soluble in water by itself. If, however, potassium
iodide is first dissolved in the water and iodine is then added, it readily goes into
solution. The finished product in this case is a true compound solution. There are other
drugs besides this one, which are soluble only in a solution of a specific salt.