Isotonic Soution
When a solvent passes through a semi permeable membrane from a dilute solution into a more concentrated one, with the result that the concentrations become equalized, the phenomenon is known as osmosis. The pressure responsible for this phenomenon is called osmotic pressure, and it proves to be caused by and to vary with the solute.
If the solute is a nonelectrolyte, its solution will contain only molecules, and the osmotic pressure of the solution will vary only with the concentration of the solute. If the solute is an electrolyte, its solution will contain ions, and the osmotic pressure of the solution will vary not only with the concentration, but also with the degree of dissociation of the solute. Therefore, substances that dissociate have a relatively greater number of particles in solution and should exert a greater osmotic pressure than could undissociated molecules.
Like osmotic pressure, the other colligative properties of solutions, namely, vapor pressure, boiling point, and freezing point, depend on the number of particles in solution. These properties, therefore, are related, and a change in any one of them will be attended by corresponding changes in the others.
Two solutions that have the same osmotic pressure are termed isosmotic. Many solutions intended to be mixed with body fluids are designed to have the same osmotic pressure for greater comfort, efficacy, and safety. A solution having the same osmotic pressure as a specific body fluid is said to be isotonic (meaning of equal tone) with that body fluid.
Solutions of lower osmotic pressure than that of a body fluid are hypotonic, whereas those having a higher osmotic pressure are hypertonic. Blood and the fluids of the eye, nose, and bowel are of principal concern to the pharmacist in the manufacture and use of preparations to be mixed with these biologic fluids, which include ophthalmic (eye), nasal (nose), parcnteral (by injection) and some enema (rectal) preparations.
PREPARATION OF ISOTONIC SOLUTIONS
The calculations involved in preparing isotonic solutions may be made in terms of data relating to the colligative properties of solutions. Theoretically, any one of these properties may be used as a basis for determining tonicity. Practically and most conveniently, a comparison of freezing points is used for this purpose. It is generally accepted that — 0.52°C is the freezing point of both blood serum and lacrimal fluid.
When one gram molecular weight of any nonelectrolyte, i.e., a substance with negligible dissociation, such as boric acid, is dissolved in 1000 g of water, the freezing point of the solution is about 1.86 C below the freezing point of pure water. By simple proportion, therefore, we may calculate the weight of any nonelectrolyre that should be dissolved in each 1000 g of water if the solution is to be isotonic with body fluids.
Boric acid, for example, has a molecular weight of 61.8, and hence (in theory) 61.8 g in 1000 g of water should produce a freezing point of — 1.86°C. Therefore:
1.86 (°C) = 61.8 (g)
0.52 (°C) x (g)
x = 17.3