Multiplying the extremes and the means, we arrive at:
400 X = 25,000
By dividing 400 into 25,000, we get:
X = 62.5%
A great majority of drugs today are dispensed in solution, primarily because they are easier to take in that state, and also because their strength can be more readily cent rolled.
Although solutions may be either liquid, gaseous, or solid, we will concern ourselves here only with liquid solution, since they are of primary importance in pharmacy.
A solution is a homogeneous mixture of two or more substances, all having completely lost their physical identity. The liquid into which the ingredients are dissolved is called the solvent, and the substances that have been dissolved in it are called the solutes.
NOTE: A solution can consist of many solutes and more than one solvent.
SOLUBILITY. The ability of a solid to dissolve in a given amount of solvent is called its volubility.
Conditions that influence volubility are as follows:
1. The degree of subdivision of the solute
2. Agitation or stirring
3. Temperature If a solution contains all of the certain solute that the solvent will hold in solution, the solution is said to be saturated. By raising the temperature of the solution, the solvent will dissolve more of the solute than could have been dissolved under normal condition. It is then said to be supersaturated.
A good place to find a drugs volubility and solution media is the USP-NF. A very good example of how this is stated is ammonium chloride, USP-NF, which reads:
One g dissolves in about 3 ml of water, in about 100 ml of alcohol, and in about 8 ml of glycerine. One g dissolves in about 1.4 ml of boiling water.
By the above it can readily be seen that ammonium chloride is very soluble in water, only slightly soluble in alcohol, and fairly soluble in glycerine.
Classes of Solutions:
1. True solutiona solution in which the particles of the solute are so small that they pass through both filter paper and animal membrane. Example: salt in water
2. Colloidal solutiona solution in which the particles of the solute will pass through filter paper and not through animal membrane.
In the preparation of solutions in pharmacy, there are three distinct types:
WEIGHT IN WEIGHT (W/W)This is an expression of concentration in terms of number of g of active ingredient per 100 g total solution.
Example: 2 g of potassium iodide in 100 g of solution (total weight) is a 2% (W/W) solution of potassium iodide.
WEIGHT IN VOLUME (W/V)This is an expression of concentration in terms of number of g of active ingredient per 100 ml of solution.
Example: 85 g of sucrose in 100 ml of total solution would result in an 85070 (W/V) solution of sucrose.
VOLUME IN VOLUME (V/V)This is an expression of concentration in terms of number of milliliters of active ingredient per 100 ml of solution.
Example: 5 ml of clove oil in 100 ml of total solution would result in a 5% (V/V) solution of clove oil.
Ratio solutions are usually prepared in strengths as follows: 1:10, 1:150, 1:1000, 1:25000. etc, using even numbers to simplify the calculations. When a solution is made by this method, the first term of the ratio expresses the part of the solute, while the second term expresses the total amount of the finished product.