How to prepare isotonic solutions.

Isotonic solutions have osmotic pressure similar to that of 0.9 % (w/v) sodium chloride solution. Osmotic pressure of a solution depends on the nature and concentrations of dissolved solutes. One method for preparing isotonic solutions is by using the E value (sodium chloride equivalent). E value of a given solute can be defined as the mass of sodium chloride that produces the same osmetic effect as 1 gram of the solute. Tables of E values of solutes and drugs can be found in physical pharmacy books. For example, assume that we have a solution of 1 % (w/v) sucrose (E value = 0.08) then the concentration of a sodium chloride solution that has the same osmetic pressure can be calculated as the following:

E value = 0.08 means that each gram of sucrose is equivalent to 0.08 gram of sodium chloride. Therefore, 0.08 % (w/v) solution of sodium chloride is equivalent (has same osmetic pressure) to 1% (w/v) sucrose solution. Moreover, to make this solution isotonic we can add some amount of sodium chloride to make the total effect equivalent to that of 0.9 % (w/v) sodium chloride solution. If the sucrose solution is prepared by dissolving 1 gram sucrose to make 100 ml solution then the amount of sodium chloride needed is 0.9 - 0.08 = 0.82 gram. Thus, the formula becomes:

Sucrose                   1 gram.
Sodium chloride       0.82 gram.
Water q.s.               100 ml.


Related posts:
Isotonic solutions in pharmaceutical preparations.
Examples on calculations for pharmaceutical isotonic solutions.