Isotonicity & E-Value Practice Problems
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Solutions placed in the eye, nose, or bloodstream should be isotonic with body fluids (equivalent to 0.9% NaCl). The sodium chloride equivalent (E-value) tells you how much NaCl produces the same osmotic effect as 1 g of a given drug, letting you calculate how much NaCl to add to make a solution isotonic.
Formula(1) drug in formulation = %×vol ÷ 100 (2) NaCl equivalent = drug g × E (3) NaCl for volume = 0.009 × vol (4) NaCl to add = step 3 − step 2
Worked example
Make 30 mL of 1% pilocarpine HCl (E = 0.24) isotonic. (1) drug = 0.3 g; (2) 0.3 × 0.24 = 0.072 g; (3) 0.009 × 30 = 0.27 g; (4) 0.27 − 0.072 = 0.198 g.
💡 The exam gives you the E-value — the skill is the 4-step setup, not memorizing E-values.