Isotonicity Calculator (Sodium Chloride Equivalent)

How it works

drug (g) = strength % ÷ 100 × volume (mL) · NaCl equivalent = drug (g) × E · NaCl for isotonicity = 0.009 × volume (mL) · NaCl to add = 0.009 × V − drug × E · other agent (g) = NaCl to add ÷ E of the agent · White–Vincent volume (mL) = drug (g) × E × 111.1

Body fluids such as tears and blood have the same osmotic pressure as 0.9% sodium chloride, so eye drops, nasal sprays and injections are made isotonic to avoid stinging and tissue damage. The sodium chloride equivalent (E-value) is the grams of sodium chloride that produce the same tonicity as 1 g of the drug. Multiply the drug weight by its E-value, subtract that from the 0.9 g of sodium chloride per 100 mL an isotonic solution needs, and add the difference as sodium chloride — or divide it by the E-value of boric acid or dextrose if sodium chloride is incompatible (for example with silver nitrate). E-values follow the standard tables in Remington and Ansel’s Pharmaceutical Calculations.

Worked example

Ephedrine sulfate 1% in 30 mL: the drug weighs 0.01 × 30 = 0.300 g and is equivalent to 0.300 × 0.23 = 0.069 g of sodium chloride. An isotonic 30 mL needs 0.009 × 30 = 0.270 g, so add 0.270 − 0.069 = 0.201 g of sodium chloride. By the White–Vincent method the drug alone would be isotonic in 0.3 × 0.23 × 111.1 = 7.67 mL of water.

Sources: Remington: The Science and Practice of Pharmacy — Tonicity and E-values (via ScienceDirect) · Wikipedia — Tonicity

Frequently asked questions