sodium chloride equivalent
When a formulator needs to make a drug solution isotonic, the easy question is: how much salt should I add? But every ingredient already in the solution contributes some of its own osmotic effect. The sodium chloride equivalent, written E, is a tidy bookkeeping number that converts any drug into the equivalent amount of common salt — it is the weight of sodium chloride that produces the same tonicity as one gram of that drug.
For example, if a drug has an E value of 0.20, then one gram of it pulls its weight osmotically like 0.20 grams of sodium chloride. To make a solution isotonic, you calculate how much salt the whole formula already provides (each ingredient's weight times its E value), compare it with the amount of salt that would be needed for isotonicity, and add the shortfall as sodium chloride or another tonicity adjuster.
E values come from each substance's effect on a colligative property — usually the freezing point depression — and are tabulated in pharmacopoeias and reference texts, often at the 0.5% or 1% concentration commonly used for the calculation.
A caveat: E values shift slightly with concentration because dissociation and interactions are not perfectly constant, so the tabulated figure is a close approximation rather than an exact constant. For precise work, the value at the relevant concentration is used.
Pilocarpine hydrochloride has an E value near 0.24, so 1 g of it contributes the tonicity of about 0.24 g of salt — a fact used when compounding isotonic eye drops.
Convert every solute into grams-of-salt, total them, and top up to isotonic.
The E value is the practical sibling of the freezing-point-depression method. Both make solutions isotonic, but the E approach lets you tally every ingredient against a single common reference — sodium chloride — which is why it is favoured in dispensing calculations.