PHARM/PCS V5 · Procedure 1
Dosage and Concentration: Drug Stock Solutions
Work out how much stock solution to draw for a dosing solution of a given molarity and volume, and how much sodium chloride makes it isotonic.
What it computes
Prepares a specified volume of drug solution at a specified molarity from a stock solution. Computes the volume of stock required and, for electrolytes, the amount of NaCl needed for isotonicity.
Inputs and outputs
You provide
- Molecular weight, desired molarity, and desired volume
- Stock concentration, as percent (g/100 mL) or g/mL
- Whether the drug is an electrolyte, and if so its dissociation factor i from Table 1.1 (1.0, 1.8, 2.6, or 3.4)
PHARM/PCS reports
- Grams of drug needed
- Volume of stock solution to draw
- For an electrolyte: the sodium chloride equivalent and the NaCl to add for isotonicity
- Volume of water to add
When to use it
At the bench, whenever you're making up a dosing or perfusion solution and need to know how much concentrated stock to draw, how much diluent to add, and (for a salt that dissociates in solution) how much sodium chloride to add so the final solution is isotonic with tissue and won't cause osmotic damage.
Reading the results
The isotonicity correction hinges on the dissociation factor (i) you pick from Table 1.1: match it to the number of ions your drug releases, since the wrong i throws off the NaCl amount. The volume returned is only as good as the stock label, so confirm the stock concentration and its units (percent vs. g/mL) first. For a non-electrolyte, ignore the NaCl output and treat it as a straight dilution.
Checked against the book
The procedure uses the simplified form of the book's Equation 1.4, E = 32.5·i/W, and reproduces the worked example for phenylephrine HCl exactly: 32.5 × 1.8 / 204 = 0.29.
This page describes the procedure. Calculations run inside the licensed PHARM/PCS V5 application. PHARM/PCS is a trademark of MCS Scientific LLC.