PHARM/PCS V5 · Procedure 23 Drug Decay and Binding
Henderson-Hasselbalch Equation
Find the ratio of ionized to unionized drug at a given pH, the starting point for questions about absorption and partitioning.
What it computes
Computes the ratio of ionized to unionized drug concentration from the drug's pK and the pH of the medium.
Inputs and outputs
You provide
- Whether the drug is a weak acid or a weak base
- The drug's pK
- The pH of the medium
PHARM/PCS reports
- The ratio of ionized to unionized drug at that pH
- A curve of percent ionized against pH, marking the pK and your pH
When to use it
When you need to know how much of a weak acid or base is ionized at a given pH, for predicting absorption, membrane permeability, or partitioning across compartments at different pH (gut, plasma, urine).
Reading the results
Mind the direction: for a weak acid, raising pH increases the ionized fraction; for a weak base, it decreases it, and confusing which you have inverts the answer. The ratio is most sensitive near pH = pK (where it's 1:1), so small pH errors matter most there.
A good first exercise
No statistics here, just the equation and two values. It makes a clean classroom demonstration of how pH governs ionization, and it sets up the absorption and partitioning questions that the pharmacokinetics procedures answer.
This page describes the procedure. Calculations run inside the licensed PHARM/PCS V5 application. PHARM/PCS is a trademark of MCS Scientific LLC.