pH

Synonyms

Potentiation of hydrogen

Definition

pH is a measure of the acidity or alkalinity of a solution. The pH scale commonly in use ranges from 0 to 14. Neutral solutions are designated as the midpoint of the scale (numerically equal to 7). As solutions become more alkaline, the number value increases; as they become more acidic, the number value decreases. Drugs tend to be weakly acidic or alkaline. The difference between the pH or a drug and the pH of the solution into which it is being dissolved can either expedite or impede the flow of a drug across a semipermeable barrier, such as the barrier between GI tract and systemic circulation or between blood and brain.

Drugs cross these barriers by a limited number of mechanisms: passive diffusion, facilitated passive diffusion, active transport, or pinocytosis. All things being equal, the drugs move toward equal distribution on both sides of a membrane. Very small molecules tend to travel across the cell membrane with little...