Application Guide
How specific surface area, tablet porosity, and pore size distribution are measured for APIs, excipients, tablets, and porous drug carriers — with the pharmacopoeial methods that apply.
For poorly soluble drugs, dissolution rate scales with the surface area exposed to the medium (the Noyes–Whitney relationship), so the specific surface area of the active ingredient after milling or micronization is a routine specification. At the tablet level, porosity controls how fast liquid penetrates, how quickly the tablet disintegrates, and how strong it is: higher compaction pressure lowers porosity and usually raises tensile strength but can slow disintegration.
Porous carriers such as mesoporous silica are also used to stabilize amorphous drugs inside pores of a few nanometres, where the pore size and pore volume determine loading capacity and whether the drug can recrystallize.
| Property | Method | Pharmacopoeial reference | Notes |
|---|---|---|---|
| Specific surface area of API / excipient powders | Gas adsorption, BET | USP <846>; Ph. Eur. 2.9.26 | Krypton at 77 K for low-area powders (below ~1 m²/g) |
| Tablet porosity and pore size distribution | Mercury intrusion | USP <267>; Ph. Eur. 2.9.32 | Also gives envelope (bulk) density |
| True (skeletal) density | Helium pycnometry | USP <699>; Ph. Eur. 2.2.42 | Combined with envelope density to calculate porosity |
| Mesoporous carriers | N2 adsorption, BJH/NLDFT | USP <268> (porosity by nitrogen adsorption–desorption) | Track pore-volume loss after drug loading |
Tablet porosity is usually calculated as ε = 1 − ρenvelope / ρtrue. Typical compressed tablets fall somewhere around 5–30 % porosity depending on formulation and compaction pressure.
Use the calculators to convert test pressures to pore sizes, or email a short description of your sample and what you need to measure.