sink condition

A sink condition is like dissolving sugar in a swimming pool rather than a thimble: the surrounding liquid is so vast that whatever dissolves barely raises the concentration, so the solid keeps dissolving steadily as if the water were always fresh.

Formally, sink conditions exist when the dissolved drug concentration in the medium stays well below its saturation solubility — by convention often kept below about one-third (sometimes one-tenth) of saturation. Because the concentration gradient (Cs − C) in the Noyes-Whitney equation barely shrinks, the dissolution rate is governed by the dosage form itself rather than by approaching the solubility ceiling.

Sink conditions are deliberately engineered into dissolution testing so that results reflect the product and mimic the body, where absorption and circulation continually remove dissolved drug. Practically, this means choosing a large enough volume of dissolution medium, or adding a surfactant or adjusting pH to raise solubility, so that a poorly soluble drug is never limited by the test vessel itself.