Two questions, two tests
Before a drug can be absorbed, it usually has to leave its dosage form and enter solution. So we ask two different questions. The disintegration test is the simpler one: does the tablet or capsule break apart into small fragments within a set time? It is a pass/fail on physical break-up, not on the drug itself. The dissolution test goes further: how much of the active drug has actually gone into solution by a given time? Disintegration is necessary but not sufficient — a tablet can crumble into particles that then sit there, undissolved.
The dissolution apparatus
The standard dissolution apparatus is a set of vessels in a 37 °C water bath, each holding a defined volume of dissolution medium that mimics gut fluid. The two everyday workhorses are USP Apparatus 1, the rotating basket, and USP Apparatus 2, the rotating paddle. In the basket, the dosage unit sits in a mesh cage that spins; in the paddle method, the unit drops to the bottom of the vessel and a paddle stirs the medium above it. Baskets suit capsules and units that would otherwise float; paddles suit most tablets. Samples are withdrawn at set times and assayed to build a release curve.
For an immediate-release product, the spec might read “not less than 80% (Q) dissolved in 30 minutes,” tested in stages (S1, S2, S3) on growing numbers of units. The shape of the curve matters as much as a single point: it is the fingerprint of how the product releases its dissolution rate.
Sink condition and why the numbers add up
A dissolution test only measures the dosage form if the medium never gets crowded with dissolved drug. We keep a sink condition: the medium volume is large enough (a common rule of thumb is at least three times the volume needed to saturate the dose) that the dissolved concentration stays far below saturation. Then dissolution is limited by the product, not by the medium running out of room. This is the Noyes-Whitney equation in action: the rate falls as the bulk concentration C rises toward the saturation solubility Cs, so a sink keeps (Cs − C) close to Cs and the test stays sensitive.
Sink condition check
Dose = 100 mg
Drug solubility (Cs) = 0.5 mg/mL
Dissolution medium = 900 mL
Volume to dissolve the whole dose at saturation:
V_sat = dose / Cs = 100 mg / 0.5 mg/mL = 200 mL
Sink ratio = medium volume / V_sat
= 900 mL / 200 mL = 4.5
Rule of thumb: ratio >= 3 -> sink condition is met (4.5 > 3). OK.
If fully dissolved, bulk concentration C reaches:
C = 100 mg / 900 mL = 0.111 mg/mL
That is only 0.111 / 0.5 = 22% of saturation, so (Cs - C)
stays near Cs and dissolution rate is not throttled by the medium.Done well, dissolution becomes more than a release gate. When the in-vitro curve tracks blood levels in people, you have an in-vitro-in-vivo correlation (IVIVC) — and the lab test starts to predict the patient. That is the prize, and the reason this single test sits at the heart of pharmaceutical quality.