The intrinsic dissolution rate of 4 substances with different solubility (borax, nicotinic acid, disodium oxalate and theophylline) was measured by means of a rotating disc technique. Compressed pellets, mounted in a holder, were rotated in the dissolution medium at various speeds. The obtained results allow the calculation of the effective diffusion coefficients of the substances under examination. Pores drilled into the surface of the pellets (pore diameter 0.20-2.00 mm) caused an increase of the dissolution rate because of changing hydrodynamics. The diameter of a drilled pore must exceed a critical value to bring about this rise in dissolution rate. Apart from the diameter, the depth of the pore is an important factor determining to what extent the dissolution rate is increased. The influence of the pore depth decreases with an increasing depth : diameter ratio. The critical pore diameter was found to be the same (about 0.2 mm) for all the substances investigated. This implies that its size depends only on the hydrodynamic conditions (that are affected by the geometric change of the solid/liquid interface), and not on the physicochemical properties of the dissolving solid. Therefore these results have a general applicability for all compounds dissolving under similar hydrodynamic conditions. © 1983.
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