Figure 3. Relationship between particle size and the time required to completely dissolve particles of a given size.
Since the dissolution rate of a loosely suspended substance will depend on the particle size and surface area of the solid, the technique of intrinsic dissolution has been developed. In this method, the solid of interest is compressed into a die and embedded in a rotational disc where only one face of the compressed solid remains exposed to the dissolution medium. Under these circumstances, the area of the solid-liquid interface must remain constant during the dissolution process.
The dissolution rate (dm/dt) of a given solid is usually directly proportional to the wetted surface area ( A) of the dissolving solid:
where J is the mass flux, or the dissolution rate per unit surface area. J is usually termed the intrinsic dissolution rate. But since dC/dt is also defined according to the Noyes-Whitney equation (48), it follows that:
where B is the mass transfer coefficient, defined as:
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