A convenient and rapid method for determining diffusion coefficients has been developed that is based on the quasi-stationary diaphra m cell method. A silver filter membrane is used to separate two compartments otthe apparatus that are stirred magnetically with synchronous motors. The advantages are the good precision of the data, running times of the experiments, and the extended repeated usability of a given membrane. HERE RECENTLY has been much interest T shown in drug dissolution rate mechanisms as well as in other diffusionally characterized transport processes. Regardless of the situation, quantification requires diffusion coefficient determination as one parameter. A method for determining the diffusion coefficient has been described by Northrup and Anson (1) and by McBain (2). Desai et al. (3) employed a method similar to the Northrup-McBain technique using sintered-glass disks to separate the drug solution from the "sink." Singh (4) found improvement by using organic polymer membrane filters.
Almost all membrane filters lack the physical strength required for repeated use. They would, after soaking in the drug solutions, lose their integrity, causing changes of area available for transport. One membrane filter,' impregnated with glass fibers, was found to maintain itself rather well for as long as 75-80 hr. of experimentation but would then slowly slough its surface. Recently, a new membrane filter was marketed, made from silver, ΒΆ and has been used with the apparatus shown in Fig. 1. This filter has tensile strength as an advantage over the other membrane filters, and the advantage of thinness over the sintered-glass disks. This allows for faster, easier wetting of all the pores. This insures having the same porosity and tortuosity in each succeeding trial, and hence, better reproducibility. Also, the thinner the membrane, assuming all other factors are constant, the faster the rate of transport through it. This advantage, however, makes the overall diffusion rate more stirring speed dependent, due to the greater importance of the aqueous diffusion layers next to the mem-