Dissolution Solutions Network

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Dissolution Testing: Sink Conditions

Posted on | January 19, 2007 | Comments Off

Here I sit in my kitchen with dishes in the sink. I am reminded of an old joke. An engineer, a physicist, and a mathematician, and a statistician are all staying at a hotel. In the middle of the night the engineer wakes up to find that his trashcan is on fire. He runs to the sink, fills his ice bucket with water and douses the flames. Then, just to be sure, he runs back to the sink, refills the bucket and dumps more water into the trashcan. With the fire out, he goes back to sleep.

A little while later, the trashcan in the physicist’s room spontaneously breaks into flame, waking the physicist. He whips out his slide rule, does some calculations, then runs to the sink, fills his bucket with exactly .75 liters of water, and douses the flames. Having put out the fire, he goes back to sleep.

A few minutes later, the mathematician wakes up to see that his trashcan is on fire. He whips out a piece of paper, scrawls out some equations, then goes back to sleep, comfortable that a solution exists.

Meanwhile, the statistician is running from room to room lighting trashcans on fire — he needed more samples….

Sink conditions refer to the excess solubilizing capacity of the dissolution medium. To calculate sink divide the concentration of the saturated solution by 3, 5 or 10, depending which limit you use, to determine the maximum working concentration of the active drug in the proposed media. Fick’s first law of diffusion states: Rate of solution=(D*A*(Cs-Cb))/h ,where D is the diffusion coefficient, A the surface area, Cs the solubility of the drug, Cb the concentration of drug in the bulk solution, and h the thickness of the stagnant layer. If Cb is much smaller than Cs then we have so-called “Sink Conditions” and the equation reduces to Rate of solution=(D*A*Cs)/h. In this case the Rate of solution is directly proportional to the concentration of the drug in bulk solution. This should give a nice sigmoid curve.


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