Figure 7.3 shows a schematic diagram of USP Apparatus 4. This apparatus consists of a flow through cell into which the dosage form is placed. Dissolution medium is continually pumped through the flow cell and the eluent analyzed for drug content. This apparatus finds its greatest utility in the testing of poorly soluble drugs as sink conditions can be maintained by continually passing fresh dissolution fluid over the dosage unit. This apparatus finds more widespread use in Europe than elsewhere.
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