In general an apparatus means a machine having a specific function. In our case, a dissolution apparatus means “a machine which may be used to determine dissolution characteristics (function) of a drug product such as a tablet or capsule”. This function (dissolution) can only be achieved if the machine is able to provide thorough but gentle (low rpm) stirring and mixing within a vessel. Another way of saying the same thing is that the machine should provide thorough but gentle product/solvent interaction at low rpms, ie, less than 100.
The reason the machines shown above may not be considered as dissolution apparatuses is that they do not provide an appropriate and required product/solvent interactions, thus drug dissolution. All these machines required very high speed (rpms) for mixing. At lower rpms the test products often lie around stagnant, thus incorrectly indicating limited or no dissolution.
A critical requirement for a dissolution apparatus is that it should be capable of providing stirring and mixing at low rpms and it must also avoid stagnation of the test product.
