The evaluation of a column-type dissolution apparatus

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Abstract
An improved column-type dissolution apparatus is described. The column of the apparatus is built from standard screw-cap connected ‘Sovirel’ glass tubes which should enable inter-apparatus variability to be reduced. The main problem with this type of device is the build up of back pressure as fragments from the disintegrating solid dosage form progressively block the filtration system, itself necessary to stop the solution process and present the solution for analysis. A simple type of light-activated switch is therefore used which works across the indicator float of a flow meter to drive a motor connected to a needle valve. As the float falls the motor is switched on to open the valve and increase the flow rate until the float rises again and causes the motor to switch off. Observation on the dissolution characteristic of dyed lactose tablets and the injection of dye solutions into the flowing stream of liquid around tablets showed that there was back-flow of solution at Reynolds Numbers below 10 but visible instability of flow at Re > 100. Over a range of flow rates between Re 10–70 the mass of drug released at a given time was a direct function of flow rate for a film coated product but an inverse function for a sugar coated tablet. The results are discussed in relation to the main problems which can be experienced with a column-type dissolution apparatus.