Media Degassing

All dissolution media should be degassed before it is placed in the dissolution vessel. Incorrectly prepared media can cause results that are either to great or too small. High results can occur when the bubbles adhere to the surface of the tablet particles, increase their buoyancy, and raise them to a higher part of the vessel where the media flow is quicker. Lower results can occur if the bubbles adhere to the surface of the basket or the tablet forming a physical barrier between the tablet and the media.

If a liquid is left to stand in air, they will form a natural equilibrium over a period of time so that a portion of the gas is dissolved in the liquid. The volume of dissolved gas is dependent on the temperature of the media and the pressure of the gas. Unlike the behaviour with solids the saturation level of gases decreases with temperature. The equilibrium value of oxygen in water drops from 8.74mg/l at 22°C to 7.73mg/l at 37°C (130% of the value at room temperature). Therefore if the media is heated it becomes supersaturated with air and air bubbles will form.

The effects on a dissolution rate can be significant, particularly with prednisone tablets. In some test results the rate of dissolution was higher by more than 30% when all the dissolved gases were removed. There is a linear relationship between the amount of dissolved gas and the rate of dissolution and a lot of low failures may be caused by the effects of unwanted gases in the media.

The release of gases from the media can be prevented if the level of gas is reduced before heating — to avoid possible problems a value at least 5% below saturation at the test temperature should be provided.

Typically the deaeration is measured by volume of dissolved oxygen present in the media as it is easy to measure directly with sensitive electrodes and has a virtually pH independent solubility in water. Oxygen is much easier to measure than nitrogen and the relationship between the level of air and that of oxygen is well correlated. It is important that the level of oxygen is always given an absolute value in parts per million (ppm). Values that are given as a percentage of dissolved air are not sufficient as these are dependent on the temperature of the media. When using an oxygen meter it is important that the media being prepared is water as an additive such as acid or buffer may adversely affect the reading.

Further Information