Dissolution of Lithium and Magnesium from Lithium Carbonate Capsules Containing Magnesium Stearate

A lithium carbonate capsule mix designed for an automatic capsule-filling machine used about 0.25% magnesium stearate as a lubricant. While the formulation ran well, it had an undesirably long disintegration time and a slow dissolution rate due to the waterproofing effect of the lubricant. Studies on the addition of sodium lauryl sulfate separately to both the formulation and the dissolution medium showed that 2 mg (0.002%) added to the capsule (12 mg/600 ml of 0.3% HCl) was more effective in improving lithium dissolution than 6 g (1%) added directly to 600 ml of medium. Furthermore, while the addition of 0.02% sodium lauryl sulfate to the dissolution medium reduced the surface tension of the medium by more than 50%, it had no effect on dissolution rate; increasing the amount of sodium lauryl sulfate from 0.03 to 0.5% in the dissolution medium had a pronounced effect on dissolution rate. Magnesium dissolution from magnesium stearate and lithium dissolution from lithium carbonate were directly related.