Martin A. Erickson III, RPh
Mr. Erickson is director of professional affairs at Gallipot Inc.
I am compounding a few tablets using a hand press;
the formulation requires calcium phosphate dibasic,
and both anhydrous and dihydrate are available. Which
should I use?
Tablet formulation can require pharmaceutical engineering
at the production level due to incompatibilities
or other difficulties. A hand press is advised here for
FDA regulatory purposes.
Anhydrous dibasic calcium phosphate USP has a molecular
weight of 136.06 and is a white, odorless, tasteless powder/crystalline
solid (density 2.89 g/cm3; decomposes at 425°C to calcium
pyrophosphate). It contains only adsorbed moisture 0.1% to 0.2%
and cannot be rehydrated to form the dihydrate. It is practically
insoluble in water, ethanol, and ether, but is soluble in dilute acids;
is nonhygroscopic and stable, but should be stored in a closed
container in a dry place; and is used as nutritional supplement for
calcium and phosphorus and as a tablet and capsule diluent.
Incompatibilities include tetracyclines. The milled material's surface
is alkaline—do not use with drugs sensitive to pH >7. The
unmilled form has an acidic surface and may affect drug stability,
especially when particle size changes. Although it flows well, high
pressure can cause lamination and "capping."
Dibasic calcium phosphate dihydrate USP has a molecular
weight of 172.09 (appearance of the anhydrate; density 2.389
g/cm3). It dehydrates, losing 2 molecules of water, below 100°C;
exhibits solubility as the anhydrous form; is nonhygroscopic and
stable, but can lose water of crystallization; and should be
stored in well-closed container in a cool, dry place. It is used as
a capsule and tablet diluent. It is probably the better choice for
your formulation. Incompatibilities include tetracyclines, ampicillin,
aspartame, aspirin, cephalexin, erythromycin, and
indomethacin. The surface is alkaline—do not use with drugs
unstable at pH >7.
Both forms are abrasive, and formulations should use a lubricant
to preserve tableting equipment.
E-mail your compounding questions to .
