At equilibrium the ionic product of a salt is called the solubility product . The solubility product is not a measure of the solubility of a salt but can be used to calculate it. Addition of a common ion affects the ionic product. This has important implications in the mouth since the concentration of calcium and phosphate ions in saliva and plaque fluid can be influenced by external factors. A variety of calcium phosphate salts are found in mineralised tissue but the major salt is an imperfect hydroxyapatite called biological apatite or, sometimes, apatite.
Biological apatite is much more soluble than hydroxyapatite.
Reliable solubility products for hydroxyapatite and fluorapatite are not available but fluorapatite is less soluble and has a lower solubility product. The variabilty of biological apatite means that there is no meaningfull solubility product. The major contaminating ions in biological apatite are strontium, barium, potassium, lead, sodium, magnesium, mono-hydrogenphosphate and carbonate. Carbonate is of major importance in apatite because it very significantly increases solubility.