Solubility of Mg-containing β-tricalcium phosphate at 25 °C

The equilibrium solubility of Mg-containing β-tricalcium phosphate (βMgTCP) with various magnesium contents was determined by immersing βMgTCP powder for 27 months in a CH3COOH–CH3COONa buffer solution at 25 °C under a nitrogen gas atmosphere. The negative logarithm of the solubility product (pKsp) of βMgTCP was expressed as pKsp = 28.87432 + 1.40348C − 0.3163C2 + 0.04218C3 − 0.00275C4 + 0.0000681659C5, where C is the magnesium content in βMgTCP (mol.%). The solubility of βMgTCP decreased with increasing magnesium content owing to the increased structural stability and possible formation of a whitlockite-type phase on the surface. As a result, βMgTCP with 10.1 mol.% magnesium had a lower solubility than that of hydroxyapatite below pH 6.0. βMgTCP was found to be more soluble than zinc-containing β-tricalcium phosphate given the same molar content of zinc or magnesium. The solubility of βMgTCP and release rate of magnesium from βMgTCP can be controlled by adjusting the Mg content by selecting the appropriate pKsp.