A Comparative Study of the Synthesis of Calcium, Strontium, Barium, Cadmium, and Lead Apatites in Aqueous Solution

Abstract

The aqueous syntheses of the hydroxy and halo apatites of calcium, strontium, barium, lead, and cadmium were explored. Because these cations represent the main group s‐ and p‐fillers and a transition metal, they present different synthetic challenges. The alkaline earth cations and lead form hydrogen phosphates at slightly acidic and slightly basic conditions, the alkaline earths form the fluorides (MF₂) in the preparation of the fluoroapatites with excess fluoride, and ammonia is required for the preparation of the cadmium apatites through decomplexation. A variety of reagents were utilized with most of the derivatives, but, in general, the source of the cation was its nitrate or halide, and the phosphate is best provided as ammonium hydrogen phosphate. The requirements for pH, heating, and reaction time were also explored. A number of literature syntheses for pure phase apatites could not be reproduced: calcium iodoapatite, strontium fluoroapatite, and cadmium hydroxyapatite. Several apatites were prepared for the first time in aqueous solution: barium fluoroapatite, lead bromoapatite, and cadmium chloroapatite. The relative ease of formation of the compounds is rationalized with arguments based upon lattice and hydration energies.

Keywords:

• Apatite
• Aqueous synthesis
• Pure phase
• XRD

Acknowledgments

The authors are indebted to the William M. and Lucille M. Hackman Scholars Program, the Howard Hughes Medical Institute, and the Camille and Henry Dreyfus Foundation for a Jean Dreyfus Boissevain Undergraduate Scholarship for Excellence in Chemistry.