The Existence of Pure‐Phase Transition Metal Hydroxy Apatites

Abstract The existence of pure‐phase transition metal hydroxy apatites of zinc, copper, manganese, and cobalt could not be verified by repeated attempts to duplicate the few literature procedures reported for their preparation in aqueous solution. Variations of temperature, time of reactions, pH, and use of decomplexation also did not produce apatitic compounds. In the case of zinc, the product resulting from the addition of zinc nitrate to ammonium dihydrogen phosphate in solutions brought to pH >9 with ammonia appears to be a zinc ammonia complex with phosphate as the counter ion. Extensive drying removed ammonia to produce the phosphate. At pH <8.5 several hydrates of zinc phosphate were formed. With copper, the use of literature procedures and variations thereon resulted in libethenite, Cu2(PO4)OH. The literature procedure for the cobalt(II) apatite using ethylenediamine produced no precipitate; at pH 5, Co3(PO4)2 · H2O was formed in the presence of ethylenediamine. The Mahapatra procedure for the manganese apatite produced a mixture of Mn3(PO4)2 · 3H2O and hureaulite, Mn5(PO4)2[PO3(OH)]2, whereas the Rao method resulted in (NH4)MnPO4 · H2O. On extensive heating all of the products decomposed to the anhydrous phosphate. The instability of the apatites in aqueous solution is attributed to their solubility, supported by an approximate calculation of ΔG dissolution for the zinc apatite. The underlying thermodynamic reason for the solubility appears to be the large negative heat of hydration of the relatively small, polarizable transition metal cations.