JOURNAL O F RESEARC H of the National Bureau of Standards - A. Ph ys ics a nd Chemistry Vol. 72A, No. 6, N ovember- December 1968 Preparation and Solubility of Hydroxyapatite E. C. Moreno,* T. M. Gregory,* and W. E. Brown* Institute for Basic Standards, National Bureau of Standards, Washington, D.C. 20234 (August 2, 1968) Two portions of a syntheti c hydroxyapatite (HA), Ca" OH(PO'}J, full y characterized by x-ray, infrared, petrographi c, and chemi cal anal yses, were heated at 1,000 °C in air and steam atmospheres, respectively. Solubility isotherms for these two samples in the syste m Ca(OH}2 -H3PO,-H20 were determined in the pH range 5 to 7 by equilibrating the solids with dilute H3 PO. solutions. Both sam­ ples of HA disso lved stoichiometrically. The activity products (Ca + +)'( OH - ) (pO~f' and their standard errors-obtained by a least squares adjustment of the measurements (Ca and P concentrations and pH of the saturated solutions) subject to the conditions of electroneutralit y, constancy of the activity product, and stoichiometric dissolution - were 3.73 ± 0.5 x 10- 58 for the steam-h eated HA and 2_5, ± 0.4 x 10- 55 for the air-heated HA. Allowance was made in the calculations for the presence of the ion pairs [CaHPO. lo and [CaH2P04 1+ The hi gher solubility product for the air-heated HA is as­ cribed either to a change in the heat of formation brought about by partial dehydration or to a state of fine subdivision resulting from a disproportionation reacti on_ The solubility product constant s were used to cal culate the points of intersection (i.e_, sin gular points) of the two HA solubility isot herms with the isotherms of CaHPO. ·2H20 and CaHPO.; it was found that the pH's of the sin gul ar points for the air-heated HA were a full unit hi gher than those of the steam-heated preparation_ Conditions are described for the precipitation of HA crystals suitabl e for solubility measuremen ts_ Key Words: Apatite; hydroxyapatite; calcium phosphates; solubili ty; olubility isotherms; solubility product. 1. Introduction based on the stoi chi ometry of the salt. Among the models th at have been advanced to explain the Hydroxyapatite (HA), CasOH(P0 4 h, has wide apparent anomalous behavior of HA in aqueous solu­ biological and industrial importance. Its physico­ tions, those advocating formation of complexes on the chemical properties, in particular solubility and de­ surface of the solid [5 , 8J have received considerable tailed crystallographic structure, are directly related attention. These models implicitly assume that to the mineralization processes in biological systems. equilibrium between the solid and the liquid phases The various reports in the literature on solubility of is not achieved with the crystalline portion of the HA, HA are contradictory. Clark [l J, Ion the basis of but instead with surface layers having quite different precipitation and dissolution experiments, concluded chemical composition from that of the bulk of the that HA has a definite solubility product. Thermody­ solid. Furthermore, the compositi on of such layers namic functions for HA (from whi ch a solubility prod­ has been assumed to be co nstant over a wid e range of uct constant can be calculated) have also been reported calcium, phos phorus, and hydrogen concentrations [2 , 3, 4]. Other reports [5- 8], however, claim that in th e liquid phase. systems saturated with respect to HA do not con­ At least part 01 th e proble m in the interpretation form to the principle of a solubility product constant of data on solubility of HA is related to the difficulty in preparing pure samples of this compound. So *Research Associates from the A me ri can Dent al Associati on of the Nati onal Bu rea u far, the best crystals have been obtained by hydro­ of Stand ards, Washington, D.C. 20234. This investi gat ion was support ed in part by Research Grant s DE- OJ8J9 and DE-02659 thermal methods, but the yields are smail, the equip­ to th e Ameri can Dental Associati on fr om th e National In stitute of Dental Research and is part of th e de nt al research program condu cted by the Nati onal Bureau of Stand ards, in ment is relatively expensive and the product may cooperation with th e Council on De ntal Research of th e Am erican De nt al Associati on­ contain extraneous phases such as ,B-calcium pyro­ th e National Instit ute of Dent al Researc h; th e Arr..y De ntal Corps; th e Aerospace Medicai Division, USAF School of Aerospace Medi cin e; and the Veterans Administration. phosphate and/or ,B-tricalcium phosphate. Direct I Figures in brackets ind icate th e literature references at th e end of thi s pape r. precipitation methods usually yield very fin ely divided 773 products, often without the stoichiometric composition the reaction vessel for petrographic examination of of HA. A particularly serious difficulty is contamination the solid and pH determination of the liquid. by precipitation of the more acid calcium phos­ Initially, 9 1 of N ammonium acetate solution were phates, among which octacalcium phosphate (OCP), boiled in the reaction vessel for about 12 hr to remove CaSH 2(P04}6 • 5H20 and its decomposition products CO2 ; then ammonia gas was passed in until the pH are the most difficult to eliminate. reached a value between 8.5 and 9, and the pumping T he purposes of thi s work were (a) to devise a sim­ of the phosphate and calcium solutions was started, ple method for preparation of HA that would yield maintaining vigorous boiling. The pH was maintained sufficiently large and pure crystals so that an un­ between 8.5 and 9.5 (measured at room temperature) ambigious characterization would be possible, (b) by an adequate flow of NH3 until the total volume of I to learn whether this compound behaves as a normal the solutions was pumped into the reaction vessel; chemical compound with unique thermodynamic then the flow of NH3 was discontinued but boiling properties when equilibrated with aqueous solutions, was continued for about 3 hr. The solid was allowed and (c) to establish its solubility isotherm and solu­ to settle, the supernatant liquid was withdrawn, bility product constant at 25°C. about 9 1 of boiled distilled water were introduced and the mixture was boiled for about 2 hr. This 2. Experimental Methods and washing operation was repeated as was deemed nec­ Procedures essary to remove the soluble salts (usually five times). Further washing was done by decantation, using boiled distilled water and short settling periods so In the method of preparation outlined here, the as to discard as much as possible the finest fraction possibility of OCP precipitation is minimized by the of the precipitate. Precautions to avoid contami­ use of relatively high temperature and pH. However, nation by atmospheric CO2 were taken during all pH values higher than 10 are avoided because they these operations. The suspension was then filtered seem to favor the formation of very finely divided and the solid was dried at 150 °C for 48 hr. As ex­ precipitates. Other conditions are (a) slow addition plained in section 3, a portion of this sample was of reagents to reduce continuous nucleation, (b) heated at 1,000 °C in air and another portion was heated high ionic strength and the presence of acetate ions at the same temperature in steam at one atmosphere. (a Ca complexing agent) to increase the solubility of HA, and thus to favor formation of larger crystals, (c) use of a large reaction vessel to perform the pre­ 2.2. Equilibrations cipitation without sacrificing yields, and (d) special Equilibrations were made in glass-stoppered bot­ precautions to avoid contamination with carbonate tles, each of which contained 5g of HA and 120 ml from the atmosphere or reagents (ammonia was used of a phosphoric acid solution, by slowly rotating end­ to control the pH instead of NaOH or KOH because over-end in a water bath at 25 ± 0.1 0C. After two the alkali hydroxides are difficult to obtain free of weeks, the suspension was transferred to a thermo­ carbonate contamination). stated saturator, described in an earlier publication [9], in which the pH of the clear filtered solution was 2.1. Prepara tion of HA measured. Samples of this solution were taken for calcium and phosphorous analyses. Standardization Freshly boiled distilled water and chemically of the pH-meter (with claimed relative accuracy pure salts were used to prepare 2.5 liter of 7.25 X 10- 2 of ± 0.0037 pH units) was made with certified NBS M (NH4}zHP04 solution and 2.5 liter of 13.33 X 10- 2 M buffer standards [10]. Ca(N0 ·4H20 solution; their pH's were adjusted 3h Phosphoric acid solutions used in the equilibra­ with ammonia gas to 8.5-9. These solutions were tions were prepared from doubly crystallized stored in borosilicate glass bottles vented through 2~P04 . H2 0 and freshly boiled conductance water. ascarite tubes. The molarities of the final solutions were determined The reaction vessel was a 22-1, 3-neck, borosili­ by chemical analyses. cate glass flask. A 4-ft condenser was attached to one neck; transfer of reagents and materials was done through the other two necks. The top of the condenser 2.3. Calcium and Phosphorus Determinations was connected by a glass tube to a 6-1, 3-neck flask containing 3 liters of 10 N NaOH solution; a check Calcium was determined colorimetrically by the valve permitting outward flow of gases was con­ indirect technique described by Banerjee et al.