M. Jemal

Synthése, caractérisation et thermochimie d’apatites calco-magnésiennes hydroxylées et fluorées

Abstract Calcium-magnesium hydroxyapatite and fluorapatite solid solutions with the general formula Ca(10−x)Mgx(PO4)6X2, where X=F or OH were synthesized by precipitation in an ammonia medium. They have been characterized by infrared spectroscopy, X ray diffraction and chemical analysis. The nature of phases precipitated and the limit of magnesium substitution and formula of solid solutions were determined. Calcium-magnesium hydroxyapatite is obtained for a magnesium atomic fraction Mg/(Ca+Mg) up to 0.066. For more amount of magnesium, whitlockite (Ca18Mg2H2(PO4)14) and MgPO4NH4·H2O appear. Fluorapatite solid solutions were obtained for fraction of magnesium up to 0.101. Beyond this limit, MgPO4NH4·H2O phase appears. Using an isoperibol calorimeter the heats of solution of these products in a 9% weight nitric acid solution were determined. Combining these results with crystallographic ones allows to determine the substitution limits of magnesium in these products. This limit is in the range 0.066–0.084 for hydroxyapatites and 0.073–0.101 for fluorapatites.

Standard enthalpies of formation and mixing of hydroxy- and fluorapaties☆

Abstract Ca, Sr, Cd and Pb hydroxy- (Hap) and fluorapatites (Fap) and (caCd) and (CaPb) Hap have been synthesized and spectroscopically and chemically characterized, then dissolved in 9 wt% nitric acid using an isoperibol calorimeter. Dissolution of secondary products and other entities lead to standard enthalpies of formation of single-cation products. These values are, respectively, −13.305, −13.373, −8.648 and −8.261 kJ mol −1 for Haps, and −13.545, −13.604, −8.795 and −8.529 kJ mol −1 for Faps. Replacement of OH by F decreases the lattice volume and increases the stability of the structure. In the Hap and Fap series, the smaller the lattice volume, the worse the stability, except for Pb products. Dissolution of solid solution Haps leads to enthalpies of mixing of the limit products. The results are in agreement with the preferable occupancy of site II by Cd or Pb up to 40% and 45%, respectively.

Thermochemistry of phosphate products. Part I: Standard enthalpy of formation of tristrontium phosphate and strontium chlorapatite

Abstract Tristrontium phosphate Sr 3 (PO 4 ) 2 (triSr) and strontium chlorapatite Sr 3 (PO 4 ) 6 Cl 2 (ClapSr) have been prepared and characterized by X-ray diffraction, IR spectroscopy and chemical analysis. Using an isoperibol calorimeter the heat of solution of these products in a 9% weight nitric acid solution were determined. They are (−110.1 ± 1.5) and (−283.6 ± 2.4) kJ mol −1 for triSr and ClapSr, respectively. Combining these results with others found by dissolving the products and entities involved in thermochemical cycles, allows to determine the standard heat of formation of these Sr-phosphates. These quantities are: Δ f H °(triSr, 298 K) = −4110 kJ mol −1 and Δ f H °(ClapSr, 298 K) = −13233 kJ mol −1 . The results are discussed on the basis of the differences in bond enthalpies.

Enthalpy of formation and enthalpy of mixing of calcium and cadmium hydroxyapatites

Abstract Calcium—cadmium hydroxyapatite solid solutions with the general formula Ca 10- x Cd x (PO 4 ) 6 (OH) 2 were prepared by a double decomposition method. The samples were characterized by X-ray diffraction, infrared spectroscopy and elemental analysis. Using an isoperibol calorimeter, enthalpies of solution of these products in 10% nitric acid solution were measured. The combination of these results with those of the limiting products x = 0 and x = 10 allows the determination of the enthalpies of mixing. Reaction schemes are proposed in order to determine the standard enthalpy of formation of cadmium and calcium hydroxyapatites.

Synthése et thermochimie de phosphates au cadmium. Partie II : Enthalpie standard de formation de fluorapatites calco-cadmiées

Abstract Cadmium–Calcium fluorapatite solid solutions with the general formula Ca 10− x Cd x (PO 4 ) 6 F 2 were prepared by a double decomposition method. The samples were characterized by X-ray diffraction, infrared spectroscopy and chemical analysis. The limit product corresponds to x =6. Using an isoperibol calorimeter, enthalpies of solution of these products in 9% weight nitric acid were measured. A thermochemical cycle is proposed and complementary processes are carried out in order to get the standard enthalpies of formation of these apatites. The variation of the enthalpy of solution versus X =Cd/(Ca+Cd) ratio, shows a minimum at about X =0,4. This is probably related to the existence, in the hexagonal apatite structure, of two types of cationic sites.

Enthalpie De Formation Et De Melange De Phosphoapatites Calco-Cadmiees Chlorees

Calcium–cadmium chlorapatites solid solutions with the general formula Ca10–xCdx(PO4)6Cl2,1≤x≤10, were prepared by solid state reaction and characterized by X-ray diffraction, infrared spectroscopy and chemical analysis. Using an isoperibol calorimeter, their enthalpies of solution in 9 mass% nitric acid were measured. In order to determine the enthalpies of formation and enthalpies of mixing, thermochemical cycles were proposed and complementary experiences were performed. The results obtained show a decrease of the enthalpy of formation with the amount of cadmium introduced in the lattice. The variation of mixing enthalpy vs. x=Cd/(Cd+Ca) shows a maximum at about x=0,4. This could be explained by the existence of two cationic sites in the phosphoapatite structure.

Dissolution de l'hydroxyapatite et du phosphate tricalcique β dans les solutions d'acide nitrique

Using an isoperibol calorimeter for rapid reactions and a Calsol type microcalorimeter for slow processes, are applied to determine the enthalpies of solution of two synthetic phosphate products in nitric acid. Namely, β tricalcium phosphate Ca3(PO4)2 and the calcium hydroxyapatite Ca10 (PO4)6 (OH)2 are measured by varying pH value of the solvent. Some dissolution mechanisms are proposed for various pH values. They are ensured by complementary reactions of solution of Ca(NO3)2, Ca(H2 PO4)2 and H3 PO4 in the same solvents. An extrapolation of solution enthalpies to pH=7 leads to the enthalpy of solution of these products in the pure water. These values are ΔsolH °=–138.3 kJ mol–1 for Ca3 (PO4)2 and –393.6 kJ mol–1 for Ca10 (PO4)6 (OH)2 .