W.E. Brown

Infra-red spectra of hydroxyapatite, octacalcium phosphate and pyrolysed octacalcium phosphate.

Infra-red spectra of hydroxyapatite, octacalcium phosphate and pyrolysed octacalcium phosphate were studied in the range 4000-400 cm−1, and probable band assignments are given. Unreported bands were found for both octacalcium phosphate (thirteen bands) and hydroxyapatite (five bands); the hydroxyapatite band at 631 cm−1 was reassigned to the librational mode of the hydroxyl group. The pyrolysis reactions of octacalcium phosphate were studied by infra-red absorption, weight loss, and pyrophosphate analysis. Simple dehydration was continuous from 50 to about 400 °C. The major products between 325 and 600 °C were hydroxyapatite and β-Ca2P2O7; between 650 and 900 °C, the products were β-Ca2P2O7 and β-Ca3 (PO4)2. Formation of β-Ca2P2O7, instead of γ-Ca2P2O7, was detected at a lower temperature (325 °C) than has been reported. The maximum amount of pyrophosphate formed, about 40 per cent at 500 °C, was intermediate between the amounts predicted by reactions postulated in other studies. From 700 to 900 °C, the pyrophosphate content approximated the expected value.

Setting Reactions and Compressive Strengths of Calcium Phosphate Cements

Setting reactions and compressive strengths of a self-hardening calcium phosphate cement (CPC) were investigated. The CPC consists of tetracalcium phosphate (TTCP) and anhydrous dicalcium phosphate (DCPA). The cement specimens were prepared by mixing 0.7 g of the powder (TTCP 72.9 wt% + DCPA 27.1 wt%) with 0.175 mL of the liquid (25 mmol/L H3PO4 and 1.32 mmol/L sodium fluoride). The specimens were removed from the molds at pre-determined time intervals after being mixed, and their compressive strengths were measured. Immediately afterward, the fractured specimens were rapidly frozen in ethanol (- 80°C), lyophilized, and examined by powder x-ray diffraction and scanning electron microscopy (SEM). The results showed that (1) hydroxyapatite was the only reaction product; (2) the reaction was nearly completed within four h, during which both the reaction product and compressive strength increased linearly with time, resulting in a strong correlation between the two; and (3) fully set CPC consisted primarily of small rod-like crystals and some platy crystals.

A mechanism for incorporation of carbonate into apatite.

SummaryOctacalcium phosphate (Ca8H2(PO4)6·5H2O) is considered to be a precursor in the formation of apatite in bones and teeth; a crucial step for incorporation of impurities appears to occur during its hydrolysis. The present study examines the role that octacalcium phosphate plays in the process of incorporation of carbonate into apatite. Chemical, X-ray diffraction, and infrared techniques were used.When octacalcium phosphate is hydrolyzed in the presence of sodium and carbonate ions in aqueous media, approximately one sodium and one carbonate ion seem to substitute for a calcium and phosphate ion, respectively, in forming apatite, and thea axis is shortened. The infrared spectrum of the product indicates that the carbonate is in the type B site, which is presumed to be a phosphate site. This mechanism is of particular importance since the presence of carbonate in human enamel appears to be related to caries susceptibility. A structural mechanism for the incorporation of impurities during hydrolysis of octacalcium phosphate is presented.

Thermodynamic Solubility Product of Human Tooth Enamel: Powdered Sample

Solubility of human dental enamel in H3PO4 was studied in the pH range of 4.5 to 7.6. Thermodynamic solubility of the enamel mineral was calculated in terms of the ion activity product, (Ca2+)5(PO4 3-)3 (OH-), for hydroxyapatite. The solubility product varied from 7.2 × 10-53 to 6.4 × 10-58 mol9 liter-9 depending on the cumulative amount of the dissolution of the solid in a series of repetitive sequences of solubility experiments.

An intermediate state in hydrolysis of amorphous calcium phosphate.

SummaryThe hydrolysis of previously prepared amorphous calcium phosphate (ACP) was studied in a solution “saturated” with ACP; this eliminated the initial consumption of acid due to ACP dissolution. The procedure established that conversion of a high-concentration ACP slurry to an apatite involves two processes: the first process consumes acid and indicates the formation of a more acidic calcium phosphate intermediary with the solubility of octacalcium phosphate (OCP); the second process consumes base and indicates the conversion of the intermediary to apatite and, possibly, direct conversion of ACP to apatite. The thermodynamic analysis of the solution composition data suggests that ACP converts into a nonstoichiometric apatite when the OCP-like intermediary is formed, and a stoichiometric apatite is formed when no OCP-like intermediary is involved.

Phosphoric Acid Conditioning of Teeth for Pit and Fissure Sealants

Phosphoric acid solutions (-50%) are used sometimes before application of pit and fissure sealants to improve the bonding between enamel and sealant. Our purpose here is to show that monocalcium phosphate monohydrate, Ca (H2P04) 2-HO, forms on the enamel surface during the acid conditioning. Several extracted human teeth were cleaned with pumice. A solution containing 50% H3PO4 then was applied to the enamel surface with cotton pellets. The acid was allowed to stay on the surface for one minute and then was washed away by rinsing the samples with dry acetone. Further drying of the samples in air revealed a white crystalline material deposited on the tooth surface. Petrographic examination and X-ray powder diffractometry indicated that this material was Ca (HPO4) 2 -HO. Similar results were obtained from samples treated with a solution containing 50% H3PO and 7% ZnO. Because of its high solubility in water, the Ca (H2PO4) 2H.0 coating would be completely washed away in the clinical situation; this would leave a clean surface available for adhesion to sealant. These results are in accord with the phase diagram (ELMORE and FARR, Ind Eng Chem 32: 580-586, 1940; BROWN and LEHR, Soil Sci 23: 3-12, 1959) for the ternary system, H3PO4Ca (OH) 2-H20, shown in the illustration. The process of hydroxyapatite, Ca5 (PO4) 30H, (or enamel) dissolution in H-3PO4 solutions (represented by the ordinate) can be divided into two steps: (1) the dissolution of Ca5(PO4),,0H; this would change the solution composition along the dissolution line (C through E) associated with the initial acid composition; (2) dissolution of Ca, (PO4) 30H and precipitation of Ca (H2PO4) 2H20 after the composition reaches the isotherm for Ca (HPO4) 2H20; the combined effect of the two reactions would tend to change the solution composition along the Ca (H2PO4) 2H20 isotherm toward greater calcium concentrations. Apatite dissolution in step 1 should proceed more rapidly than in step 2, because in the latter it is impeded by the Ca (H2PO) 2H20 coating that would form. Therefore, if enamel

Reaction of Dicalcium Phosphate Dihydrate with Fluoride

The intermediate formation of CaHPO4 · 2H2O and its subsequent conversion to Ca5 (PO4)3F or CaF2 may be a major factor in increased fluoride uptake when tooth enamel is pretreated with acid solutions before being exposed to fluoride. To obtain information concerning the form of fluoride incorporation, the reactions of CaHPO 4 ·2H2O with solutions containing various amounts of F-and PO4 3- ions were studied.

Calcium phosphate saturation levels in ultrafiltered serum.

SummaryCalcifications occurring in arteriosclerotic plaque and other pathological deposits are important health concerns, and the nature of these deposits and their mechanisms of formation warrant investigation. Crystals of the relevant calcium phosphates were equilibrated with the undiluted ultrafiltered human serum (u.f.s.) at 37°C by constant stirring and periodically removing samples for calcium and phosphate analysis and for pH measurement. The solubility measurements were carried out both with and without a 5.5% CO2 atmosphere, the physiological partial pressure of CO2. The apparent ion activity products of well-crystallized dicalcium phosphate dihydrate (DCPD), octacalcium phosphate (OCP), and hydroxyapatite (OHAp) equilibrated, in u.f.s. were calculated from the calcium and phosphate concentrations and pH in each case for comparison with their known, solubility products. In this way the well-crystallized calcium phosphates serve as fiducial solubility standards, thereby minimizing errors due to complexing of calcium and phosphate ions by u.f.s. constituents. Under 5.5%, CO2 native u.f.s. was found to be substantially undersaturated with respect to DCPD, slightly supersaturated with respect to OCP, and highly supersaturated with respect to OHAp. The ion activity product of DCPD in DCPD-saturated u.f.s. was 2.4×10−7, and the ion activity product of OCP in OCP-saturated u.f.s. was 4×10−49, slightly above their solubility products (Ksp(DCPD)=2.3×10−7, Ksp(OCP)=2.5×10−49). The ion activity products of DCPD and OCP in u.f.s. under CO2 indicate that the concentrations of calcium and phosphate complexing agents (except bicarbonate) are quite low. The u.f.s. remained supersaturated with respect to OHAp even after 2 months of equilibration. This is attributed to the presence of crystal growth inhibitors in u.f.s.

Basic Biological Sciences Hydrolysis of Dicalcium Phosphate Dihydrate in the Presence or Absence of Calcium Fluoride

Effects of temperature (25 and 37°C), pH (4.9-10.5), and CaF2 on CaHPO4·2H2O (DCPD) hydrolysis were studied in a pH-stat. Octacalcium phosphate (OCP) was the product at pH 6.2-6.8 and 25-37°C; thermodynamically stable apatitic compounds were formed at higher pH and/or higher temperature. In the presence of CaF2, apatite was the product, its crystallinity improved, and the fluoride content increased as pH of the reaction decreased. The results demonstrate the remarkable ability of fluoride to promote the hydrolysis of an acidic calcium phosphate, DCPD, to apatite.

The effects of magnesium and fluoride on the hydrolysis of octacalcium phosphate

The adsorption of Mg ions on octacalcium phosphate (OCP) and its effect on OCP hydrolysis, with and without F, were studied. The Mg adsorption isotherm was fitted by the Langmuir model with an affinity constant of 0.74 ml/mumol and maximum number of sites, 31.19 mumol/g. The hydrolysis rates were measured in a pH stat by titration of base and were strongly temperature dependent. The products were examined by X-ray diffraction and chemical analysis. OCP hydrolysis takes place in two stages: the fast initial process, which is attributed to the surface topotactical conversion, followed by the main, slower process, which involves the nucleation and crystal growth. Mg ions, as 1 mmol/l MgCl2, prevented the initial surface reaction and decreased the nucleation rate dramatically and the growth rate slightly; F increased the rates of surface reaction and both the nucleation and crystal growth processes. The Ca/P ratio (1.53) and the line broadening in the X-ray diffraction patterns of the apatitic products were not significantly affected by the F. Mg also did not affect the Ca/P ratio and the line broadening at (002) diffraction, but decreased the line broadening at (310) diffraction.