Synthetic hydroxyapatite-solubility product and stoichiometry of dissolution

Enhanced Release of Calcium Ions from Hydroxyapatite Nanoparticles with an Increase in Their Specific Surface Area

Synthetic calcium phosphates, e.g., hydroxyapatite (HAP) and tricalcium phosphate (TCP), are the most commonly used bone-graft materials due to their high chemical similarity to the natural hydroxyapatite-the inorganic component of bones. Calcium in the form of a free ion or bound complexes plays a key role in many biological functions, including bone regeneration. This paper explores the possibility of increasing the Ca2+-ion release from HAP nanoparticles (NPs) by reducing their size. Hydroxyapatite nanoparticles were obtained through microwave hydrothermal synthesis. Particles with a specific surface area ranging from 51 m2/g to 240 m2/g and with sizes of 39, 29, 19, 11, 10, and 9 nm were used in the experiment. The structure of the nanomaterial was also studied by means of helium pycnometry, X-ray diffraction (XRD), and transmission-electron microscopy (TEM). The calcium-ion release into phosphate-buffered saline (PBS) was studied. The highest release of Ca2+ ions, i.e., 18 mg/L, was observed in HAP with a specific surface area 240 m2/g and an average nanoparticle size of 9 nm. A significant increase in Ca2+-ion release was also observed with specific surface areas of 183 m2/g and above, and with nanoparticle sizes of 11 nm and below. No substantial size dependence was observed for the larger particle sizes.

Do Soluble Phosphates Direct the Formose Reaction towards Pentose Sugars?

The formose reaction has been a leading hypothesis for the prebiotic synthesis of sugars such as ribose for many decades but tends to produce complex mixtures of sugars and often tars. Channeling the formose reaction towards the synthesis of biologically useful sugars such as ribose has been a holy grail of origins-of-life research. Here, we tested the hypothesis that a simple, prebiotically plausible phosphorylating agent, acetyl phosphate, could direct the formose reaction towards ribose through phosphorylation of intermediates in a manner resembling gluconeogenesis and the pentose phosphate pathway. We did indeed find that addition of acetyl phosphate to a developing formose reaction stabilized pentoses, including ribose, such that after 5 h of reaction about 10-fold more ribose remained compared with control runs. But mechanistic analyses using liquid chromatography-mass spectrometry showed that, far from being directed towards ribose by phosphorylation, the formose reaction was halted by the precipitation of Ca²⁺ ions as phosphate minerals such as apatite and hydroxyapatite. Adding orthophosphate had the same effect. Phosphorylated sugars were only detected below the limit of quantification when adding acetyl phosphate. Nonetheless, our findings are not strictly negative. The sensitivity of the formose reaction to geochemically reasonable conditions, combined with the apparent stability of ribose under these conditions, serves as a valuable constraint on possible pathways of sugar synthesis at the origin of life.

Quantification of local zinc and tungsten deposits in bone with LA-ICP-MS using novel hydroxyapatite-collagen calibration standards

Tungsten has recently emerged as a potential toxicant and is known to heterogeneously deposit in bone as reactive polytungstates. Zinc, which accumulates in regions of bone remodeling, also has a heterogenous distribution in bone. Determining the local concentrations of these metals will provide valuable information about their mechanisms of uptake and action. A series of bone (BN), 7:3 hydroxyapatite:collagen (HC), and hydroxyapatite (HA) standards were spiked with tungsten and zinc and used as calibration standards for laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis of bone tissue. The analytical performance of these standards was studied and validated at different step sizes using NIST SRM 1486 Bone Meal. The effect of matrix-matched calibration was assessed by comparing the calibration with BN and HC standards, which incorporate both inorganic and organic components of bone, to that of HA standards. HC standards were found to be more homogenous (RSD < 10%) and provide a linear calibration with better accuracy (R2 > 0.994) compared to other standards. The limits of detection for HC at a 15 μm step size were determined to be 0.24 and 0.012 μg g-1 for zinc and tungsten, respectively. Using this approach, we quantitatively measured zinc and tungsten deposits in the femoral bone of a mouse exposed to 15 μg mL-1 tungsten for four weeks. Localized concentrations of zinc (942 μg g-1) and tungsten (15.7 μg g-1) at selected regions of enrichment were substantially higher than indicated by bulk measurements of these metals.

Membrane technologies in toilet urine treatment for toilet urine resource utilization: a review

Membrane technologies have broad potential in methods for separating, collecting, storing, and utilizing urine collected from toilets. Recovering urine from toilets for resource utilization instead of treating it in a sewage treatment plant not only reduces extra energy consumption for the degradation of N and P but also saves energy in chemical fertilizer production, which will contribute to carbon emission reduction of 12.19-17.82 kg kgN -1 in terms of N alone. Due to its high efficiency in terms of volume reduction, water recycling, nutrient recovery, and pollutant removal, membrane technology is a promising technology for resource utilization from urine collected from toilets. In this review, we divide membrane technologies for resource utilization from urine collected from toilets into four categories based on the driving force: external pressure-driven membrane technology, vapor pressure-driven membrane technology, chemical potential-driven membrane technology, and electric field-driven membrane technology. These technologies influence factors such as: recovery targets and mechanisms, reaction condition optimization, and process efficiency, and these are all discussed in this review. Finally, a toilet with source-separation is suggested. In the future, membrane technology research should focus on the practical application of source-separation toilets, membrane fouling prevention, and energy consumption evaluation. This review may provide theoretical support for the resource utilization of urine collected from toilets that is based on membrane technology.

The usage of calcium phosphate systems for onsite defluoridation treatment

Over 200 million people in over 35 countries are affected by excessive fluoride in their waters. For people that do not have access to a centralized water treatment plant, there is a need for an on-site defluoridation system that requires no special operational expertise, does not use hazardous chemicals, and is sustainable by the local population. 8 different calcium phosphate precipitation systems were analyzed and tested for fluoride removal effectiveness. An effective system would have final fluoride concentrations less than 1.5 mg/L and final solutions with pH within drinkable limits. Phosphoric acid with the addition of a calcium carbonate source was found to have a 99.8% fluoride removal rate. Monosodium phosphate with addition of slaked lime was also found to be effective with a 99.98% fluoride removal rate. An optimal slaked lime to monosodium phosphate ratio that achieved effective fluoride removal and neutral pH was found. With 0.45 g of Ca(OH)₂ and 1 g of NaH₂PO₄, initial fluoride concentrations up to 100 mg/L or more could be reduced to near zero concentrations, and a volume of approximately 337 mL of water with a concentration of 5 mg/L F^- could to be reduced to less than 1.5 mg/L F^-.

Accuracy of Thermodynamic Databases for Hydroxyapatite Dissolution Constant

Discrepancies have been noted in the solubility constant values of calcium phosphate minerals between various databases employed in widely used aqueous speciation calculation software programs. This can cause serious errors in the calculated speciation of waters when using these software programs. The aim of this communication was to bring to light these discrepancies. Experimental determinations of the hydroxyapatite (HAP) solubility product vary by as much as 10 orders of magnitude as a result of experimental challenges related to the presence of impurities in the HAP used, incongruent dissolution, and the contamination of solutions with dissolved carbon dioxide. It is suggested that the value used in the database Thermo.dat is consistent with experimental data devoid of common experimental problems, whereas other common databases use values that lead to a vastly overestimated solubility of HAP.

Technological Challenges of Phosphorus Removal in High-Phosphorus Ores: Sustainability Implications and Possibilities for Greener Ore Processing

With the present rates of iron ore consumption, currently unusable, high-phosphorus iron ore deposits are likely to be the iron ores of the future as higher-grade iron ore reserves are depleted. Consequently, the design and timely development of environmentally-benign processes for the simultaneous beneficiation of high-phosphorus iron ores and phosphorus recovery, currently a technological challenge, might soon become a sustainability challenge. To stimulate interest in this area, phosphorus adsorption and association in iron oxides/hydroxyoxides, and current efforts at its removal, have been reviewed. The important properties of the most relevant crystalline phosphate phases in iron ores are highlighted, and insights provided on plausible routes for the development of sustainable phosphorus recovery solutions from high-phosphorus iron ores. Leveraging literature information from geochemical investigations into phosphorus distribution, speciation, and mobility in various natural systems, key knowledge gaps that are vital for the development of sustainable phosphorus removal/recovery strategies and important factors (white spaces) not yet adequately taken into consideration in current phosphorus removal/recovery solutions are highlighted, and the need for their integration in the development of future phosphorus removal/recovery solutions, as well as their plausible impacts on phosphorus removal/recovery, are put into perspective.

Application of high resolution DLP stereolithography for fabrication of tricalcium phosphate scaffolds for bone regeneration

Bone regeneration requires porous and mechanically stable scaffolds to support tissue integration and angiogenesis, which is essential for bone tissue regeneration. With the advent of additive manufacturing processes, production of complex porous architectures has become feasible. However, a balance has to be sorted between the porous architecture and mechanical stability, which facilitates bone regeneration for load bearing applications. The current study evaluates the use of high resolution digital light processing (DLP) -based additive manufacturing to produce complex but mechanical stable scaffolds based on β-tricalcium phosphate (β-TCP) for bone regeneration. Four different geometries: a rectilinear Grid, a hexagonal Kagome, a Schwarz primitive, and a hollow Schwarz architecture are designed with 400 μm pores and 75 or 50 vol% porosity. However, after initial screening for design stability and mechanical properties, only the rectilinear Grid structure, and the hexagonal Kagome structure are found to be reproducible and showed higher mechanical properties. Micro computed tomography (μ-CT) analysis shows <2 vol% error in porosity and <6% relative deviation of average pore sizes for the Grid structures. At 50 vol% porosity, this architecture also has the highest compressive strength of 44.7 MPa (Weibull modulus is 5.28), while bulk specimens reach 235 ± 37 MPa. To evaluate suitability of 3D scaffolds produced by DLP methods for bone regeneration, scaffolds were cultured with murine preosteoblastic MC3T3-E1 cells. Short term study showed cell growth over 14 d, with more than two-fold increase of alkaline phosphatase (ALP) activity compared to cells on 2D tissue culture plastic. Collagen deposition was increased by a factor of 1.5-2 when compared to the 2D controls. This confirms retention of biocompatible and osteo-inductive properties of β-TCP following the DLP process. This study has implications for designing of the high resolution porous scaffolds for bone regenerative applications and contributes to understanding of DLP based additive manufacturing process for medical applications.

Functionalization of titania nanotubes with electrophoretically deposited silver and calcium phosphate nanoparticles: Structure, composition and antibacterial assay

Herein TiO₂ nanotubes (NTs) were fabricated via electrochemical anodization and coated with silver and calcium phosphate (CaP) nanoparticles (NPs) by electrophoretic deposition. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) revealed that Ag and CaP NPs were successfully deposited onto the TiO₂ NTs. Using X-ray diffraction, only anatase and Ti were observed after deposition of Ag and CaP NPs. However, X-ray photoelectron spectroscopy (XPS) analysis revealed that the binding energy (BE) of the Ag and CaP NP core levels corresponded to metallic Ag, hydroxyapatite and amorphous calcium phosphate, based on the knowledge that CaP NPs synthesized by precipitation have the nanocrystalline structure of hydroxyapatite. The application of Ag NPs allows for decreasing the water contact angle and thus increasing the surface free energy. It was concluded that the CaP NP surfaces are superhydrophilic. A significant antimicrobial effect was observed on the TiO₂ NT surface after the application of Ag NPs and/or CaP NPs compared with that of the pure TiO₂ NTs. Thus, fabrication of TiO₂ NTs, Ag NPs and CaP NPs with PEI is promising for diverse biomedical applications, such as in constructing a biocompatible coating on the surface of Ti that includes an antimicrobial effect.

Adsorption behavior of methacryloyloxydecyl dihydrogen phosphate on an apatite surface at neutral pH

This study aimed to quantify the adsorption affinity of neutralized 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP-N) toward hydroxyapatite (HA) and dicalcium phosphate dihydrate (DCPD) at pH 7.0 by employing the Langmuir isotherm model. Furthermore, the effects of inorganic phosphate (Pi) and fluoride (F(-) ) ions on the adsorption of 10-MDP-N onto HA and DCPD were examined. Fixed amounts of HA and DCPD powders were suspended in different concentrations of 10-MDP-N solutions and were incubated for 18 h. Equilibrated concentrations of 10-MDP-N were measured by spectrophotometry and the adsorption affinity was estimated using the Langmuir model. Moreover, the adsorption was examined by zeta-potential analysis. The results indicated that significant Langmuir correlation was noted in both substrates, along with an increasing negative zeta-potential; however, in DCPD the correlation was less strong. The addition of 1.0 mM Pi slightly delayed the adsorption of 10-MDP-N onto both substrates, whereas 3.0 mM Pi drastically delayed adsorption onto HA but completely inhibited adsorption onto DCPD. Up to 50 ppm, F(-) enhanced the adsorption onto HA, and the adsorption plateaued at higher concentrations of F(-) , whereas no obvious influence of F(-) on the adsorption onto DCPD was noted.

Properties of titanium-alloyed DLC layers for medical applications

DLC-type layers offer a good potential for application in medicine, due to their excellent tribological properties, chemical resistance, and bio-inert character. The presented study has verified the possibility of alloying DLC layers with titanium, with coatings containing three levels of titanium concentration prepared. Titanium was present on the surface mainly in the form of oxides. Its increasing concentration led to increased presence of titanium carbide as well. The behavior of the studied systems was stable during exposure in a physiological saline solution. Electrochemical impedance spectra practically did not change with time. Alloying, however, changed the electrochemical behavior of coated systems in a significant way: from inert surface mediating only exchange reactions of the environment in the case of unalloyed DLC layers to a response corresponding rather to a passive surface in the case of alloyed specimens. The effect of DLC layers alloying with titanium was tested by the interaction with a simulated body fluid, during which precipitation of a compound containing calcium and phosphorus - basic components of the bone apatite - occurred on all doped specimens, in contrast to pure DLC. The results of the specimens' surface colonization with cells test proved the positive effect of titanium in the case of specimens with a medium and highest content of this element.

Calcium phosphate ceramics in bone tissue engineering: a review of properties and their influence on cell behavior

Calcium phosphate ceramics (CPCs) have been widely used as biomaterials for the regeneration of bone tissue because of their ability to induce osteoblastic differentiation in progenitor cells. Despite the progress made towards fabricating CPCs possessing a range of surface features and chemistries, the influence of material properties in orchestrating cellular events such as adhesion and differentiation is still poorly understood. Specifically, questions such as why certain CPCs may be more osteoinductive than others, and how material properties contribute to osteoinductivity/osteoconductivity remain unanswered. Therefore, this review article systematically discusses the effects of the physical (e.g. surface roughness) and chemical properties (e.g. solubility) of CPCs on protein adsorption, cell adhesion and osteoblastic differentiation in vitro. The review also provides a summary of possible signaling pathways involved in osteoblastic differentiation in the presence of CPCs. In summary, these insights on the contribution of material properties towards osteoinductivity and the role of signaling molecules involved in osteoblastic differentiation can potentially aid the design of CPC-based biomaterials that support bone regeneration without the need for additional biochemical supplements.

Effects of different crosslinking conditions on the chemical-physical properties of a novel bio-inspired composite scaffold stabilised with 1,4-butanediol diglycidyl ether (BDDGE)

Serious cartilage lesions (Outerbridge III, IV) may be successfully treated with a three-layered gradient scaffold made by magnesium-doped hydroxyapatite and type I collagen, manufactured through a bio-inspired process and stabilised by a reactive bis-epoxy (1,4-butanediol diglycidyl ether, BDDGE). Each layer was analysed to elucidate the effects of crosslinking variables (concentration, temperature and pH). The chemical stabilisation led to an homogeneous and aligned collagenous matrix: the fibrous structures switched to a laminar foils-based arrangement and organic phases acquired an highly coordinated 3D-organization. These morphological features were strongly evident when crosslinking occurred in alkaline solution, with BDDGE concentration of at least 1 wt%. The optimised crosslinking conditions did not affect the apatite nano-crystals nucleated into self-assembling collagen fibres. The present work allowed to demonstrate that acting on BDDGE reaction parameters might be an useful tool to control the chemical-physical properties of bio-inspired scaffold suitable to heal wide osteochondral defects, even through arthroscopic procedure.

Dissolution mechanism of calcium apatites in acids: A review of literature

Eight dissolution models of calcium apatites (both fluorapatite and hydroxyapatite) in acids were drawn from the published literature, analyzed and discussed. Major limitations and drawbacks of the models were conversed in details. The models were shown to deal with different aspects of apatite dissolution phenomenon and none of them was able to describe the dissolution process in general. Therefore, an attempt to combine the findings obtained by different researchers was performed which resulted in creation of the general description of apatite dissolution in acids. For this purpose, eight dissolution models were assumed to complement each other and provide the correct description of the specific aspects of apatite dissolution. The general description considers all possible dissolution stages involved and points out to some missing and unclear phenomena to be experimentally studied and verified in future. This creates a new methodological approach to investigate reaction mechanisms based on sets of affine data, obtained by various research groups under dissimilar experimental conditions.

Solubility of calcium fluoride and fluorapatite by solid titration

OBJECTIVES

The solubility isotherms, S, of two compounds detected after topical fluoride treatment - calcium fluoride (CaF(2)) and fluorapatite (FAp) - are of fundamental interest in saliva chemistry and in the context of reduction of acid dissolution of teeth, whether through the process of caries or from exogenously ingested acids. Solid titration has shown its reliability and reproducibility for complicated systems that are not suitable for study by the traditional excess-solid method. The primary aim of this work was to ascertain S[CaF(2)] and S[FAp].

METHODS

Solid titration was used for CaF(2) (pH 2.3-9.5) and FAp (pH 2.8-5.1) in 100mM KCl solution at 37.0+/-0.1 degrees C, and further to determine the apparent solubility of hydroxyapatite (HAp) in the presence of 1mM fluoride (pH 3.2-4.8).

RESULTS

Peculiar results were obtained at first which were attributed to the adsorption or reaction of fluoride with the reaction vessel glass surface interfering with the intended solution equilibria. Wax-lined glass apparatus resolved this problem. The solubility isotherm of CaF(2) was then as theoretically expected at pH<8, but above this point a new solid species (CaFOH) was postulated to account for the data. The position of FAp was as expected relative to HAp, being about 0.63x less soluble. FAp was the only detectable equilibrium solid at pH 3.2, 3.6 and 4.1. The apparent solubility of HAp was depressed somewhat by the presence of 1mM fluoride.

CONCLUSION

The solid titration method was again found to be reliable once glass interferences were eliminated. The interaction of fluoride with borosilicate glass may have affected other work in the field; such work may therefore require re-evaluation. The S[FAp] is very similar to that of HAp determined by solid titration. Excess-solid method results are strongly discrepant from the present determination and may not be reliable, primarily due to lack of solution speciation data for that calculation.

Calcium phosphate coating formed in infusion fluid mixture to enhance fixation strength of titanium screws

A novel technique was developed to coat a calcium phosphate (CaP) layer on titanium screws with a titanium oxide surface layer, using infusion fluids officially approved for clinical use. A calcium-containing solution, a phosphate-containing solution, and a sodium bicarbonate solution prepared from the infusion fluids were mixed at a Ca/P molar ratio of 2.0. Each screw was immersed in 10 mL of the resulting mixture at 37 degrees C for 2 days. A low-crystalline apatite layer (Ca/P molar ratio = 1.681 +/- 0.038) was formed on the screws. The layer consisted of a few 100 nm diameter particles fixed on the screw surface. In animal experiments, the screws were percutaneously implanted in both proximal tibial metaphyses of rabbits. The insertion torque was not significantly different between the CaP-coated screws (0.132 +/- 0.002 Nm, n = 10) and uncoated screws (0.140 +/- 0.002 Nm, n = 10) (p = 0.5785). After the insertion torque test, the apatite layer remained on the surface of the screws, which means that the apatite layer survived the friction of screw insertion. The extraction torque of the screws in the CaP-coated group (0.239 +/- 0.066 Nm, n = 19) was significantly higher (by 29.9%) than that in the uncoated group (0.184 +/- 0.062 Nm, n = 18) 4 weeks after the operations (p = 0.0132). Histologically, a larger amount of new bone formation was observed around the CaP-coated screws than that around the uncoated screws. Even after the removal of the screw, the CaP layer remained on the screw at the site where soft tissues were attached. The coating technique with the use of the infusion fluids is an effective method of improving bone-screw interface strength.

Solubility of hydroxyapatite by solid titration at pH 3–4

OBJECTIVES

The solubility isotherm (S) of hydroxyapatite (HAp) is of fundamental importance to saliva chemistry, dental caries and related contexts. It has previously been shown that the locus of the S[HAp] is substantially lower than is commonly reported, and of different slope, probably due to HAps incongruent dissolution. The aim of the present study was to determine the S[HAp] over a wider pH range and to identify the precipitate formed at equilibrium in HAp solid titration.

METHODS

The solid titration technique of Leung and Darvell (Leung VW-H, Darvell BW. Calcium-phosphate system in saliva-like media. J Chem Soc Faraday Trans 1991;87(11):1759-64.) was used to investigate the solubility behaviour of HAp at 37.0+/-0.1 degrees C in 100 mM aqueous KCl. The pH range studied overlapped that of earlier work from pH 3.6 to 5.2, for a reproducibility check and validation, and extended to pH approximately 2.9. XRD and EDX were used to identify the precipitates. SEM and TEM were used to observe the morphology.

RESULTS

The previous S[HAp] reported by Chen et al. (Chen Z-F, Darvell BW, Leung VW-H. Hydroxyapatite solubility in simple inorganic solutions. Arch Oral Biol 2004;49(5):359-67.) was reconfirmed. An abrupt change of slope of S[HAp] was detected at pH approximately 3.9. No other phase than HAp was found at pH 3.2, 3.6 and 4.1. In particular, brushite (dicalcium phosphate dihydrate) was not detected, even below pH 3.9, where instead calcium-deficient HAp was formed.

CONCLUSION

The solid titration method was reconfirmed as reproducible and to yield HAp from pH 2.9 to 5.2. The expected brushite did not appear, but rather a stable calcium-deficient HAp was consistently formed. The chemistry of calcium phosphates needs to be reevaluated.

Apparent solubility distributions of hydroxyapatite and enamel apatite

Samples of human dental enamel and hydroxyapatite were equilibrated at 5 mg/40 ml for 9 days at 37 degrees C with acetate buffers adjusted to a range of saturations with respect to hydroxyapatite. Sigmoidal apparent solubility distributions, in which the fraction dissolved was plotted against--log(ion activity product for hydroxyapatite) (pIHA), were constructed. About 10% of the hydroxyapatite and 14% of the enamel was very soluble, dissolving even at pIHA 55. The apparent solubility distributions for both solids were invariant with pH (4.5, 5.0, 5.5), within experimental error, showing that solubility was controlled by a phase with the stoichiometry of hydroxyapatite, probably in the form of a surface layer or complex on the crystals, in agreement with other studies on carbonate-apatites and bone mineral. The pIHA at 50% dissolution was employed as an average value. The pIHA (50%) values for pooled data (58.76 for enamel and 60.17 for hydroxyapatite) were lower than the respective pIHA previously measured by conventional equilibration techniques. However, the average pIHA measured for enamel was higher than that obtained by the same technique in another study, possibly because of differences in specimen preparation and equilibration time. The possible implications of the findings for understanding the process of dental caries are discussed.

Zinc-Containing Calcium Phosphate Ceramics with a (Ca+Zn)/P Molar Ratio of 1.67

Zinc-containing calcium phosphate ceramics with a (Ca+Zn)/P molar ratio of 1.67 (ZnHAP ceramics) were synthesized to clarify the main phase for zinc release. ZnHAP ceramics with a zinc content at or more than 0.20 wt% contained zinc-containing tricalcium phosphate. The ion activity product of monophasic ZnHAP ceramic containing at or less than 0.13 wt% of zinc corresponded to that of pure HAP ceramic. The presence of tricalcium phosphate (TCP) phases was necessary condition for ZnHAP ceramics to release zinc. The coexistence of TCP phases in the ZnHAP ceramics meant that a phase with a (Ca+Zn)/P molar ratio higher than 1.67 also coexisted. Although the phase was assumed to be zinc oxide, zinc was not concentrated anywhere in the ZnHAP ceramics including grain boundaries.

Hydroxyapatite solubility in simple inorganic solutions

OBJECTIVE

To use solid titration and laser-scattering end-point detection to determine the hydroxyapatite (HAP) solubility isotherm at 37 degrees C in a plain KCl solution, with CO(2) present, and with additional phosphate.

DESIGN

Pulverised HAP solid was used to perform a titration to saturation. A very sensitive detector capable of detecting the light scattered at small forward angles from a laser beam was adopted to monitor the onset of precipitation (or failure to dissolve) near the equilibrium point. Each addition of HAP solid caused a step increase in scattered light and this signal decreased quasi-exponentially with time. When a steady but elevated scattering was obtained, small portions of 1M HCl were added to lower the pH by about 0.5-2 units, according to need, to dissolve excess solid and allow a further titration. The scatter signal and pH data, plotted against the amount of HAP added, were used to estimate the actual end-point by interpolation and thus construct each point on the solubility isotherm.

RESULTS

The solubility isotherm for HAP in plain KCl solution in the absence of CO(2) obtained now differs substantially from the results of previous solubility studies (i.e. much lower). The increase in solubility due to CO(2) reported earlier was confirmed, while the effect of excess phosphate was found to be to increase the apparent solubility of HAP, contrary to elementary mass-action expectations.

CONCLUSIONS

The solid titration method is a more reliable approach than that of conventional excess solid method with respect to the determination of the HAP-carrying capacity. The solubility of HAP appears to be substantially lower than previously reported, and a reconsideration of all aspects of the system is warranted.

Metastable Equilibrium Solubility Distribution of Carbonated Apatite as a Function of Solution Composition

Previous studies have shown that carbonated apatites (CAPs) exhibit the phenomenon of metastable equilibrium solubility (MES) in weak acid media. The purpose of the present investigation was to examine two questions: first, whether the MES concept is applicable to a broader range of solution conditions and, second, whether a driving force function associated with a surface complex having a constant stoichiometry governs the dissolution of CAP and, if so, what is this stoichiometry. CAP preparations with carbonate contents of 1.8-5.7 wt% (synthesized by hydrolysis of dicalcium phosphate anhydrate in solutions of varying bicarbonate levels or by direct precipitation from supersaturated calcium/phosphate/carbonate solutions) were studied as follows. MES distributions for each of the CAP preparations were determined by equilibrating the CAP under stirred conditions in a series of acetate buffers (0.10 M) containing various levels of calcium and phosphate in the pH range 4.5-6.5 and a solution calcium/phosphate ratio in the range 0.1-10. The amount dissolved in each instance was regarded as the fraction of the CAP possessing an MES value greater than that corresponding to the ion activity product (IAP) of the equilibrating solution. The solution IAPs were calculated from the solution compositions using plausible calcium phosphate stoichiometries, viz., dicalcium phosphate dihydrate, octacalcium phosphate, tricalcium phosphate, hydroxyapatite, carbonated apatite (based on the bulk composition of the particular CAP involved in the experiment), and tetracalcium phosphate. The fraction of CAP dissolved was plotted against the solution IAPs for each experimental set using each of the six assumed stoichiometries for the surface complex. The results demonstrated that the MES concept was applicable to all of the CAP preparations in media of various solution compositions and different pH levels. The most important new outcome of this study was that MES profiles for each of the CAP preparations in all of the experiments were found to be superimposable when solution IAPs were calculated using the hydroxyapatite stoichiometry, while such was not the case when other stoichiometries were used to calculate the solution IAPs. Copyright 1999 Academic Press.

Solubility product of OH-carbonated hydroxyapatite

Information on the solubility of OH-carbonated hydroxyapatite, Ca10(PO4)6(CO3)x(OH)2-2x, previously has not been available. In the present study the solubility product (Ksp) of OH-carbonated hydroxyapatite was measured in a 0.1 M acetic acid and sodium acetate buffer solution in a pH range of 4.0-5.8 at a CO2 partial pressure of 10(-3.52) atm. The equilibrium solubility increased with the increase of carbonate content. The Ksp values decreased with the decrease of pH. For example, Ksps were 10(-119), 10(-123), and 10(-130) for pure hydroxyapatite at pH 4.9, 4.5, and 4.1, respectively. The decrease of Ksp was not accounted for by calcium-carbonate complexation. Ksp measured at isoelectric points (L) was expressed as pL = 118.65 - 0.47316 x (CO2 wt%)2.4176. From this formula, the L values were calculated for pure and fully carbonated hydroxyapatite as 10(-118.7) and 10(102.8), respectively. The L value for pure hydroxyapatite agreed with values measured under carbonate-free conditions. Therefore, the L values were regarded as the Ksp for OH-carbonated hydroxyapatite excluding errors arising from carbonate contamination in the solution.

The hydroxyapatite solubility product of human dental enamel as a function of pH in the range 4.6-7.6 at 20 degrees C

Samples of 200 mg powdered enamel apatite were suspended in 10 ml of aqueous solutions made from phosphoric acid and calcium hydroxide for 60 days at 20 degrees C. Calcium and phosphate concentrations were determined by spectroscopy; pH and the concentrations of fluoride and carbonate by electrometry. The solubility product of hydroxyapatite varied linearly from 10-56.9 at pH 4.6 to 10-52.8 at pH 7.6. The carbonate concentration in the aqueous phase was too low to account for calcium carbonate complex formation of significance to the solubility product. After the equilibration, X-ray diffraction analysis of the powder showed that the salt was unchanged pure apatite with no trace of a transformation to brushite or any other calcium phosphate. A re-equilibration of some of the powder samples for another 60 days produced similar solubility products. Thus the change of the solubility product was most likely attributable to a pH-induced change of the ionic composition of the enamel crystal surface.

Stability and mutual conversion of enamel apatite and brushite at 20 degrees C as a function of pH of the aqueous phase

By calculation, apatite is more soluble than brushite at low pH and less soluble at high pH. Apatite, therefore should be able to transform spontaneously to brushite at low pH and brushite to apatite at high pH. The aim was to describe this mutual conversion as related to the aqueous phase composition. Powdered enamel, brushite or 1:1 mixtures of the two salts were suspended in distilled water for up to 12 weeks at 20 degrees C, pH was adjusted to 4, 5, 6, 7, 8, 9 or 10 by drops of perchloric acid or potassium hydroxide. The calcium and the phosphate concentrations and pH were determined, and the nature of the calcium phosphate powder was examined by X-ray diffraction. At pH 8 and above, brushite was invariably converted to apatite, whilst it was transformed to octacalcium phosphate at pH 7. In the pH range 6-4 brushite was not converted to apatite and enamel apatite was not transformed to brushite spontaneously within 2 months. In the enamel apatite suspensions, the apatite ion product altered with pH, which explained why apatite did not transform to brushite at pH 5-4. At pH 3.7, however, the enamel apatite was converted to brushite. No transformation of apatite to brushite was identified in apatite-brushite mixtures at pH 6-4. Supplementary experiments showed that ethanol, used as an agent for removal of water, salted out the water-free dicalcium phosphate, monetite.