Solid Titration Research Articles

Effect of Magnesium on the Solubility of Hydroxyapatite

As an abundant cation in the human body, magnesium plays important roles in physiological and pathological processes. In particular, magnesium deficiency is associated with osteoporosis. Hence, Mg may be involved in the biodegradation and reconstruction of bone mineral. Previously, Mg‐containing hydroxyapatite (HAp) has been synthesized as a model for bone mineral, and its properties, such as crystal size and crystallinity, have been examined. However, the effect of solution magnesium on the effective solubility of hydroxyapatite has not been studied in detail. Here, for a series of magnesium concentrations in the background solution, the mass solubility of hydroxyapatite was determined through the solid titration method at pH 3–5 and 37 °C. A general, quasilogarithmic increase in solubility was found with increasing magnesium concentration. Whether this is due to nucleation inhibition or solution complexation has yet to be ascertained, although the latter is likely on simple chemical grounds. As the direct effect of magnesium was to increase the solubility of HAp in a concentration‐dependent manner, this cannot be the direct cause of osteoporosis in the event of magnesium deficiency.

The effect of excess phosphate on the solubility of hydroxyapatite

The solubility of hydroxyapatite (HA) is critical in fields such as medicine, dentistry and geochemistry. Previously, it had been found that it was apparently slightly increased with 1mmol/L excess phosphate present. This study was to determine the solubility of HA over the pH range 3.3–5.4 with a series of concentrations of such ‘excess’ phosphate with solid titration (ST), and to identify the precipitate formed at equilibrium with SEM, TEM, EDX, FTIR and XRD. For [PO4]XS=0–0.3mmol/L, results followed closely the already-reported ST ‘low’ solubility isotherm. At [PO4]XS=0.5mmol/L, the solubility surface switched abruptly to a ‘high’ position that could not be reconciled with either the ‘low’ isotherm or conventional calculations. Thus, HA solubility is critically dependent on the presence of excess phosphate. Such excess may account for the discrepancy between ST and excess-solid results, although the crystallographic explanation is as yet lacking.

The effect of strontium incorporation into hydroxyapatites on their physical and biological properties

In this study, the effects of a series of strontium (Sr)-substituted HA ceramics (0, 1, 5, and 10 mol % Sr substitution) were tested on their physical and biological properties. The crystal structure, composition, and solubility were investigated by TEM, XRD, and solid titration solubility isotherms, respectively. In addition, rat MSCs were cultured with culture media containing ions released from the strontium-substituted HA ceramics as they dissolved. MTT test, alkaline phosphatase activity, and osteoblast transcription factor gene (cbfa1) expression were conducted at different time points. Our results suggested that HA with Sr substitution may change its physical properties, especially its solubility, and consequently enhance undifferentiated MSCs into osteoblast lineage. The results from this and the previous study suggested that 5-10 mol % Sr substitution into HA may be a suitable choice for its use in bone regenerative field.

Effect of Carbonate on Hydroxyapatite Solubility

Carbonate is present in biological apatites (BAp) (enamel, dentine, bone, and pathological calcifications) by substitution at phosphate and hydroxide sites, tending to increase its solubility in comparison with pure hydroxyapatite (HAp). The role of solution carbonate, however, is poorly understood. Using the solid titration method, it was found that the apparent solubility of HAp increased greatly with an increase in pCO2, and was sensitively-dependent at low values. No other phase was formed at the end point except HAp, and in particular no dicalcium phosphate dihydrate (DCPD) was found. However, no carbonate was detected in the end-point solid except for a small amount at pH ∼ 7.4, pCO2 = 0.01−1.0 bar. The implications of these results need further investigation, bearing as they do on a central topic in oral chemistry.

Solubility of sparingly-soluble electrolytes–a new approach

The technique of solid titration was developed to explore the true solution equilibria for complex solids and solutions, and in particular for biominerals. It has been shown that the conventional method of equilibration over a large excess of solid is inappropriate for the investigation of solubility when there is incongruent dissolution. The new method shows great sensitivity, reliability and reproducibility, and can be applied to many systems in medical and dental contexts, as well as general chemistry, geology, and the pharmaceutical industry.

Solid Titration of Octacalcium Phosphate

Octacalcium phosphate (OCP) is of considerable importance as a precursor in the formation of dental enamel and an intermediate phase in the precipitation of hydroxyapatite (HAp) in bone. However, agreement is poor on the solubility product (pK_sp), possibly due to the formation of the more stable phase HAp. The system was investigated using solid titration, which has shown reliability in work on HAp and related fluoride minerals, with OCP in 100 mM KCl at 37.0 ± 0.1°C. The constitution of the end point precipitate was determined by X-ray diffraction and selected-electron area diffraction; the particle morphology and elements present were examined by high-resolution field emission scanning, transmission electron microscopy and energy-dispersive X-ray analysis. The titration curve for OCP was found for pH ∼3.4–7.4. The precipitate was HAp at pH 3.6 and 4.5; no residual OCP or other phase was detected. Dicalcium phosphate dihydrate (DCPD) was then found to form at pH 3.6 on further addition of OCP titrant after equilibrium had been achieved, possibly due to easier nucleation at lower pH. However, markedly crystalline HAp was formed in equilibrium for OCP titration with HAp seeding, verifying HAp as the more stable phase. A solubility isotherm for OCP was not obtained as HAp appears to be less soluble in the pH range studied. This adds weight to the view that HAp may be the most stable phase of all calcium phosphates, with further doubt being cast on DCPD being the most stable phase below pH 4.2. However, metastable DCPD may form in an Ostwald succession, depending on supersaturation and nucleation conditions.

Solubility of Dicalcium Phosphate Dihydrate by Solid Titration

Solid-titration results for hydroxyapatite (HAp), octacalcium phosphate, β-tricalcium phosphate and tetracalcium phosphate have shown that the only stable phase in 100 mmol·l^-1 KCl at 37°C is HAp. In particular, dicalcium phosphate dihydrate (DCPD) did not form at pH <4.2 (where it is otherwise believed to be stable) except as a metastable phase under conditions of slight supersaturation. The behaviour of DCPD itself under the same conditions requires checking. Solid titration was used to determine the apparent solubility of DCPD in a 100-mmol·l^–1 KCl solution at 37.0 ± 0.1°C over the pH range 3.2–11.6. The constitution of the precipitate was determined by X-ray diffraction, particle morphology was observed by scanning and transmission electron microscopy, and the precipitate Ca/P ratio was calculated by energy-dispersive X-ray analysis. The titration curve for DCPD was substantially lower than the position reported elsewhere. DCPD was the only identified phase at equilibrium at pH 3.60 and 4.50, but HAp was formed after seeding with 1 mg HAp at DCPD equilibrium at pH 4.47, 3.60 and 3.30. It is concluded that the titration curve observed for DCPD corresponds to the solubility isotherm for the phase, but that this represents a metastable equilibrium. HAp is more stable than DCPD, particularly below pH 4.2. The implications for calcium phosphate studies are profound as the reverse is generally believed to be true. Thus, solubility results and the nature of the carious lesion need reconsideration.

Solubility of Bovine-Derived Hydroxyapatite by Solid Titration, pH 3.5−5

Bovine-derived hydroxyapatite (BDHA) is commonly used in treating alveolar bony defects. There is lack of agreement in the literature as to its rate of degradation in vivo, and little is known about its solubility in vitro, which is relevant to the mechanism of degradation of calcium phosphate biomaterials in general. Therefore, this study was undertaken to measure the effective solubility isotherm of BDHA using the established solid-titration method and to identify the equilibrium solid phase.

Solubility of strontium-substituted apatite by solid titration

Solid titration was used to explore the solubility isotherms of partially (Srx-HAp, x = 1, 5, 10, 40, 60 mol.%) and fully substituted strontium hydroxyapatite (Sr-HAp). Solubility increased with increasing strontium content. No phase other than strontium-substituted HAp, corresponding to the original titrant, was detected in the solid present at equilibrium; in particular, dicalcium hydrogen phosphate was not detected at low pH. The increase in solubility with strontium content is interpreted as a destabilization of the crystal structure by the larger strontium ion. Carbonated HAp was formed in simulated body fluid containing carbonate on seeding with Sr10-HAp, but the precipitate was strontium-substituted on seeding with Sr-HAp. Strontium-substituted HAp might be usable as a template for the growth of new bone, since nucleation appears to be facilitated.

Calcium Phosphate Solubility: The Need for Re-Evaluation

The determination of the solubility of calcium phosphates by the conventional large excess of solid method has been demonstrated to be inappropriate. The problem lies in incongruent dissolution, leading to phase transformations, and lack of detailed solution equilibria: all calculations have been based on simplifications, which are only crudely approximate. The absolute solid-titration approach shows excellent reliability and reproducibility. Using solid titration, the true solubility isotherm of hydroxyapatite (HAp) has been found to lie substantially lower than previously reported. In addition, contrary to wide belief, dicalcium phosphate dihydrate (DCPD) is not the most stable phase below pH ∼4.2, where calcium-deficient HAp is less soluble. The misunderstanding here arises from the metastability of DCPD, which nucleates much more easily than HAp at low pH. Such results indicate that the Ca−P system is in need of complete reappraisal. The solid-titration method can be extended to other complex systems.

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 100 mM KCl solution at 37.0 ± 0.1 °C, and further to determine the apparent solubility of hydroxyapatite (HAp) in the presence of 1 mM 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 0.63× 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 1 mM 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.

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 °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 ∼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 ∼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.

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 °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 1 M 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.

Identification and Determination of Organohalogen Compounds in Swimming Pool Water

Abstract Volatile organohalogen compounds were determined in a swimming pool using three methods: determination of volatile organic halogen (VOX) after sorption on XAD-4 solid sorbent, thermal desorption and coulometric titration of the halides formed; identification and determination of the individual organohalogen compounds after sorption on XAD-4, solvent extraction and chromatographic determination; direct injection of aqueous samples to the chromatographic column. The concentration of organohalogen compounds varied from 35.9 to 99.7 μg/dm3 for trichloromethane, 22.0 to 57.7 μg/dm3 for dichloromethane, 2.4 to 27.4 μg/dm3 for trichloroethylene, 2.3 to 14.7 μg/dm3 for bromodichloromethane and 4.2 to 7.6 μg/dm3 for tetrachloromethane. The results obtained by the three methods are consistent in principle.

Calcium phosphate system in saliva-like media

Despite the well documented importance of carbon dioxide dissolved in physiological media, consideration of its effect on calcium phosphate solubilities has been neglected, an omission now shown to be serious. A solid titration technique has been developed to avoid a number of the difficulties inherent in solubility studies of calcium phosphates; this employed a graphical method based on pH and calcium-ion electrode potential to identify breakpoints. The solubility isotherm at 37.0 °C for hydroxyapatite has thus been determined for the chosen saliva-like medium. A numerical model of the system provided corroboration for the results obtained. An approximately 10-fold increase in the calcium-carrying capacity of that medium was then demonstrated with CO2 present at a fixed partial pressure of 0.035 bar, a result not explicable in terms of known equilibria. Two new complexes, CaH2PO4H2CO+3(β= 54) and CaH2PO4HCO3(β= 43), are postulated to account for the data.