Interaction of Citric or Hydrochloric Acid with Calcium Fluorapatite: Precipitation of Calcium Fluoride

The interaction of citric acid (H₃Ci) with calcium fluorapatite(Ca₁₀ F₂(PO₄)₆) was explored for two reasons: (i) to determine the role of the acid in the dissolution process and hence in the mechanism of tooth fluoridation and (ii) to determine whether there is any formation of calcium citrate. It was found that the concentration of calcium or fluoride ions is not stoichiometric with respect to that of phosphate ions in the solution and the stoichiometric deficiency of the amount of fluoride ions in the solution is twice that of the calcium ions and this demonstrates that some calcium fluoride is precipitated. The interaction may be represented by the following chemical equation (pH ≈ 2.5):where x (presently < 1) is related to the amount of CaF₂ that has precipitated and may be calculated from the experimental ratio of Ca to P in the solution. In order to establish that the interaction occurs generally with all acids, the reaction of hydrochloric acid with fluoroapatite was studied, and exactly analogous behavior was observed. These facts are also in accord with the solubility of fluorapatite and calcium fluoride. When the amounts of Ca and P in solution are corrected for the precipitated CaF₂, the ratio of Ca to P becomes stoichiometric (= 1.67). Preliminary X-ray analysis of the reacted fluorapatite showed that it contained calcium fluoride. The precipitated CaF₂ may act as a reservoir for the subsequent fluoridation of the mineral and may inhibit bacterial action in the mouth. The concentration of citrate ions does not change in solution, and no formation of calcium citrate is observed.     

锌负极材料锌酸钙的晶体形貌和物化性质研究

0引言 碱性可充锌基电池因其具有能量密度高、无环境污染而引起人们的广泛重视.上世纪70年代掀起了二次锌电极研究的热潮.但电极材料的形变、枝晶等问题依旧影响了锌电极的开发利用.     

Synthesis and Physicochemical Study of Chitosan-Containing Calcium Hydroxylapatites

The CaCl₂-(NH₄)₂HPO₄-(C₆H₁₁NO₄)_n-NH₃-H₂O system at 25°C was studied by the solubility (Tananaev’s residual concentrations) technique and pH measurements. The parameters providing for the coprecipitation of nanocrystalline (12.5–18.7 nm) calcium and chitosan hydroxylapatites were found. Calcium-deficient chitosan hydroxylapatites Ca_9.8(PO₄)₆(OH)_1.6 · xC₆H₁₁NO₄ · yH₂O, where x = 0.075 or 0.37 and y = 5.8 or 6.2, and stoichiometric calcium hydroxylapatites Ca₁₀(PO₄)₆(OH)₂ · xC₆H₁₁NO₄ · yH₂O, where x = 0.075, 0.1, 0.2, 0.37, 0.5, or 0.75 and y = 5.7–7.5, were synthesized. Solid phases were characterized by chemical analysis, X-ray powder diffraction, thermogravimetric analysis, and IR spectroscopy.     

Calcium hydroxylapatite/methylcellulose nanocomposite: Synthesis and properties

Organomineral nanocomposites (OMCs) of calcium hydroxylapatite Ca₁₀(PO₄)₆(OH)₂ (HA) and natural methylcellulose biopolymer [C₆H₇O₂(OH)_{3 ? x} (OCH₃) _x ] _n (MC) were prepared by coprecipitation from aqueous solution in the Ca(OH)₂-H₃PO₄-[C₆H₇O₂(OH)_3?x (OCH₃) _x ] _n -H₂O system under biomimetic conditions (37°C). Synthesis products were identified by X-ray powder diffraction, IR spectroscopy, thermal analysis, scanning and transmission microscopy, and electron diffraction. The compositions and structural features of the OMCs and the crystallographic parameters, sizes, and morphology of HA nanoparticles in the OMCs were determined. The HA nanoparticles in the OMCs were found to interact with MC molecules to form agglomerates with sizes on the order of 150–200 nm.     

Calcium Thiolates and Selenolates: Synthesis and Structures

The synthesis and structural characterization of a family of calcium thiolates and selenolates is described. In the solid state the compounds adopt either contact pairs, as observed in Ca(THF)₄(SMes*)₂ ( 1 ), (Mes* 2,4,6‐tBu₃C₆H₂), and Ca(THF)₄(SeMes*)₂, ( 2 ), or separated ions as shown in [Ca(18‐crown‐6)(HMPA)₂][SeMes*]₂ ( 3 ). The two different ion association modes are induced by addition of specific donors. The compounds were prepared by metalation involving the reaction of elemental calcium dissolved in dry liquid ammonia with either HSMes* or Mes*SeSeMes*. All compounds were characterized by X‐ray crystallography, NMR and IR spectroscopy.     

Synthesis of Calcium(II) Amidinate Precursors for Atomic Layer Deposition through a Redox Reaction between Calcium and Amidines

We have prepared two new Ca^II amidinates, which comprise a new class of ALD precursors. The syntheses proceed by a direct reaction between Ca metal and the amidine ligands in the presence of ammonia. Bis(N,N′‐diisopropylformamidinato)calcium(II) ( 1 ) and bis(N,N′‐diisopropylacetamidinato)calcium(II) ( 2 ) adopt dimeric structures in solution and in the solid state. X‐ray crystallography revealed asymmetry in one of the bridging ligands to afford the structure [(η²‐L)Ca(μ‐η²:η²‐L)(μ‐η²:η¹‐L)Ca(η²‐L)]. These amidinate complexes showed unprecedentedly high volatility as compared to the widely employed and commercially available Ca^II precursor, [Ca₃(tmhd)₆]. In CaS ALD with 1 and H₂S, the ALD window was approximately two times wider and lower in temperature by about 150 °C than previously reported with [Ca₃(tmhd)₆] and H₂S. Complexes 1 and 2 , with their excellent volatility and thermal stability (up to at least 350 °C), are the first homoleptic Ca^II amidinates suitable for use as ALD precursors.     

Sol-gel bioactive glass-ceramics Part I: Calcium phosphate silicate/wollastonite glass-ceramics

In this work we present experimental results about synthesis, structure evolution and in vitro bioactivity of new calcium phosphate silicate/wollastonite (CPS/W) glass-ceramics. The samples obtained were synthesized via polystep sol-gel process with different Ca/P+Si molar ratio (R). The structure of the materials obtained was studied by XRD, FTIR spectroscopy and SEM. XRD showed the presence of Ca₁₅(PO₄)₂(SiO₄)₆, β-CaSiO₃ and α-CaSiO₃ for the sample with R=1.89 after thermal treatment at 1200°C/2h. The XRD results are in good agreement with FTIR analysis. SEM denotes that apatite formation can be observed after soaking in simulated body fluid (SBF).      

Reaction Mechanism in the Mixtures of Calcium Polyphosphate with Apatite and Accompanying Minerals on Heating

Abstract

On obtaining defluorinated feeding phosphates from Kovdor apatite the system of Ca₁₀(PO₄)₆(OH)_1,4F_0,6-CaCO₃-(Ca,Mg)CO₃-Mg₂SiO₄-Ca(H₂PO₄)₂·H₂O-Mg(H₂PO₄)₂·xH₂O with mole correlation (CaO+MgO)/P₂O₅V3 is subjected to thermal treatment. On heating up to 500°C calcium and magnesium hydrophosphates turn into polyphosphates M(PO₃)₂ which in accordance with the increase of the temperature react with other components of the system. To establish the mechanism and conditions for reactions, thermal interactions in the mixtures of Ca(PO₃₎₂ and Ca₂P₂O₇ with apatite, phorsterite, dolomite and calcite when (CaO+MgO)/P₂O₅=3 have been investigated. Methods of chemical, thermal, chromatographic, X-ray and IR-spectroscopy analysis were used.     

Nitrate-Citrate Sol-Gel Synthesis of Hydrated Calcium Aluminate and Sorption Materials on Its Basis

Sol-gel technique was used to develop a method for synthesis of hydrated tricalcium aluminate Ca₃[Al(OH)₆]₂ and surface-layered gas-chromatographic sorbents on its basis. The materials obtained were examined by X-ray diffraction analysis, scanning electron microscopy, X-ray fluorescence microanalysis, adsorption porosimetry, and gas chromatography. It was found that cubic Ca₃[Al(OH)₆]₂ is the main phase in the surface layer of the sorption materials. The Hammett indicator method was used to examine the acid-base properties and the variation of the content of active centers between sorbents obtained in different ways. The chromatographic retention parameters were determined for test compounds, and the polarity and selectivity of the sorbents under study was evaluated. It was shown that Chromaton N-AW modified with hydrated calcium aluminate can be used, with addition of SE-30 stationary liquid phase, to separate complex mixtures of organic compounds.     

钙掺杂介孔氧化锆的合成及其表征

室温下1.5 mmol·L^-1硫酸铵溶液体系中,以阳离子型表面活性剂十六烷基三甲基溴化胺为模板,四水硫酸锆为无机前驱体,按照配位体辅助模板机理合成介孔氧化锆前驱体。通过液相后移植工艺实现了钙对氧化锆的掺杂改性。借助XRD、TEM、UV-Vis、XPS、N₂吸-脱附及室温荧光光谱(RTPL)等方法对样品进行了表征分析。研究表明,钙离子进入氧化锆骨架结构中,掺杂改性后的氧化锆具有很强的荧光特性,其孔径尺寸在2.2 nm左右。     

Calcium ion selective membrane electrode

The calcium salts of the mono- and diesters of [4-(1,1,3,3-tetramethylbutyl)phenyl phosphoric acid] have been prepared, and the individual esters as well as mixtures of the esters have been used with several varieties of polyvinyl chloride to construct macro membrane electrodes selective to calcium ions. These electrodes have been calibrated by using solutions of CaCl₂ and Ca ion buffers. The mixed ester electrodes showed Nernstian response in the concentration range 10^-1 to 10^-7M; the diester electrodes showed Nernstian response down to 7.9 x 10^-8M. The detection limit of the mixed ester electrode was 10^-8M, whereas that of the diester electrode was 7.9 x 10^-9M. Contrary to these results, the monoester electrodes showed unsatisfactory behavior. The responses of both the mixed ester and diester electrodes to calcium ions were not affected by the presence of sodium, potassium, or other divalent ions. Only ferric and lanthanum ions showed interferences with the electrode response to calcium ions. p]The electrode response was independent of pH in the approximate range 5–8 at a CaCl₂ concentration of 10^-4M. As the Ca ion concentration was increased, the range of pH independence widened to approximately 4–8. The dynamic response time constant of the mixed ester electrode was in the range 0.7–1.5 sec, whereas that of the diester electrode was in the range 0.5–0.75 sec.     

Nanocrystalline Calcium Carbonate Hydroxyapatites Containing Multiwall Carbon Nanotubes: Synthesis and Physicochemical Characterization

Nanocomposites (NCs) based on carbonated calcium hydroxyapatite (CHA) (bioapatite, an analogue of the inorganic component of mammalian bone tissue), carbonate apatite (Ca₁₀(PO₄)₆CO₃, CA), and multiwall carbon nanotubes (CNTs) are prepared in the system CaCl₂–(NH₄)₂HPO₄–NH₄HCO₃–NH₃–CNT–H₂O (25°C) by coprecipitation of calcium and phosphorus salts with CNTs from aqueous solutions. The physicochemical properties of nanocomposites are studied as dependent on their formation conditions and composition using the solubility (residual concentrations) method and pH measurements. The composition, crystal structure, morphology, spectroscopic and thermal characteristics of the synthesized CHA/CNT and CA/CNT NCs are determined using chemical analysis, X-ray powder diffraction, thermal analysis, and IR spectroscopy. Either CHA/CNT NCs of composition Ca₁₀(PO₄)₆(CO₃)_x(OH)_2–2х · yCNT · zH₂O, where х = 0.2; 0.5; 0.8; y = 1, 2, 3; z = 6.8–10.8, or (when х = 1) CA/CNT NCs of composition Ca₁₀(PO₄)₆CO₃ · yCNT · zH₂O, where y = 1–3; z = 6.9–10.8, are formed as the carbonate and CNT contents of the NC increase. Our results favor the understanding of the effect of carbonization and CNTs on the metabolic formation of native bone tissue apatite and can be used for the design of efficient ceramics for bone implants.     

Calcium Hydride Catalyzed Highly 1,2‐Selective Pyridine Hydrosilylation

Reaction of the calcium hydride complex (DIPPnacnac‐CaH?THF)₂ with pyridine is much faster and selective than that of the corresponding magnesium hydride complex (DIPPnacnac = [(2,6‐iPr₂C₆H₃)NC(Me)]₂CH). With a range of pyridine, picoline and quinoline substrates, exclusive transfer of the hydride ligand to the 2‐position is observed and also at higher temperatures no 1,2→1,4 isomerization is found. The heteroleptic product DIPPnacnac‐Ca(1,2‐dihydropyridide)?(pyridine) shows fast ligand exchange into homoleptic calcium complexes and therefore could not be isolated. Calcium hydride reduction of isoquinoline gave well‐defined homoleptic products which could be characterized by X‐ray diffraction: Ca(1,2‐dihydroisoquinolide)₂?(isoquinoline)₄ and Ca₃(1,2‐dihydroisoquinolide)₆?(isoquinoline)₆. The striking selectivity difference in the dearomatization of pyridines by Mg or Ca complexes could be explained by DFT theory and was utilized in catalysis. Whereas hydroboration of pyridine with pinacol borane with a calcium hydride catalyst gave only minor conversion, the hydrosilylation of pyridine and quinolines with PhSiH₃ yields exclusively 1,2‐dihydropyridine and 1,2‐dihydroquinoline silanes with 80–90 % conversion. Similar results can be achieved with the catalyst Ca[N(SiMe₃)₂]₂?(THF)₂. These calcium complexes represent the first catalysts for the 1,2‐selective hydrosilylation of pyridines.     

The alkoxide sol–gel process in the calcium phosphate system and its applications†

A single calcium glycolate was synthesized. The alkoxide was stable under ambient atmosphere. The calcium glycolate, phosphoric acid and P(OH) _x (OEt)_{3− x} were used as the precursors. Acetic acid was used as a reagent to modify the calcium glycolate and to change the acidity of the mixtures of the precursors. Mixtures of the calcium glycolate and phosphoric acid in a Ca/P ratio of 1.67 showed unusual sol–gel behavior. A transparent gel could be formed depending on the content of acetic acid and the extent of stirring. The behavior is attributed to a high viscosity and a large molecular size of the ethylene glycol solvent, leading to a strong dependence of the reactions in the mixtures on the diffusion process, greatly affected by stirring. When the mixtures of the calcium glycolate and PO(OH) _x (OEt) _{3− x} contained acetic acid at an acetic acid/Ca ratio of 3, stable alkoxide solutions with Ca/P ratios of 1.0, 1.5 and 1.67 could be formed. Different calcium phosphate compounds and hydroxyapatite coatings on alumina substrates could easily be formed from the alkoxide solutions. The chemical homogeneity provided by the alkoxide route leads to easy formation of the required products. Copyright © 1999 John Wiley & Sons, Ltd.     

Calcium alkoxyalanates : I. Synthesis and physicochemical characterization

Soluble calcium alkoxyalanates, Ca[AlH_4?n(OR)_n]₂, in which n ranges from 1 to 3, generally complexed with tetrahydrofuran, have been obtained by partial alcoholysis of calcium alanate with various branched aliphatic alcohols and with 2-methoxyethanol in toluene. With a few exceptions X-ray powder diffraction patterns and infrared AlH absorptions indicate that the calcium alkoxyalanates are individual molecular species.     

Calcium Carbonate@silica Composite with Superhydrophobic Properties

In this paper, spherical calcium carbonate particles were prepared by using CaCl₂ aqueous solution + NH₃·H₂O + polyoxyethylene octyl phenol ether-10 (OP-10) + n-butyl alcohol + cyclohexane inverse micro emulsion system. Then, nanoscale spherical silica was deposited on the surface of micron calcium carbonate by Stöber method to form the composite material. Scanning electron microscope (SEM), X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS) were used to characterize the morphology and structure of the composite material. It is found that the surface of the composite material has a micro-nano complex structure similar to the surface of a “lotus leaf”, making the composite material show hydrophobicity. The contact angle of the cubic calcium carbonate, spherical calcium carbonate and CaCO₃@SiO₂ composite material were measured. They were 51.6°, 73.5°, and 76.8°, respectively. After modification with stearic acid, the contact angle of cubic and spherical CaCO₃ were 127.1° and 136.1°, respectively, while the contact angle of CaCO₃@SiO₂ composite was 151.3°. These results showed that CaCO₃@SiO₂ composite had good superhydrophobicity, and the influence of material roughness on its hydrophobicity was investigated using the Cassie model theory.     

Solubility of Crystalline Calcium Isosaccharinate

The solubility of calcium isosaccharinate Ca(ISA)₂(c) was determined at 23°C as a function of pH (1–14) and calcium ion molality (0.03–0.52). The similarity of solubility from the over- and undersaturation directions for different equilibration periods indicated that equilibrium in these solutions was reached rapidly (< 7 days) and that these data can be used to develop thermodynamic equilibrium constants. The solubility data were interpreted using the Pitzer ion–interaction model. The logarithms of the thermodynamic equilibrium constants determined from these data were 1.30 for the dominant reaction at pH < 4.5 [Ca(ISA)₂(c) + 2H⁺ Ca²⁺ + 2HISA(aq)], and –2.22 for the dominant reaction at 4.5 [Ca(ISA)₂(c)+ Ca(ISA)₂(aq)]. In addition, the logarithm of the dissociation constant of HISA [HISA(aq) ISA- + H⁺] was calculated to be –4.46.     

Molecular Calcium Hydride: Dicalcium Trihydride Cation Stabilized by a Neutral NNNN‐Type Macrocyclic Ligand

Hydrogenolysis of bis(triphenylsilyl)calcium containing the neutral NNNN‐type macrocyclic amine ligand Me₄TACD [Ca(Me₄TACD)(SiPh₃)₂] ( 2 ), gave the cationic dinuclear calcium hydride [Ca₂H₃(Me₄TACD)₂](SiPh₃) ( 3 ), characterized by NMR spectroscopy, single‐crystal X‐ray analysis, and DFT calculations. Compound 3 reacted with deuterium to give the deuteride [D₃]‐ 3 .     

Synthesis and Reactivity of Discrete Calcium Imides

Protonolysis of dibenzylcalcium with triphenylsilylamine affords a thf‐coordinated tetrametallic calcium imide with a heterocuboid core structure. The use of calcium bis(tetramethylaluminate) as a precursor for tandem salt metathesis/protonolysis reactions with alkali metal amides of 2,6‐diisopropylaniline and triphenylsilylamine provides access to Lewis acid stabilized monocalcium imides of the type [(thf)₄Ca(μ₂‐NR)(μ₂‐Me)AlMe₂]. Treatment of [(thf)₄Ca(μ₂‐NSiPh₃)(μ₂‐Me)AlMe₂] with phenylsilane results in H?Si addition across the Ca?N imido bond, producing a homoleptic calcium amidoaluminate complex and putative CaH₂ whereas reaction with phenylacetylene leads to protonation of an AlMe moiety to yield the dimeric complex [(thf)Ca{NSiPh₃}{AlMe₂(CCPh)}]₂.     

Calcium Hydride Cation [CaH]+ Stabilized by an NNNN‐type Macrocyclic Ligand: A Selective Catalyst for Olefin Hydrogenation

Reaction of dibenzyl calcium complex [Ca(Me₄TACD)(CH₂Ph)₂], containing the neutral NNNN‐type macrocyclic ligand Me₄TACD (Me₄TACD=1,4,7,10‐tetramethyl‐1,4,7,10‐tetraazacyclododecane), with triphenylsilane gave the cationic dinuclear calcium hydride [Ca₂H₂(Me₄TACD)₂](PhCHSiPh₃)₂ which was characterized by NMR spectroscopy and single‐crystal X‐ray diffraction. The cation can be regarded as the ligand‐stabilized dimeric form of hypothetical [CaH]⁺. Hydrogenolysis of benzyl calcium cation [Ca(Me₄TACD)(CH₂Ph)(thf)]⁺ gave dicationic calcium hydrides [Ca₂H₂(Me₄TACD)₂][BAr₄]₂ (Ar=C₆H₄‐4‐^tBu; C₆H₃‐3,5‐Me₂) containing weakly coordinating anions. In THF, they catalyzed the isotope exchange of H₂ and D₂ to give HD and the hydrogenation of unactivated 1‐alkenes.