The precipitation of sparingly-soluble alkaline-earth metal and transition metal oxalates and mixed metal oxalates from aqueous solution: Ionic equilibria,crystalline phases and precipitation mechanisms

The precipitations of alkaline-earth metal and transition metal oxalates, from aqueous solution and from excess alkali metal oxalate solution, are surveyed: also the coprecipitations of transition metal oxalates together with alkaline-earth metal oxalates. The ionic equilibria that influence these precipitations are examined, the crystalline precipitate phases are tabulated and precipitation mechanisms are analysed.     

Growth and characterization of calcium hydrogen phosphate dihydrate crystals from single diffusion gel technique

Calcium hydrogen phosphate dihydrate (CaHPO₄·2H₂O, CHPD) a dissolved mineral in urine is known to cause renal or bladder stones in both human and animals. Growth of CHPD or brushite using sodium metasilicate gel techniques followed by light and polarizing microscopic studies revealed its structural and morphological details. Crystal identity by powder x‐ray diffraction confirmed the FT‐IR and FT‐Raman spectroscopic techniques as alternate methods for fast analysis of brushite crystals which could form as one type of renal stones. P‐O‐P asymmetric stretchings in both FT‐IR (987.2, 874.1 and 792 cm^‐1) and FT‐Raman (986.3 cm^‐1, 1057.6 cm^‐1 and 875.2 cm^‐1) were found as characteristics of brushite crystals. Differential Scanning Calorimetry (DSC) analysis revealed brushite crystallization purity using gel method by studying their endothermic peaks. This study incorporated a multidisciplinary approach in characterizing CHPD crystals grown in vitro to help formulate prevention or dissolution strategy in controlling urinary stone growth. Initial studies with 0.2 M citric acid ions as controlling agent in the nucleation of brushite crystals further support the presented approach. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The crystallisation of alkaline-earth metal phosphates from alkaline-metal halide and alkali metal phosphate melts: Some preliminary solubility-temperature phase diagram studies

The solubility v temperature phase diagrams, for magnesium and calcium meta-, pyroand orthophosphate solutions in the alkalineearth metal halide melts and in different alkali metal phosphate melts, have been analysed: these are the pseudo-binary sections of the ternary P₂O₅ MO MX₂ and P₂O₅ MO Na(K)₂O systems at MO/P₂O₅= 1, 2, and 3, respectively. The temperature ranges and yields for crystallisations of alkalineearth, metal phosphates from these melt solutions are discussed.     

Growth of octacalcium phosphate and dicalcium phosphate dihydrate crystals in silica gels

The crystallization of octacalcium phosphate (OCP) and dicalcium phosphate dihydrate (DCPD) have been studied at ambient temperature in silica gel using double diffusion system. The first stage of evolution of the OCP spherulites as well as their growth have been observed by optical microscopy. The growth rate of the OCP spherulites under stable external conditions was constant. The formation of DCPD crystals has been observed in the last stages of experiments.     

The precipitation of sparingly-soluble acid,neutral and basic metal salts from aqueous solution: Ionic equilibria at different pHs,precipitate phases and precipitation mechanisms

The precipitations of the sparingly-soluble acid, neutral and basic salts of the divalent cations beryllium, magnesium, calcium, strontium, barium, manganese (II), iron (II), cobalt (II), nickel (II), copper (II), zinc, cadmium, mercury (II), tin (II) and lead (II) with the inorganic anions sulphate, chromate, molybdate, monohydrogen phosphate, phosphate, and carbonate (from supersaturated aqueous solutions) are surveyed. The different types of ionic equilibria (cation hydrolysis, anion hydrolysis, ion-pair formation) that may influence these precipitations, at different pHs and ionic concentrations, are examined. The crystalline phases precipitated at different pHs are tabulated and the precipitation mechanisms (at different pHs) are analysed.     

FTIR spectroscopic,thermal and growth morphological studies of calcium hydrogen phosphate dihydrate crystals

Calcium hydrogen phosphate dihydrate (CHPD or DCPD) is found quite frequently in urinary calculi (stones). The CHPD crystals were grown by the single diffusion gel growth technique in sodium metasilicate gel. The crystals were found to be having platelet and broad needle type morphologies. The crystals were analyzed by FTIR spectroscopy. The thermal properties were studied by employing the thermogravimetric analysis. Various kinetic and thermodynamic parameters for dehydration were estimated. The selected platelet was studied by SEM for the growth morphologies indicating that the crystals grew in the form of leaflets having prominent (010) faces. This was in agreement with earlier reported studies. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The precipitation of sparingly-soluble metal ‘oxinates’ induction periods and nucleation rates

The precipitation of barium, strontium, calcium, magnesium, zinc, cadmium, lead, cobalt, nickel and copper 8-quinolinolates (‘oxinates’) was studied in buffer solutions (pHs from 10 to 4.5) at 22 °C: the metal cation and overall ‘oxinate’ with ‘oxine’ concentrations were varied from 0.0005 to 0.010 M (and saturation rations varied from 3 to 3.000). The induction periods, before the main growth surge, were measured at different metal oxinate concentrations (C) and saturation ratios S = (C)/S · P_r^1/3. Nucleation rates were determined from combined measurements of induction periods and final crystal numbers. Nucleation rates varied with saturation ratios according to the relation \documentclass{article}\pagestyle{empty}\begin{document}$ R_n = k_1 \cdot S^8 {\rm\ nuclei cm}^{ - {\rm 3}} {\rm sec}^{ - {\rm 1}} $\end{document} Induction periods decreased with increasing concentration and supersaturation; over the range t > 3.000 to < 1 sec, . Where t and t were factors that depended on nucleation rates and metal salt solubility. t values decreased and t values increased with decreasing solubility. Precipitation occurred through homogeneous nucleation. Low nucleation rates and prolonged induction periods in solutions of medium to high supersaturation were related to slow rate-determining stages in nucleus formation and to complex formation between metal cation and oxinate anion.     

The annealing of alkaline-earth metal polymetaphosphate powders (I). Crystallisation and sintering processes

Amorphous barium, strontium, calcium and magnesium polymetaphosphate powders (MP₂O₆)_n, n = 20 were prepared by dehydration of the corresponding polymetaphosphate hydrate precipitates. These powders were annealed by different continuous and isothermal heat treatments over the temperature range 450° to 700 °C, the glass transition temperatures T_g to above (T_g + 120) °C. The morphologies at different degrees of crystallisation were studied by scanning electron microscopy. For the main crystallisation process (ten to sixty-seventy percent crystallisation), the powder particles retained their original pea-pod form; then after seventy percent crystallisation, these crystallised particles sintered laterally to lozenge-shaped twin-hexagonal crystals of lengths 0.5 to 3 μm. Differential thermal analysis confirmed that a markedly exothermic crystallisation process (overall enthalpy changes from about 30 to 45 kJ mol⁻¹) was occurring within the powder particles. Crystallisation rates varied from < 0.005 min⁻¹ at temperatures near T_g to > 0.5 min⁻¹ at higher temperatures; the activation energies for this process varied from 360 to 560 kJ mol⁻¹. The completely annealed crystals were studied by scanning electron microscopy, X-ray diffraction and further differential thermal analysis to 1000 °C. The X-ray diffraction d value patterns, the fusion temperatures and the enthalpies of fusion were all in close agreement with the literature values for the corresponding beta alkaline-earth metal polymetaphosphates prepared by melt crystallisation.     

The precipitation of alkaline-earth metal and transition metal oxalates from aqueous solution. Induction periods and nucleation rates

The precipitation of magnesium, calcium, strontium and barium oxalates and of manganous, ferrous, cobalt, nickel and copper oxalates was studied from equivalent aqueous solutions at 22°C: the initial overall concentrations (C) generally varied from 0.001 to 0.2 M and the saturation ratios (S_mox) varied from <10 to >3000. The induction periods before the main growth surge were measured and nucleation rates were determined from final crystal numbers and induction periods. Precipitation occurred through homogenous nucleation: the critical nuclei in supersaturated alkaline-earth metal oxalate solutions were formed by aggregation of 6–8 M⁺⁺Ox⁻ ion-pairs while the critical nuclei in supersaturated transition metal oxalate solutions were formed by aggregation of 6–8 MOx complexes (to units of 3–4 M⁺⁺MOx₂⁻ ion-pairs). Over the range studied, the nucleation rates then varied with saturation ratios according to the relation, Nucleation rates at any saturation ratio decreased in the order Mg > Sr, Ba > Ca and Fe > Mn > Co, Cu > Ni; that is, generally in the order of increasing M⁺⁺–Ox⁻ and M⁺⁺–MOx₂⁻ bond strengths and increasing surface energies of the metal oxalate crystals. Induction periods decreased with increasing-concentration and saturation ratio; over The factors t _C1 and t _S1 depended in turn on the ‘rate constants’ for nucleation and growth during the induction periods and on metal oxalate solubilities.     

The precipitation of sparingly-soluble metal chromates from aqueous solution in the presence of chelating organic anions: Induction periods and nucleation rates

The precipitation of barium, zinc, lead, copper and silver chromates was studied in the presence of the chelating organic anions gluconate, tartrate, citrate, aspartate, glutamate and E.D.T. Aate. Generally, these chelators complex the metal cation and reduce the free metal cation and mean metal chromate concentrations in solution. The rates of nucleation with microcrystallite formation were then markedly reduced and the induction periods increased from below 1 sec to well above 100,000 sec with increasing chelator/cation concentration ratio; the experimental results were expressed by the relations; The coefficients F at any metal chromate concentration were related in turn to the stability constants of the metal-chelator complexes.     

The precipitation of iron (II) and nickel oxalates with alkaline-earth metal oxalates: Induction periods,nucleation rates and mechanisms

The coprecipitations of magnesium (and barium) iron(II) and nickel oxalate dihydrates were studied from excess magnesium and barium oxalate solutions. Nucleation rates were estimated from the induction periods for the coprecipitations. The nucleation rates in excess magnesium oxalate solutions first decreased with increasing excess oxalate anion concentration to minimum values and then increased with increasing magnesium cation concentration. At low to intermediate Mg/Fe(II) (and Mg/Ni) ratios the main nuclei were FeOx (and NiOx); at intermediate Mg/Fe(II) (and Mg/Ni) ratios the main nuclei were probably MgOx FeOx (and MgOx NiOx) mixtures and/or solid solutions of compositions Mg_αFe_{1 α}Ox (and Mg_αNi_{1 α}Ox); at high Mg/Fe(II) (and Mg/Ni) ratios the main nuclei were MgOx. The nucleation rates in excess barium oxalate solutions were similar to those for the precipitation of barium oxalate from supersaturated equivalent solutions. The main nuclei in most systems were BaOx.     

The precipitation of alkaline-earth metal molybdate powders from aqueous solution. Crystal numbers,final morphology and sizes

The precipitation of barium, strontium and calcium molybdates was studied from neutral equivalent solutions of concentrations from 0.0004 to 0.4 M at 25 °C. Crystal growth started after induction periods; the precipitiations were heterogeneously nucleated at low supersaturations and homogeneously nucleated at medium to high supersaturations. Barium molybdate was precipitated as tetragonal bipyramids, strontium molybdate generally as prisms and calcium molybdate as platelets. Crystal numbers at medium to high supersaturations increased with increasing inital metal molybdate concentrations according to the relation, The final crystal lengths in this range than decreased from maximum values (at the critical concentrations) with increasing initial metal molybdate concentrations according to the relation. Generally, for precipitation from solutions at any concentration, larger crystals were obtained in the precipitates of the salt of higher solubility.     

Optimization of the mineral content in polymeric gels: The effect of calcium to phosphate molar ratio

The influence of calcium to phosphate (Ca/P) molar ratio on the extent of mineralization in a model (poly)acrylamide gel was investigated under simulated physiological conditions. We hypothesized that the optimal growth of hydroxyapatite crystals will take place at the stoichiometric Ca/P molar ratio of 1.67. Phosphate ions were incorporated during the polymerization of the gel and mineralization was initiated by submersion of the gel in calcium acetate solution. Ca/P molar ratios were varied in the range of 0.5–5.0. The mineralized gel was characterized by Raman spectroscopy, scanning electron microscopy (SEM) and mineral weight fraction analysis via ashing. Raman spectra captured across the bulk of the gels indicated the presence of mineral at the core section. The phosphate symmetric stretching peak was observed in the range of 955–960 cm⁻¹ which is characteristic of hydroxyapatite. SEM images showed that crystals formed at Ca/P=2.0 were denser and larger in size than at other molar ratios. In agreement with SEM images, the dry weight fraction of mineral reached the maximum at the molar ratio of 2.0 and the extent of mineralization rapidly declined as the molar ratio diverged from 2.0. Also, the crystallinity of the mineral was optimum at the molar ratio of 2.0. Thus it appears that for effective mineralization, the molar ratio of the two ions needs to be in excess of the stoichiometric requirement, suggesting that ions are expended in processes other than the formation and growth of hydroxyapatite crystals. Therefore, the optimal level of mineralization in biomimetic-based growth of calcium phosphate crystals in sol–gel environment requires consideration of a range of molar ratios as opposed to using the molar ratios corresponding to that of the crystal species intended to grow.     

The precipitation of alkaline-earth metal and transition metal ‘oxinate’ powders from aqueous solution. Crystal numbers and final sizes

The precipitation of barium strontium, calcium, magnesium, zinc, cadmium and lead, manganese, cobalt, nickel and copper 8-quinolinolates (‘oxinates’) was studied from equivalent solutions, at pHs from 4.5 to 10, by optical microscopy: the metal cation and overall ‘oxinate’ with ‘oxine’ concentrations were varied from 0.0002 to 0.020 M (while the mean metal oxinate concentrations varied from 10⁻⁷ to 0.001 M). Crystal growth started after induction periods; the precipitations were heterogeneously nucleated at low supersaturations and homogeneously nucleated at medium to high supersaturations. The final precipitate crystal numbers depended on the number of nuclei formed during the induction periods. Crystal numbers at medium to high supersaturations increased with increasing initial metal oxinate concentration according to the relation, The final crystal lengths in this supersaturation range then decreased (from maximum values) with increasing initial mean metal oxinate concentration according to the relation, For precipitation from solutions of any concentration at any pH, smaller crystals were generally obtained in the precipitates from solutions of the metal oxinate of lower solubility.     

The crystallisation of alkaline-earth metal tungstates (molybdates,chromates, and sulphates) from metal chloride melts

This paper presents studies on the crystallisation of alkaline-earth metal tungstates, molybdates, chromates and sulphates by slow cooling of solutions in lithium chloride and alkaline-earth metal chloride melts at 600° to 1100°C. Solubility — temperature diagrams were prepared for this temperature range. The effects of solute — solvent interaction, crystallisation temperature range and rate of cooling on crystal form and size were investigated. Final crystal size increases with reduced rate of cooling and with increasing crystallisation temperature; barium sulphate crystallisation from lithium chloride melt is anomalous.     

The precipitation of alkaline-earth metal polymetaphosphate hydrate powders from aqueous solution. Nucleation and crystal growth processes,final precipitate compositions and crystal sizes

The precipitation of barium, strontium, calcium and magnesium polymetaphosphate hydrates was studied from aqueous solutions of initial metal salt concentrations from 0.001 to 3 M at 20 °C; equivalent sodium polymetaphosphate solutions were added to the alkaline-earth metal chloride solutions. Precipitate compositions were determined by chemical analysis, paper chromatography, potentiometric analysis, thermogravimetric and differential thermal analysis and infra-red spectrophotometry; final crystallite morphologies and sizes were studied by scanning electron microscopy and X-ray powder diffraction. Nucleation rates and nucleus numbers (at the end of the induction periods) were very high; crystal numbers varied from 10¹⁴ to 10¹⁵ at the critical concentrations to above 10¹⁷ per 1. solution. Crystal growth rates were also very high and varied as the fourth power of the initial metal salt concentration. High molecular-weight metal polymetaphosphate hydrates were precipitated from the more dilute solutions (0.001 to 0.025 M) while increasing amounts of the more soluble intermediate and low molecular-weight products were precipitated from the more concentrated solutions. Washing with cold water removed the tri- and tetralinear and cyclic phosphate products. The magnesium salts were not precipitated even from 3 M aqueous solutions. The precipitates from aqueous (NaPO₃(I))_n (n = 12) solutions had the compositions (BaP₂O₆ · 2.5 H₂O)₆, (SrP₂O₆ · 3 H₂O)_n and (CaP₂O₆ · 4 H₂O)_n while the magnesium salt precipitate from 20 percent aqueous acetone solution had the composition (MgP₂O₆ · 4 H₂O)_n, the precipitate n values varied from 19 to 13. The precipitates from aqueous (NaPO₃(II))_n (n = 20) solutions contained 0.5n to n additional adsorbed water molecules; these precipitate n values varied in turn from 40 to 26. The final precipitate powders consisted of ‘spherules’ of highly microcrystalline or amorphous polymer glass; the spherule diameters were about 0.2 μm at the critical concentrations and decreased to below 0.05 μm with increasing solution concentrations.     

The precipitation of alkaline-earth metal-nickel,aluminium (chromium (III) and iron (III)), silicyl(titanyl and zirconyl) hydroxide,carbonate and oxalate precipitate precursors for alkaline-earth metal oxyanion salt ceramics and allied materials – a review of small scale laboratory studies and industrial patents

The alkaline-earth metal – nickel, aluminium, chromium III, iron III, silicyl, titanyl and zirconyl hydroxide, carbonate and oxalate coprecipitates are the precursors for the preparation of the corresponding alkaline-earth metal oxyanion salt refractories, magnetics, dielectric and optical ceramics, cements and allied materials. The small-scale laboratory studies on the coprecipitations of these precursor materials, their compositions and the coprecipitation mechanisms are reviewed and the relevant industrial patents are described (140 references).     

Crystal structures of double calcium and alkali metal phosphates Ca10 M(PO4)7(M = Li, Na, K)

Crystal structures of double calcium and alkali metal phosphates described by the general formula Ca₁₀ M(PO₄)₇(M = Li, Na, K) have been studied by the Rietveld method. The lattice parameters are a = 10.4203(1) and c = 37.389(1) Å (for M = Li), a = 10.4391(1) and c = 37.310(1) Å (for M = Na), and a = 10.4229(1) and c = 37.279(2) Å (for M = K); sp. gr. R3c, Z = 6. The specific features of the distribution of alkali metal cations over the structure positions are discussed.     

Thermodynamic analysis of solute-solvent interactions in the solubility-temperature relations of alkaline-earth metal tungstates and sodium tungstate

This paper presents a study of thermodynamic analysis of the solubility-temperature phase diagrams for solutions of calcium, strontium and barium tungstate in sodium tungstate melts in the temperature range 660 to 1200 °C. At temperatures 1000 °C and above, the solutions were ideal but below 1000 °C the solutions became non-ideal and the non-ideality increased with decreasing temperature. At any mole fraction concentration of the solute the excess free energies of mixing and the activity coefficients increased in the order CaWO₄ > SrWO₄ > BaWO₄, whereas the excess chemical potentials decreased in the order CaWO₄ < SrWO₄ < BaWO₄.