Growth zones and their boundaries in GaPO4

Gallium (ortho)-phosphate (GaPO₄) is a piezoelectric material which is grown from hydrothermal solutions. The growth zones, i.e. parts of the crystal grown by different faces, and their boundaries can influence the quality of the grown material. Thus, a theoretical and experimental study of form, size and properties of growth zones and their boundaries in GaPO₄ was performed. A program was developed with which the zones in the crystal as well as in any plane cutting through it can be calculated and outlined, simulating the facetted growth using growth rates determined from the crystal habit. This program was applied to spontaneous crystals as well as to crystals growm from seeds. Experimentally, growth zone boundaries which correspond to accumulation of OH were determined by scanning infrared absorption measurements.     

Thermodynamics of aqueous phosphate solutions: Apparent molar heat capacities and volumes of the sodium and tetramethylammonium salts at 25°C

A Picker flow microcalorimeter and a flow densimeter were used to obtain apparent molar heat capacities and apparent molar volumes of aqueous solutions of Na₃PO₄ and mixtures of Na₂HPO₄ and NaH₂PO₄. Identical measurements were also made on solutions of tetramethylammonium salts to evaluate the importance of anion-cation interaction. The experimental apparent molar properties were analyzed in terms of a simple extended Debye-Hückel model and the Pitzer ion-interaction model, both with a suitable treatment for the effect of chemical relaxation on heat capacities, to derive the partial molar properties of H₂PO ₄ ^– (aq), HPO ₄ ^2– (aq) and PO ₄ ^3– (aq) at infinite dilution. The volume and heat capacity changes for the second and third ionization of H₃PO₄(aq) have been determined from the experimental data. The importance of ionic complexation with sodium is discussed.     

Corrosion Inhibition of Mild Steel in Acidic Solutions Using 1,2,4-Triazolo[1,5-<Emphasis Type="Italic">a</Emphasis>]pyrimidine

The inhibiting action, thermodynamics, and adsorptive properties of 1, 2, 4-triazolo[1, 5-a]pyrimidine (TP) have been investigated for the corrosion of mild steel in 0.5 M H₂SO₄ and 0.5 M H₃PO₄ solutions by means of potentiodynamic, electrochemical impedance spectroscopy techniques and quantum chemical calculations. Results obtained revealed that TP is more effective in 0.5 M H₃PO₄ than in 0.5 M H₂SO₄. Theoretical fitting of different adsorption isotherms such as Langmuir, Flory–Huggins, Temkin, and the kinetic-thermodynamic models were tested. The obtained experimental data fitted all the applied adsorption isotherms except Langmuir. The thermodynamic activation parameters were calculated. The potential of zero charge was also determined using AC measurements to clarify the nature of surface charge of the mild steel in both acidic solutions. Quantum chemical parameters were calculated and explained. The data explained that the inhibition of mild steel in both acidic solutions takes place through physicochemical adsorption mechanism.     

Thermodynamics of electrolytes. VI. Weak electrolytes including H3PO4

Equations previously developed and widely applied to the thermodynamic properties of strong electrolytes are extended to solutions involving a dissociation equilibrium. Excellent agreement is obtained with the data for pure phosphoric acid to 6M and for phosphate buffer solutions. The parameters of the strong electrolyte components of the buffer solutions are taken from other work, and the remaining parameters for H⁺, H₂PO ₄ ^– , and H₃PO₄ are evaluated, including a pK of 2.146. The present method avoids ambiguities which formerly arose in treating weak acids with as small pK as this.     

On α-β phase transition in cristobalite-type Al1−xGaxPO4 (0.00?x?1.00)

The results of variable temperature powder X-ray diffraction and differential thermal analysis (DTA) studies on the orthorhombic (α) low-cristobalite to cubic (β) high-cristobalite phase transition for Al_1−xGa_xPO₄, (0.00?x?1.00) are presented. These studies reveal that all these compositions undergo reversible phase transitions from orthorhombic to cubic form at higher temperature. The high-temperature behavior of GaPO₄ is observed to have a different behavior compared to all other compositions in this series. Orthorhombic low-cristobalite-type GaPO₄ transforms to cubic high-cristobalite form at ∼605 °C. Above ∼700 °C, the cubic high-cristobalite-type GaPO₄ slowly transforms to trigonal quartz type structure. At about 960 °C, the quartz type GaPO₄ transforms back to the cubic high-cristobalite form. During cooling cycles the cubic phase of GaPO₄ reverts to trigonal quartz type phase. However, annealing of GaPO₄ at higher temperatures for longer duration can stabilize the orthorhombic low cristobalite phase. The phase transition temperatures and associated enthalpies are related to the change in unit cell volume and the orthorhombicity of the respective low cristobalite lattice.     

Computational chemistry and molecular simulations of phosphoric acid

Quantum mechanics (QM) calculations, molecular dynamics (MD) simulations using the condensed‐phase optimized molecular potentials for atomistic simulation studies (COMPASS) force field, and the atom‐centered density matrix propagation (ADMP) approach have been used to investigate properties of phosphoric acid (PA). QM using B3LYP/6‐31++G(d,p) density functional theory were used to calculate gas‐phase proton affinities and interaction energies of PA and its derivatives. Detailed single coordinate driving, followed by quadratic synchronous transit optimization was used to determine energy barriers for different proton transfer (PT) pathways. Determined energy barrier heights in ascending order are (unit: kJ/mol): H₃O⁺→H₃PO₄ (0); H₄P₂O₇→H₃PO₄ (2.61); H₃PO₄→H₂PO (5.31); H₄PO→H₃PO₄ (～7.33); H₃PO₄→H₄P₂O₇/H₃PO₄→H₃PO₄ (15.99); H₄P₂O₇→H₂O (28.61); H₃PO₄→H₂O (47.14). The COMPASS force field was used to study condensed‐phase properties of PA. Good agreement between experimental data and MD results including density, radial distribution functions, and self‐diffusion coefficient at different temperatures provides validation of the COMPASS force field for PA. Finally, preliminary ADMP studies on a cluster of three PA molecules shows that the ADMP approach can reasonably describe the PT and self‐dissociation processes in PA. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Rb(GaPO4)2(OH)(H2O)·H2O: a hydrated rubidium gallium phosphate analogue of GaPO4·2H2O and leucophosphite

Crystals of the title hydrated rubidium gallium phosphate, rubidium aqua‐μ₃‐hydroxo‐di‐μ‐phosphato‐digallium hydrate, were synthesized hydro­thermally at 453 K under autogenous pressure. The solid crystallizes in the monoclinic system and its structure was determined from single‐crystal X‐ray diffraction analysis. It is similar to dihydrated gallium phosphate, GaPO₄·2H₂O, which is isostructural with the mineral leucophosphite. The structure is built up from a three‐dimensional anionic framework composed of corner‐linked octameric Ga₄(PO₄)₄(OH)₂(H₂O)₂ units. The Ga atom is in an octahedral coordination. Connection of the Ga₄P₄ species generates eight‐ring channels, in which are encapsulated the Rb⁺ cations and water mol­ecules.     

Electrosynthesis of vanadophosphate by anodic oxidation of vanadium in phosphoric acid solutions

Anodic polarisation of vanadium has been studied in aqueous H₃PO₄ solutions. In all solutions, from 1 to 14 M H₃PO₄, vanadium oxidation occurred at +0.2 V/SCE. A passivation behaviour was observed. In all the cases, the polarisation curve showed a current peak followed by a current plateau. The peak current was dependent upon phosphoric acid concentration. It decreased with the increase of acid concentration. Current oscillations occurred for vanadium anode for lowest concentrated solutions. All the electrochemical observations agreed with the formation of an anodic electrodeposit of vanadophosphate. The yellowish product isolated after controlled potential oxidation has been identified to VOPO₄,2H₂O.     

Coupled diffusion produced by hydrolysis of aqueous potassium phosphate

Conductimetric and diaphragm cell techniques have been used to measure diffusion of aqueous potassium phosphate solutions at 25°C from 0.01 to 0.10 mol-dm^–3 (M). A significant portion of the aqueous K₃PO₄ component diffuses as equimolar amounts of potassium hydrogen phosphate and potassium hydroxide produced by hydrolysis: K₃PO₄+H₂O=K₂HPO₄+KOH. Because OH^– diffuses more rapidly than HPO ₄ ^2– , the total flow of KOH exceeds the flow of K₂HPO₄. The extra flow of KOH constitutes coupled transport of a second solute component. Ternary diffusion coefficients that describe interacting flows of K₃PO₄ and KOH components are reported. At low concentrations where phosphate is strongly hydrolyzed, the molar flux of the KOH component produced by diffusion of K₃PO₄ is six times larger than the flux of the K₃PO₄ component. Binary diffusion coefficients for aqueous K₂HPO₄ solutions are also reported. It is shown that ternary transport coefficients for K₃PO₄ solutions can be estimated from the properties of binary solutions of K₂HPO₄ and KOH.     

Formation of titanium phosphate composites during phosphoric acid decomposition of natural sphene

Decomposition of mineral sphene, CaTiOSiO₄, by H₃PO₄ is investigated in detail. During the dissolution process, simultaneous calcium leaching and formation of titanium phosphate (TiP) take place. The main product of decomposition is a solid titanium phosphate-silica composite. The XRD, solid-sate NMR, IR, TGA, SEM and BET data were used to identify and characterize the composite as a mixture of crystalline Ti(HPO₄)₂·H₂O and silica. When 80% phosphoric acid is used the decomposition degree is higher than 98% and calcium is completely transferred into the liquid phase. Formation of Ti(HPO₄)₂·H₂O proceeds via formation of meta-stable titanium phosphate phases, Ti(H₂PO₄)(PO₄)·2H₂O and Ti(H₂PO₄)(PO₄).The sorption affinities of TiP composites were examined in relation to caesium and strontium ions. A decrease of H₃PO₄ concentration leads to formation of composites with greater sorption properties. The maximum sorption capacity of TiP is observed when 60% H₃PO₄ is used in sphene decomposition.The work demonstrates a valuable option within the Ti(HPO₄)₂·H₂O-SiO₂ composite synthesis scheme, to use phosphoric acid flows for isolation of CaHPO₄·2H₂O fertilizer.     

Material properties of GaPO4 and their relevance for applications

A set of physical constants of gallium(ortho-)phosphate GaPO₄ has been determined in recent years. These constants allow to calculate precisely the properties of the most important applications of GaPO₄ such as BAW and SAW devices.     

Solid solutions between β-Ca3(PO4)2 and sodium-containing whitlockite

Mixtures of CaHPO₄, CaCO₃, and Na₂CO₃ were heated at 870°C under steam or under dry CO₂ until phase composition and weight were constant. According to chemical analysis and X-ray diffractometry the stability field of the β-Ca₃(PO₄)₂ phase is limited by the molar P/Ca ratio of 0.664 ± 0.003 and 0.675 ± 0.010 irrespective of the partial water vapour pressure. A continuous series of solid solutions was found between β-Ca₃(PO₄)₂ and a new whitlockite with the composition Ca₁₀Na(PO₄)₇. The IR spectrum of these solid solutions shows that the point symmetry of the PO₄ groups and their environment increases with increasing sodium content. This is in agreement with data published about the structure of β-Ca₃(PO₄)₂ and whitlockite. The composition of these solid solutions suggests that Na⁺ ions can replace H⁺ ions in the whitlockite structure. Carbonate and pyrophosphate ions are not incorporated in these whitlockites.     

Synthesis of cerium orthophosphates with monazite and rhabdophane structure from phosphoric acid solutions in the presence of hydrogen peroxide

It has been proposed to conduct the synthesis of cerium(III) orthophosphates by reacting cerium(IV) compounds with hydrogen peroxide in the presence of concentrated orthophosphoric acid at ambient temperature. It has been shown that the reaction of H₂O₂ with CeO₂ suspensions in H₃PO₄ medium produces CePO₄ · xH₂O (rhabdophane structure), while that with CeO₂ solutions in concentrated H₃PO₄ results in CePO₄ (monazite structure).     

Speciation of Sb(III) and Sb(V) by pH-control using three inorganic acids (hydrochloric, phosphoric and sulphuric)

Summary A method is described for the speciation of Sb(III) and Sb(V) using HG-AAS. The efficiency of stibine generation using different pH, from Sb(III) and Sb(V) solutions, was tested. At high pH-values Sb(V) is not reduced to form stibine, Sb(III) being selectively determined. The three acids HCl, H₂SO₄ and H₃PO₄ at controlled pH were used to generate stibine, H₃PO₄ being the most satisfactory for antimony speciation. The interference of Sb(V) was studied for the case of Sb(III) determination with stibine generation in H₃PO₄ medium (pH 1.81). The speciation of Sb(III) and Sb(V) is possible up to a ratio of 1:9.     

Apparent molar heat capacities of aqueous solutions of phosphoric acid and sulfur dioxide from 303 to 623 K and a pressure of 28 MPa

Heat capacities of aqueous solutions of phosphoric acid from 0.1 to 0.8 mol- kg^-1 and sulfur dioxide from 0.2 to 0.9 mol-kg^-1 have been measured with a flow heat-capacity calorimeter from 303 to 623 K and a pressure of 28 MPa. At the lowest molality single-solute solutions as well as mixtures of either H₃PO₄ or SO₂ with HC1 were measured to repress dissociation. Calculated apparent molar heat capacities were corrected for dissociation reactions and the chemical relaxation effect. Experimental results for mixtures were analyzed using Young’s rule. Standard state partial molar heat capacities of H₃PO₄(aq) and SO₂(aq) were obtained by extrapolation to infinite dilution. A few measurements of the densities of aqueous H₃PO₄ and SO₂ were made at 25°C and a pressure of 28 MPa.     

Transport and thermal characteristics of Cs<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Rb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>H<Subscript>2</Subscript>PO<Subscript>4</Subscript>

The transport and thermal properties of Cs_{1 − x} Rb _x H₂PO₄ in a wide range of compositions were studied. The binary salts Cs_{1 − x} Rb _x H₂PO₄ (x = 0–0.9) contain solid solutions with a structure of CsH₂PO₄. The binary salts were synthesized by mechanically mixing the starting components and growing crystals by isothermal evaporation from aqueous solutions. The properties of Cs_{1 − x} Rb _x H₂PO₄ salts obtained by different procedures were found to differ considerably. At higher rubidium contents in compounds obtained by mechanical mixing, the superionic transition temperature rose insignificantly, the high-temperature phase conductivity decreased twofold, the low-temperature conductivity increased within the limits of the order of magnitude, and the system of hydrogen bonds was slightly weakened. In Cs_{1 − x} Rb _x H₂PO₄ crystals grown from solutions, the temperature of the superionic transition decreased along with its slowing down, and the low-temperature conductivity increased by more than three orders of magnitude because of the higher contents of residual acid aqueous centers in the structure of the salt. These systems are characterized by increased thermal stability.     

Physicochemical Properties of the H3PO4-Dimethylformamide System

The viscosity, density, and electrical conductivity of H₃PO₄-dimethylformamide solutions and the enthalpies of solution and mixing of the components were measured in the entire composition range in the interval 25-65°C. It was concluded that phosphoric acid dissociates in dimethylformamide and that complexes of the presumed composition 2H₃PO₄·DMF are formed.     

Gas chromatographic analysis of gas emissions containing impurities of hydrocyanic acid and carbon oxysulfide

Gas chromatography was used for studying the retention of HCN, COS, H₂S, H₂O, CO₂, CO, and H₂ on organic porous polymer sorbents Chromosorb-104 and Hayesep C either unmodified or modified with different amounts of H₃PO₄. The effect of water on the signal of the thermionic detector was studied, and the conditions of the determination of 6–23 ppm HCN in aqueous solutions were found: column (3 m × 2 mm) with Hayesep C containing 15 wt % H₃PO₄. A procedure was developed for the determination of 15–1000 ppm COS in the presence of high concentrations (up to 1 vol %) of H₂S on a column (3 m × 2 mm) packed with Chromosorb-104 modified with 0.5 wt % H₃PO₄ with a flame photometric detector (396 nm). A basic scheme was proposed for the gas chromatographic analysis of the products of the catalytic detoxication of gas emissions in the process of coal gasification.     

Polarographic analysis. III

Summary Reduction of U^VI in H₃PO₄ solution passes to U^IV, the behaviour of the polarogram depends upon the acidity of the medium. In 0.1 M H₃PO₄ solution, a single wave is obtained which is characterised by a minimum due to the formation of an insoluble U^III phosphate. In 0.6 M H₃PO₄ and 1 M H₃PO₄ solutions two waves appear. The two waves join together to form a single wave representing the reduction of U^VI to the U^IV, by increasing the acidity of the medium or by the addition of gelatin. The diffusion coefficient of the phosphate complex varies with the concentration of H₃PO₄. It possesses a maximum value of 6.25×10^–6 cn²/sec in 0.5 M H₃PO₄ solution in absence of gelatin and of 5.59×10^–6 cm²/sec in presence of 0.005% gelatin.Part II: See Z. analyt. Chem. 176, 250 (1960).     

Interaction model for the volumetric properties of weak electrolytes with application to H3PO4

The Pitzer ion-interaction model for the thermodynamic properties of a weakly dissociating neutral solute is extended to include solution densities and compressibilities. Excellent agreement is obtained with the literature data for pure aqueous phosphoric acid to 8m. The first and second pressure derivatives of the interaction parameters for H⁺, H₂PO ₄ ^? , and H₃PO₄ are evaluated, in addition to the limiting partial molar volumes and adiabatic compressibilities of the ionized and unionized acids. The present method requires no additional data or extraneous assumption regarding the properties of the free electrolyte in extrapolating to infinite dilution. The quantities evaluated are used to estimate the pressure dependence of the ionization constant, activity coefficients, and speciation to 1 kbar. The estimated effect of pressure on aqueous phosphoric acid ionization is in excellent agreement with experimental data.