Precise Conductance Measurements on Dilute Aqueous Solutions of Sodium and Potassium Hydrogenphosphate and Dihydrogenphosphate

Precise conductance measurements are reported on dilute aqueous solutions of the sodium and potassium salts of orthophosphoric acid at 25 ^∘C. Conductance measurements on solutions of electrolytes such as these phosphate salts that exist in solution as complicated mixtures of ions have previously proved difficult to interpret. To overcome this, a mathematical method has been developed to calculate the concentrations of all the species in the aqueous system M₃PO₄/M₂HPO₄/M₂HPO₄/H₃PO₄ (M = Na or K) over a continuous range of stoichiometries. The Lee–Wheaton conductance equation has been used to interpret the conductance of these multicomponent solutions in terms of the limiting ionic conductances and concentrations of all the ions in the solution. The limiting molar conductances of the ions H₂PO₄ ⁻ and HPO₄ ²⁻ and the ion-pair formation constants of these ions with sodium and potassium ions were determine This work has enabled the accurate determination of solution parameters for the important hydrogenphosphate ions in water and provides an excellent example of the use of an advanced conductance theory in the analysis of the conductance of multicomponent electrolyte systems.     

Acoustic absorption in aqueous potassium phosphate

Measurements of acoustic absorption and velocity as a function of frequency and concentration in KH₂PO₄–K₂HPO₄ buffers at 4°C and pH 5-7 are reported. The dependence of the observed acoustic relaxation parameters on concentration is consistent with that to be expected from perturbation of a monomer-dimer equilibrium with an equilibrium constant [for 2H₂PO ₄ ^– (H₂PO₄)₂ ^2–] of 0.21 M^–1, a bimolecular rate constant of 5×10⁸ M^–1-sec^–1 and a standard volume change of –5 cm³ mole. The equilibrium constant for H₂PO ₄ ^– + HPO₄ ^2–H₃(PO₄)₂ ^3– is estimated to be 0.7 M^–1.     

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.     

Thermodynamics of aqueous solutions of poly ethylene glycol di-methyl ethers in the presence or absence of ammonium phosphate salts

Precise measurements of density and sound velocity at different temperatures ranging from 283.15 to 308.15 K for solutions of PEGDME250, PEGDME500 and PEGDME2000 in water and of PEGDME500 in aqueous solutions of 0.500 mol kg⁻¹ ammonium di-hydrogen phosphate ((NH₄)H₂PO₄) and di-ammonium hydrogen phosphate ((NH₄)₂HPO₄), binodal curves at temperature ranges 293.15-318.15 K for the aqueous PEGDME500 + (NH₄)₂HPO₄, PEGDME500 + (NH₄)₃PO₄, PEGDME2000 + (NH₄)H₂PO₄, PEGDME2000 + (NH₄)₂HPO₄, PEGDME2000 + (NH₄)₃PO₄ and PPG400 + (NH₄)₂HPO₄ two-phase systems, and liquid-liquid equilibrium data at temperature ranges 298.15-318.15 K for the aqueous PEGDME500 + (NH₄)₂HPO₄ and PEGDME2000 + (NH₄)₂HPO₄ two-phase systems have been taken. From the experimental density and sound velocity data, the apparent specific volume, excess specific volume, isentropic compressibility and isentropic compressibility deviation values have been determined and the effect of temperature, charge on the anion of electrolytes and molar mass of PEGDME on the volumetric and compressibility properties of the investigated polymer solutions as well as on the salting-out effect of PEGDMEs produced by ammonium phosphate salts has been studied.     

Reactions of Phosphate Radicals with Monosubstituted Benzenes. A Mechanistic Investigation

The kinetics and reaction mechanism of phosphate radicals with substituted benzenes, PhX (X=OH, Me, H, Cl, MeO, and CHO), were studied by flash photolysis and continuous irradiation of aqueous solutions containing potassium peroxodiphosphate (K₄[(PO₃)₂O₂]). The rate constants for the reactions of phosphate radicals with the aromatic contaminants PhX were measured and the reaction intermediates and products detected. A correlation between the logarithm of the rate constant for every substrate with H₂PO₄^., HPO₄^.?, and PO₄^.2? and the logarithm of the dissociation constant K_a for H₃PO₄, , and was found. A general mechanism is proposed (Scheme 1), which accounts for the experimental results.     

Mechanism of solid-state conversion of non-stoichiometric hydroxyapatite to diphase calcium phosphate

Two non-stoichiometric hydroxyapatites (n-HA) with Ca/P molar ratios of 1.50 and 1.58 and one stoichiometric hydroxyapatite (s-HA) with Ca/P = 1.67 were prepared from chemically pure CaHPO₄·2H₂O and KOH. After sintering at 1050 °C for 4 h, n-HA with Ca/P = 1.50 was transformed into -Ca₃(PO₄)₂, n-HA with Ca/P = 1.58 was converted to diphase calcium phosphate (DCP), while s-HA underwent no chemical transformations. The sintered and unsintered samples of hydroxyapatite were studied by IR spectroscopy, chemical analysis, and X-ray diffraction analysis. The crystallite dimensions were calculated, and a model for the DCP structure was proposed. The mechanism of the solid-state n-HA to DCP conversion was proposed on the basis of this model and published values of the volume diffusion coefficients of the OH^–, Ca²⁺, and PO₄ ^3– ions at 1000 °C.     

Solubility and Phase Behavior of Cr(III) Oxides in Alkaline Media at Elevated Temperatures

A platinum-lined, flowing autoclave facility is used to investigate the solubility behavior of Cr₂O₃ and FeCr₂O₄ in alkaline sodium phosphate, sodium hydroxide, and ammonium hydroxide solutions between 21 and 288°C. Baseline Cr(III) ion solubilities were found to be on the order of 0.1 nmolal, which were enhanced by the formation of anionic hydroxo and phosphato complexes. At temperatures below 51°C, the activity of Cr(III) ions in aqueous solution is controlled by a Cr(OH)₃·3H₂O solid phase rather than Cr₂O₃; above 51°C the saturating solid phase is -CrOOH. Measured chromium solubilities were interpreted via a Cr(III) ion hydrolysis/complexing model and thermodynamic functions for the hydrolysis/complexing reaction equilibria were obtained from least-squares analyses of the data. The existence of four new Cr(III) ion complexes is reported: Cr(OH)₃(H₂PO₄)^–, Cr(OH)₃(HPO₄)^2–, Cr(OH)₃(PO₄)^3–, and Cr(OH)₄(HPO₄)-(H₂PO₄)^4–. The last species is the dominant Cr(III) ion complex in concentrated, alkaline phosphate solutions at elevated temperatures.     

Chromium(III) Hydroxide Solubility in The Aqueous Na+-OH?-H2PO?4-HPO2?4-PO3?4-H2O System: A Thermodynamic Model

Chromium(III)-phosphate reactions are expected to be important in managing high-level radioactive wastes stored in tanks at many DOE sites. Extensive studies on the solubility of amorphous Cr(III) solids in a wide range of pH (2.8–14) and phosphate concentrations (10^–4 to 1.0 m) at room temperature (22±2)°C were carried out to obtain reliable thermodynamic data for important Cr(III)-phosphate reactions. A combination of techniques (XRD, XANES, EXAFS, Raman spectroscopy, total chemical composition, and thermodynamic analyses of solubility data) was used to characterize solid and aqueous species. Contrary to the data recently reported in the literature,⁽¹⁾ only a limited number of aqueous species [Cr(OH)₃H₂PO^–₄, Cr(OH)₃(H₂PO₄)^2–₂), and Cr(OH)₃HPO^2–₄] with up to about four orders of magnitude lower values for the formation constants of these species are required to explain Cr(III)-phosphate reactions in a wide range of pH and phosphate concentrations. The log K^o values of reactions involving these species [Cr(OH)₃(aq)+H₂PO^–₄⇌Cr(OH)₃H₂PO^–₄; Cr(OH)₃(aq)+2H₂PO^–₄⇌Cr(OH)₃(H₂PO₄)^2–₂; Cr(OH)₃(aq)+HPO^2–₄⇌Cr(OH)₃HPO^2–₄] were found to be 2.78±0.3, 3.48±0.3, and 1.97±0.3, respectively.     

Equations for calculation of the pH of buffer solutions containing sodium or potassium dihydrogen phosphate, sodium hydrogen phosphate, and sodium chloride at 25°C

Published thermodynamic data measured in aqueous mixtures of sodium or potassium dihydrogen phosphate with hydrogen phosphate and chloride at 25°C were used to test recently developed methods for calculation of the pH of phosphate buffer solutions. Equations for ionic activity coefficients are used in these methods. It is shown that all data used in the tests up to an ionic strength of about 0.5 mol-kg^-1 can be accurately predicted by the two methods recommended. In one of these methods, equations of the Hückel type are used for ionic activity coefficients and in the other equations of the Pitzer type. Several sets of phosphate buffer solutions are recommended,e.g., for calibrations of glass electrode cells. In the recommended sets, the pH of the buffer solutions can be calculated either by the Hückel or Pitzer method, and the pH predictions of these methods agree in most cases within 0.005 at least up to ionic strengths of about 0.2 mol-kg^-1. The pH values of the two primary pH standards endorsed by IUPAC based on aqueous mixtures of KH₂PO₄ and Na₂HPO₄,i.e., pH values of 6.865 and 7.413, can also be accurately predicted by the equations recommended in this study.     

The enthalpies of dilution of phosphate solutions at 30°C

The enthalpies of dilution of aqueous solutions of HCl, H₃PO₄, NaOH, NaH₂PO₄, Na₂HPO₄ and Na₃PO₄ in the molality range 0.1 to 1.0 mole-kg^–1 have been determined at 30°C. The relative apparent molal enthalpies _L of HCl, NaOH, NaH₂PO₄ and Na₂HPO₄ have been determined with the aid of an extended form of the Debye-Hückel limiting law. The relative apparent molal enthalpies for Na₃PO₄ solutions have been corrected for hydrolysis. A value of H _H ^o =9525±150 cal-mole^–1 was determined for the heat of hydrolysis of PO ₄ ^–3 . This value gives H ₃ ^o =3815±150 cal-mole^–1 for the ionization of H₂PO ₄ ^– , which is in good agreement with the value of H ₃ ^o =3500±500 cal-mole^–1 determined directly by Pitzer at 25°C. The relative apparent molal enthalpies for H₃PO₄ solutions have been corrected for ionization. A value of H ₁ ^o =–1900±150 cal-mole^–1 was obtained for the heat of ionization of H₃PO₄ to H⁺+H₂PO ₄ ^– . This value is in good agreement with the value of H ₁ ^o =–2031 cal-mole^–1 at 30°C determined by Harned and Owen from the temperature coefficient of the equilibrium constant and H ₁ ^o =–1950±80 cal-mole^–1 at 25°C determined from calorimetry by Pitzer.     

Voltammetric Study of the Influence of Various Phosphate Anions on Silver Nanoparticle Oxidation

The antibacterial properties of silver are strongly controlled by the redox couple of silver/silver(I). This work reports the influence of phosphate anions on silver nanoparticle oxidation, which is important given the abundance of phosphate species in biological systems. The three different species of anions were found to have a varying degree of influence on silver oxidation with the order PO₄³⁻>HPO₄²⁻>H₂PO₄⁻. It was found that in the presence of phosphate anions, the silver oxidation potential shifts to a less positive value, which indicated the increasing ease of the oxidation reaction of silver. Given that the interplay between silver and its cation is crucial to its antibacterial properties and significant concentrations of the HPO₄²⁻ anion are present at biological pH (near neutral), it is essential that the influence of the dibasic anion (HPO₄²⁻) on silver oxidation dynamics be considered for biological systems.     

Hartley–Crank Equations for Coupled Diffusion in Concentrated Mixed Electrolyte Solutions. The CaCl2 + HCl + H2O System

Nernst—Planck equations and ionic conductivities are used to calculate accurate limiting interdiffusion coefficients D _ik ^o for mixed electrolyte solutions. The electrostatic mechanism for coupled electrolyte diffusion is investigated by calculating the electrostatic contribution to each D _ik ^o coefficient to give the flux of each electrolyte driven by the electric field, which is generated by the migration of ions of different mobilities. Ternary diffusion coefficients are measured for dilute aqueous K₂SO₄ + KOH and Li₂SO₄ + LiOH solutions. Because of the different mobilities of K⁺ and Li⁺ ions relative to SO ₄ ^2– ions, diffusing K₂SO₄ drives cocurrent flows of KOH, but diffusing Li₂SO₄ drives counterflows of LiOH. To describe coupled diffusion in concentrated mixed electrolyte solutions, the Hartley–Crank theory is used to correct the limiting D _ik ^o coefficients for nonideal solution behavior, viscosity changes, ionic hydration, and the zero-volume flow constraint. Diffusion coefficients predicted for concentrated aqueous CaCl₂ + HCl solutions are compared with recently reported data. The large amount of HCl cotransported by the diffusing CaCl₂ is attributed to the salting out of HCl by CaCl₂ and to the migration of H⁺ ions in the diffusion-induced electric field, which slows down the Cl^– ions and speeds up the less-mobile Ca²⁺ ions to maintain electroneutrality along the CaCl₂ gradient.     

Zinc(II) oxide solubility and phase behavior in aqueous sodium phosphate solutions at elevated temperatures

A platinum-lined, flowing autoclave facility is used to investigate the solubility/phase behavior of zinc(II) oxide in aqueous sodium phosphate solutions at temperatures between 17 and 287°C. ZnO solubilities are observed to increase continuously with temperature and phosphate concentration. At higher phosphate concentrations, a solid phase transformation to NaZnPO₄ is observed. NaZnPO₄ solubilities are retrograde with temperature. The measured solubility behavior is examined via a Zn(II) ion hydrolysis/complexing model and thermodynamic functions for the hydrolysis/complexing reaction equilibria are obtained from a least-squares analysis of the data. The existence of two new zinc(II) ion complexes, Zn(OH)₂(HPO₄)^2– and Zn(OH)₃(H₂PO₄)^2–, is reported for the first time. A summary of thermochemical properties for species in the systems ZnO–H₂O and ZnO–Na₂O–P₂O₅–H₂O is also provided.     

Solubility behavior of titanium(IV) oxide in alkaline media at elevated temperatures

A platinum-lined, flowing autoclave facility was used to investigate the solubility behavior of titanium dioxide (TiO₂) in aqueous sodium phosphate, sodium hydroxide and ammonium hydroxide solutions between 17 and 288°. Baseline Ti(IV) solubilities were found to be on the order of one nanomolal, which were enhanced by the formation of anionic hydroxo- and phosphato-complexes. The measured solubility behavior was examined via a titanium(IV) ion hydrolysis/complexing reaction equilibria were obtained from a least squares analysis of the data. The existence of three new Ti(IV) ion complexes is reported for the first time: Ti(OH)₄(HPO₄)^2–, Ti(OH)₅(H₂PO₄)^2– and Ti(OH)₅(HPO₄)^3–. The triply-charged anionic complex was the dominant Ti(IV) species in concentrated, alkaline phosphate solutions at elevated temperatures. This complex is expected to exhibit C.N.=4 (i.e., Ti(OH)₂OPO ₄ ^3– ). A summary of thermochemical properties for species in the systems TiO₂-H₂O and TiO₂-P₂O₅-H₂O is also provided.     

Electroconduction of Potassium Orthophosphate Modified with Triple-Charged Cations

Solid electrolytes in the systems K_{3 – 3x} Me _x PO₄ (Me = Sc, Y, In, La, Nd, Gd, Tb) are synthesized. Their phase composition and the temperature and concentration dependences of their electroconductivity are studied. In all the systems there form solid solutions based on K₃PO₄, which have a high potassium cation conductance. The latter is due to the formation of potassium vacancies at substitutions 3K⁺ Me³⁺ and, at lower temperatures, to stabilization of a high-temperature -modification of potassium orthophosphate. The electroconductivity of synthesized solid solutions, which equals (4–7) × 10^–3 and 10^–1 S cm^–1 at 300 and 700°C, is similar to that of solid electrolytes K_{3 – x} P_{1 – x} E _x ^VIO₄ and K_{3 – 4x} E _x ^IVPO₄.     

Ternary mutual diffusion coefficients of KCl-LaCl3-water mixtures from taylor dispersion profiles

The Taylor dispersion (peak-broadening) method is used to measure ternary mutual diffusion coefficients at 25°C for fifteen different compositions of the system KCl–LaCl ₃ -water at ionic strengths from 0.02 to 0.24 mol-dm^–3. The diffusion coefficients are evaluated by least-squares analysis of the refractive index profiles across the dispersed solute peaks. Diffusing LaCl ₃ is found to cotransport significant amounts of KCl. Monte Carlo simulations are used to estimate the precision of ternary diffusion coefficients determined by the dispersion method. Tracer diffusion coefficients are also reported for La ³⁺ ions in aqueous KCl solutions and for K⁺ ions in aqueous LaCl ₃ solutions.     

Evaluation of the impact of phosphate salts on the formation of ionic-liquid-based aqueous biphasic systems

The present study aims at evaluating the capability of phosphate-based salts, whose anions can coexist in water depending on the media pH, to promote aqueous biphasic systems (ABS) formation with 1-butyl-3-methylimidazolium-based ionic liquids, as well as to infer on the influence of the ionic liquid anion in the overall process of liquid–liquid demixing. In this context, novel phase diagrams of ABS composed of several imidazolium-based ionic liquids and three phosphate salts and a mixture of salts (K₃PO₄, K₂HPO₄, K₂HPO₄ + KH₂PO₄, and KH₂PO₄) were determined by the cloud point titration method at 298 K and atmospheric pressure. The corresponding tie-line compositions, tie-line lengths, and pH values of the coexisting phases were also determined. The ionic liquids ability to promote ABS is related with the hydrogen-bond basicity of the composing anion – the lower it is the higher the ability of the ionic fluid to undergo liquid–liquid demixing. Moreover, similar patterns on the ionic liquids sequence were observed with the different phosphate salts. The phosphate anion charge plays a determinant role in the formation of ABS. The two-phase formation aptitude (with a similar ionic liquid) decreases in the rank: K₃PO₄ > K₂HPO₄ > K₂HPO₄ + KH₂PO₄ > KH₂PO₄. Yet, besides the charge of the phosphate anion, the pH and ionic strength of the aqueous media also influence the phase separation ability.     

Magnetite solubility and phase stability in alkaline media at elevated temperatures

A platinum-lined flowing autocláve facility was used to investigate the solubility behavior of magnetite (Fe₃O₄) in alkaline sodium phosphate and ammonium hydroxide solutions between 21 and 288°C. Measured iron solubilities were interpreted via a Fe(II)/Fe(III) ion hydroxo-, phosphato-, and ammino-complexing model and thermodynamic functions for these equilibria were obtained from a least-squares analysis of the data. A total of 14 iron ion species were fitted. Complexing equilibria are reported for 8 new species: Fe(OH)(HPO₄)^–, Fe(OH)₂(HPO₄)^2–, Fe(OH)₃(HPO₄)^2–, Fe(OH)(NH₃)⁺, Fe(OH)₂(PO₄)^3–, Fe(OH)₄(HPO₄)^3–, Fe(OH)₂(H₂PO₄)^–, and Fe(OH)₃(H₂PO₄)^3–. At elevated temperatures, hydrolysis and phosphato complexing tended to stabilize Fe(III) relative to Fe(II), as evidenced by free energy changes fitted to the oxidation reactions.

<Emphasis Type="Italic">Ab initio</Emphasis> Study of Al(III) Adsorption on Stepped 100 Surfaces of KDP Crystals

Crystals of potassium dihydrogen phosphate (KH₂PO₄, KDP) are grown in large scale for use as nonlinear material in laser components. Traces of trivalent metal impurities are often added to the supernatant to achieve habit control during crystal growth, selectively inhibiting the growth of the {100} face. Model systems representing AlPO₄-doped KDP {100} stepped surfaces are prepared and studied using ab initio quantum methods. Results of Hartree–Fock partial optimizations are presented, including estimated energies of ion pair binding to the steps. We find that the PO₄^3– ion takes a position not unlike that of a standard phosphate in the crystal lattice, while the aluminum atom is displaced far from a K⁺ ion position to establish coordinations with the PO₄^3– ion and to bind with another lattice-bound phosphate. Our optimized structures suggest that it is the formation of a fourth coordination of Al(III) to a third phosphate ion from solution, or perhaps from a nearby position in the lattice, that disrupts further deposition, pinning the steps.     

Ion chromatographic determination of hydrolysis products of hexafluorophosphate salts in aqueous solution

In this work, hydrolysis of three different hexafluorophosphate salts in purified water was investigated. Aqueous samples of lithium hexafluorophosphate (LiPF₆), sodium hexafluorophosphate (NaPF₆) and potassium hexafluorophosphate (KPF₆) were prepared and stored for different times. Ion chromatography (IC) with UV as well as non-suppressed and suppressed conductivity detection was used for the analysis of the reaction products. For the detection and identification of the formed decomposition products, an IC method using IonPac AS14A 250 mm × 4.0 mm i.d. column and 2.5 mM KHCO₃–2.5 mM K₂CO₃ eluent was established. Besides hexafluorophosphate, four other anionic species were detected in fresh and matured aqueous solutions. The hydrolysis products fluoride (F⁻), monofluorophosphate (HPO₃F⁻), phosphate (HPO₄²⁻) and difluorophosphate (PO₂F₂⁻) were found and were unambiguously identified by means of standards or electrospray ionization mass spectrometry (ESI-MS). It was shown that stability of hexafluorophosphate solutions depends on the nature of the counter ion and decreases in the order potassium > sodium > lithium.