Investigation of Acid-Base Equilibria for Symmetrically Substituted <Emphasis Type="Italic">P,P</Emphasis>′-Dialkyl Partial Esters of Bisphosphonic Acids

Bisphosphonic acids are powerful metal chelating reagents with a wide spectrum of applications. A series of partial ester derivatives of bisphosphonic acids was synthesized in order to investigate the physico-chemical changes induced by partial protection of the acid groups. The acid dissociation constants K₁ and K₂ for a homologous series of short-chain aqueous-soluble dibasic bisphosphonic acids, i.e., the P,P′-diethyl methylene-, ethylene- and propylene-bisphosphonic acid partial esters, H₂DEt[MBP], H₂DEt[EBP] and H₂DEt[PrBP], respectively, were determined by potentiometric titration and ³¹P NMR spectrometry. All three diethyl esters are rather acidic with pK₁ < 1.2 and pK₂ < 2.2. The influence of the separation between the phosphonate groups on ligand acidity and successive pK_a values is rationalized in terms of solvation, inductive and electrostatic effects.     

Spectrophotometric investigation of protolytic equilibria of rutin

The dissociation constants of rutin in aqueous-methanol medium (6040 v/v), the values of which are pK₁=–2.92±0.06, pK₂=6.72±0.11, pK₃=8.26±0.05, pK₄=12.57±0.09, were estimated by using the spectrophotometric method. They were ascribed to the dissociation of protonated oxygen atom at position 1 and then to hydroxyl groups at positions 7, 4', 5, respectively. Resonance structures of H₃L^– ion of rutin were suggested and, by using them, the greater dissociability of the hydroxyl group at position 7 in relation to the –OH group at position 4' was explained.     

Role of Acid-Base Equilibria in the Reduction of Selenious(IV) Acid

ABSTRACT

Electroanalytical methods have been widely used for determination of Se(IV), but the nature of the reduction processes involved is not well understood. Polarographic reduction occurs in three waves (i₁, i₂, and i₃) the height of which changes with pH. We proved that in wave i₁, H₃SeO₃ ⁺ is reduced, in i₂ H₂SeO₃, and in i₃ HSeO₃ ^-. SeO₃ ^2? is not reducible. All reductions involve a transfer of six electrons and yield selenides. Limiting currents are controlled by the rate of protonation. As proton donors, in addition to H₃O⁺, the acid forms of the buffer present also act. Limiting currents increase markedly with increasing concentration of the buffer. Tenfold increase in buffer concentration can result in up to 200% increase in limiting current.     

Pr4(SeO3)2(SeO4)F6 und NaSm(SeO3)(SeO4): Selenit‐Selenate der Selten‐Erd‐Elemente

Pr₄(SeO₃)₂(SeO₄)F₆ and NaSm(SeO₃)(SeO₄): Selenite‐Selenates of Rare Earth Elements Light green single crystals of Pr₄(SeO₃)₂(SeO₄)F₆ have been obtained from the decomposition of Pr₂(SeO₄)₃ in the presence of LiF in a gold ampoule. The monoclinic compound (C2/c, Z = 4, a = 2230.5(3), b = 710.54(9), c = 835.6(1) pm, β = 98.05(2)°, R_all = 0.0341) contains two crystallographically different Pr³⁺ ions. Pr(1)³⁺ is attached by six fluoride ions and two chelating SeO₃^2– groups (CN = 10), Pr(2)³⁺ is surrounded by four fluoride ions, three monodentate SeO₃^2– and two SeO₄^2– groups. One of the latter acts as a chelating ligand, so the CN of Pr(2)³⁺ is 10. The selenite ions are themselves coordinated by five and the selenate ions by four Pr³⁺ ions. The coordination number of the F^– ions is three and four, respectively. The linkage of the coordination polyhedra leads to cavities in the crystal structure which incorporate the lone pairs of the selenite ions. The reaction of Sm₂(SeO₄)₃ and NaCl in gold ampoules yielded light yellow single crystals of NaSm(SeO₃)(SeO₄). The monoclinic compound (P2₁/c, Z = 4, a = 1066.9(2), b = 691.66(8), c = 825.88(9) pm, β = 91.00(2)°, R_all = 0.0530) contains tenfold oxygen coordinated Sm³⁺ ions. The oxygen atoms belong to five SeO₃^2– and two SeO₄^2– ions. Two of the SeO₃^2– groups as well as one of the SeO₄^2– groups act as a chelating ligand. The sodium ions are surrounded by five SeO₄^2– ions and one SeO₃^2– group. One of the selenate ions is attached chelating leading to a coordination number of seven. Each selenite group is coordinated by six (5 × Sm³⁺ and 1 × Na⁺), each selenate ion by seven cations (5 × Na⁺ and 2 × Sm³⁺).     

Raman spectral study of the constitution and equilibria of nitric acid and d-nitric acid

From relative integrated intensity measurements of the symmetric stretching vibration of nitrate ion in nitric acid solutions (both HNO ₃ /H ₂ O and DNO ₃ /D ₂ O), the mass law concentration quotients, Q were obtained as functions of concentration. By extrapolation the limiting dissociation constants were estimated to be 24.4 and 15 respectively at 25°C. It is shown that this constant refers to a process in which the ion pair H ₃ O⁺ NO ₃ ^– is in equilibrium with the dispersed, solvated H ₃ O⁺ and NO ₃ ^– ions.     

The pK * for the dissociation of TRISH+ in NaClO4 media

The stoichiometric dissociation constant, pK^*, of TRISH⁺ has been determined from emf measurements in NaClO₄ solutions to 6.0m at 25°C. The results have been used to derive Pitzer coefficients for the interaction of TRISH⁺ with ClO ₄ ^– . The coefficients have been compared to the values in NaCl solutions. The values of pK^* for TRISH⁺ can be used to calibrate pH electrodes in NaClO₄ solutions using TRIS buffers.     

Thermal Decomposition Studies of Mixed Rare Earth Hydrogen Selenite Crystals

Mixed rare earth hydrogen selenite crystals, neodymium praseodymium hydrogen selenite (Nd_xPr_1−x(HSeO₃)(SeO₃)⋅2H₂O), Neodymium samarium hydrogen selenite (Nd_xSm_1−x(HSeO₃)(SeO₃)⋅2H₂O) and praseodymium samarium hydrogen selenite (Pr_xSm_1−x(HSeO₃)(SeO₃)⋅2H₂O) were prepared by gel diffusion technique. Simultaneous thermogravimetric and differential thermal analysis were carried out on the grown crystals. Decomposition is observed to occurs in six steps, which gives the evidence of successive losses of H₂O and SeO₂. The final product due to decomposition is a mixed rare earth oxides. FT-IR spectrum of the crystal samples heated at different temperatures complemented to the TG-DTA results. This revised version was published online in July 2006 with corrections to the Cover Date.     

Crystal structure,thermal behavior,and infrared absorption spectrum of cesium hydrogen selenite-selenious acid (12) CsHSeO3 · 2H2SeO3

The present work describes the crystal structure, thermal behavior, and infrared absorption spectrum of cesium hydrogen selenite-selenious acid (12), CsHSeO₃ · 2H₂SeO₃. This compound crystallizes in the monoclinic crystal system withP2₁c-C⁵_2b (Z = 4) as the space group. The unit cell dimensions are as follows:a = 8.9897(20), b = 8.5078(21), c = 12.6476(31)A˚, and β = 95.141(19)°. The crystal structure consists of discrete H₂SeO₃ molecules which are weakly hydrogen bonded to form layers which are further connected by (HSeO₃)⁻ ions with much stronger hydrogen bonds. The hydrogen atoms show no disorder within the hydrogen bonds. The Cs⁺ ions are coordinated to oxygens from both selenious acid molecules and hydrogen selenite ions. The thermal decomposition of CsHSeO₃ · 2H₂SeO₃ in air starts with incongruent melting due to rupture of hydrogen bonds at 310 K and is followed later by the formation of cesium diselenite phase. At higher temperatures (700 K) this compound decomposes with oxidation of selenium to yield cesium selenate. Both deformation and stretching vibrations of SeOH groups from both (HSeO₃)⁻ ions and H₂SeO₃ molecules can be found in the IR absorption spectrum of CsHSeO₃ · 2H₂SeO₃. This confirms the ordered position of hydrogen atoms in hydrogen bonds. The OH vibrations corresponding to hydrogen bonded species can be found also.     

The solubility product of crystalline ferric selenite hexahydrate and the complexation constant of FeSeO 3 +

The aqueous solubility of Fe₂(SeO₃)₃·6H₂O(c) was studied in deionized water adjusted to a range in pH values from 0.77 to 5.1 and in Na₂SeO₃ solutions ranging in concentrations from 0.0002 to 0.02 mol-dm^?3. The studies were conducted from both the undersaturation and oversaturation directions, with equilibration periods ranging from 7 to 1725 days. Stoichiometric dissolution of the solid was observed in solutions with pH values up to nearly 4. In general, concentrations of both Se and Fe decreased as pH increased from 1 to 4. Analyses of the equilibrated suspensions confirmed the equilibrium solid to be Fe₂(SeO₃)₃·6H₂O(c) and the aqueous Se to be selenite. Pitzer's ion-interaction model was used with selected ion pairs to interpret the solubility data. The logarithm of the solubility product of ferric selenite $$Fe_2 (SeO_3 )_3 .6H_2 O(c) \begin{array}{*{20}c} \to \\ \leftarrow \\ \end{array} 2Fe^{3 + } + 3SeO_3^{2 - } + 6H_2 O$$ was found to be ?41.58±0.11. This value is less than any reported in the literature for a ferric selenite by more than 10 orders of magnitude. The solubility data and calculations show an extremely strong interaction between aqueous Fe³⁺ and SeO ₃ ^2? ; interpretation of these data requires the inclusion of FeSeO ₃ ⁺ i.e. $$Fe^{3 + } + SeO_3^{2 - } \begin{array}{*{20}c} \to \\ \leftarrow \\ \end{array} FeSeO_3^ + , log K = 11.15 \pm 0.11$$      

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.     

Basicity of 2-phenyl-5-R-1,3,4-oxadiazoles

We have studied the basicity of 2-phenyl-5-R-1,3,4-oxadiazoles (R = H, Me, CH₂Ph, t-Bu, CH₂Cl, CCl₃, CF₃) in aqueous sulfuric acid solutions. These compounds are weak organic bases (pK_BH ⁺ is −1.8 to −5.2). The values of pK_BH ⁺ determined on the H₀ and X acidity function scales agree well with each other. The substituent at the 5 position has a substantial effect on the basicity of the 1,3,4-oxadiazole ring. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 748–756, May, 2006.     

Protonated silanoic acid HSi(OH)2+ and its neutral counterpart: a tandem mass spectrometric and CBS-QB3 computational study

Protonated silanoic acid, HSi(OH)₂⁺, 1a ⁺, is cleanly generated by the dissociative electron ionization of triethoxysilane, HSi(OC₂H₅)₃, and tetraethoxysilane, Si(OC₂H₅)₄. This follows from tandem mass spectrometric experiments and CBS-QB3 model chemistry calculations. The calculations predict that 1a ⁺ (ΔH_f(298 K) = 205 kJ mol⁻¹) is separated by high barriers from its isomers HOSiOH₂⁺, 1b ⁺ and HSi(O)OH₂⁺, 1c ⁺. Low-energy (metastable) ions 1a ⁺ dissociate by loss of H₂O via the pathway 1a ⁺ → 1b ⁺ → SiOH⁺ + H₂O. Analysis of the metastable peak for this process confirms that the isomerization step 1a ⁺ → 1b ⁺ is rate determining. The calculations further predict that the incipient ions 1b ⁺ communicate via a low barrier with the proton-bound dimer SiO···H···OH₂⁺, 1d ⁺. This dimer ion is much lower in energy than its counterpart OSi···H···OH₂⁺, 1e ⁺, which is calculated to be only marginally stable. A comparison of the potential energy diagram for the silicon-containing ions 1a ⁺– 1e ⁺ with that of their carbon analogues reveals that the dissociation chemistries of HSi(OH)₂⁺ and HC(OH)₂⁺ are only superficially similar. Neutralization–reionization experiments confirm the theoretical prediction that the HSi(OH)₂^• radical (ΔH_f(298 K) = −455 kJ mol⁻¹) is a stable species in the rarefied gas phase. However, owing to a mismatch of Franck–Condon factors a large fraction of the neutralized ions dissociates by loss of H^• yielding Si(OH)₂. Copyright © 2004 John Wiley & Sons, Ltd.     

Polarography of Uranium Chelate with 2,3-Cresotic Acid

The polarography of uranyl ion in 2,3-cresotic acid solution has been studied at 25°C under varying conditions of ligand concentration and pH. The ligands species were proved to be a 2,3-cresotate anion. The half-wave potential vs. pH value interpreted on the basis of pK value for the acid ionization, and resulted in agreement with the deduction. The mole ratio of metal to ligand was found to be 1:1 and 1:2 by conductometric titration. At pH < pK₁, the complex species of UO₂(H₂A)²⁺ and UO₂(HA)⁺ was identified. At pK₁ < pH < pK₂, the co-existence of UO₂(HA)⁺, UO₂(OH)(HA)₂^? and UO₂(A)₂^2– was confirmed. At pH > pK₂, the complex species of UO₂(OH) (A)₂^3– was formed.     

Unexplored metallic cation effect in the acidity constants of p-t-butylthiacalix[4]arene,p-t-butylcalix[4]arene and p-t-butylcalix[6]arene in aqueous-organic media

The influence of the metallic cation of the base (Li⁺, Na⁺ or K⁺) was determined on the acid–base constants of p-t-butylthiacalix[4]arene (TC4), p-t-butylcalix[4]arene (CA4) and p-t-butylcalix[6]arene (CA6) in ethanol/water in an large interval of pH values by potentiometry and spectrophotometry. The pK_a values determined by both methods correlate very well and these are characteristic for each macrocycle with influence of the cation of the base without a straight evidence of an effect by the size of the metallic cation. In the case of TC4, pK_a1 and pK_a2 were lower to Li⁺ and Na⁺ than with K⁺. For CA4, an effect of K⁺ on the pK_a2 with respect to Li⁺ was observed. A very different behaviour was observed for CA6 with Li⁺ and K⁺ showing a lower pK_a2 and a higher pK_a3 than with Na⁺. These effects were interpreted on the basis of the interaction/complexation of each cation with each macrocycle.     

Determination of pK-Values of Some Barbituric Acid Derivatives

Abstract

Practical dissociation constants (μ = 0.1, 25°C) of the following barbituric acids (pK₁, pK₂) were determined spectrophotometrically: 5-pivaloyloxy-5-(1-phenylethyl) - (7.0; 12.1); 5-hydroxy-5-(1-phenyethyl) - (7.8; 12.0); N-methoxymethy 1 -5-phenyl-5-ethyl (7.3₅; -); N-methyl -5-phenyl -5-ethyl - (7.8; -); 5-phenyl-5-ethyl - (7.3₅; 12.3). Structures were attributed to individual ionic forms involved.     

Nd2(SeO3)2(SeO4) · 2H2O – a Mixed‐Valence Compound containing Selenium in the Oxidation States +IV and +VI

Pale pink crystals of Nd₂(SeO₃)₂(SeO₄) · 2H₂O were synthesized under hydrothermal conditions from H₂SeO₃ and Nd₂O₃ at about 200 °C. X‐ray diffraction on powder and single‐crystals revealed that the compound crystallizes with the monoclinic space group C 2/c (a = 12.276(1) Å, b = 7.0783(5) Å, c = 13.329(1) Å, β = 104.276(7)°). The crystal structure of Nd₂(SeO₃)₂(SeO₄) · 2H₂O is an ordered variant of the corresponding erbium compound. Eight oxygen atoms coordinate the Nd^III atom in the shape of a bi‐capped trigonal prism. The oxygen atoms are part of pyramidal (Se^IVO₃)^2? groups, (Se^VIO₄)^2? tetrahedra and water molecules. The [NdO₈] polyhedra share edges to form chains oriented along [010]. The selenate ions link these chains into layers parallel to (001). The layers are interconnected by the selenite ions into a three‐dimensional framework. The dehydration of Nd₂(SeO₃)₂(SeO₄) · 2H₂O starts at 260 °C. The thermal decomposition into Nd₂SeO₅, SeO₂ and O₂ at 680 °C is followed by further loss of SeO₂ leaving cubic Nd₂O₃.     

One‐Electron Oxidation and Sensitivity of Uric Acid in On‐Line Electrochemistry and in Electrospray Ionization Mass Spectrometry

The sensitivity of detection of uric acid (H₂U) in positive ion mode electrospray ionization mass spectrometry (ESI MS) was enhanced by uric acid oxidation during electrospray ionization. With a carrier solution of pH 6.3>pK_a1=5.4 of H₂U, protonated unoxidized uric acid [H₂U+H]⁺ (m/z 169) was detected together with the protonated uric acid dimer [2H₂U+H]⁺ (m/z 337). The dimer likely forms by 1e^? oxidation of urate (HU^?) followed by rapid radical dimerization. A covalent structure of the dimer was verified by H/D exchange experiments. Efficiency of 2e^?, 2H⁺ oxidation of uric acid is low during ESI in pH 6.3 carrier solution and improves when a low on‐line electrochemical cell voltage is floated on the high voltage of the ES in on‐line electrochemistry ESI MS (EC/ESI MS). The intensity of the uric acid dimer decreases with an increase in the low applied voltage. In a carrier solution with 0.1 M KOH, pH 12.7>pK_a2=9.8 of H₂U, allantoin (Allnt) (MW 158.04), the final 2e^?, 2H⁺ oxidation product of uric acid, was detected as a potassium complex [K(Allnt)+K]⁺ (m/z 235) and the [2H₂U+H]⁺ dimer was not detected. In direct ESI MS analysis of 1000‐fold diluted urine [NaHU+H]⁺ (pK_sp NaHU=4.6) was detected in 40/60 (vol%) water/methanol, 1 mM NH₄Ac, pH ca. 6.3 carrier solution. A new configuration of the ESI MS instrument with a cone‐shaped capillary inlet significantly enhanced sensitivity in ESI and EC/ESI MS measurements of uric acid.     

Controlling the Self‐Assembly of a Mixed‐Metal Mo/V–Selenite Family of Polyoxometalates

Five mixed‐metal mixed‐valence Mo/V polyoxoanions, templated by the pyramidal SeO₃^2? heteroanion have been isolated: K₁₀[Mo^VI₁₂V^V₁₀O₅₈(SeO₃)₈]?18 H₂O ( 1 ), K₇[Mo^VI₁₁V^V₅V^IV₂O₅₂(SeO₃)]?31 H₂O ( 2 ), (NH₄)₇K₃[Mo^VI₁₁V^V₅V^IV₂O₅₂(SeO₃)(Mo^V₆V^V‐ O₂₂)]?40 H₂O ( 3 ), (NH₄)₁₉K₃[Mo^VI₂₀V^V₁₂V^IV₄O₉₉(SeO₃)₁₀]?36 H₂O ( 4 ) and [Na₃(H₂O)₅{Mo_18?xV_xO₅₂(SeO₃)} {Mo_9?yV_yO₂₄(SeO₃)₄}] ( 5 ). All five compounds were characterised by single‐crystal X‐ray structure analysis, TGA, UV/Vis and FT‐IR spectroscopy, redox titrations, and elemental and flame atomic absorption spectroscopy (FAAS) analysis. X‐ray studies revealed two novel coordination modes for the selenite anion in compounds 1 and 4 showing η,μ and μ,μ coordination motifs. Compounds 1 and 2 were characterised in solution by using high‐resolution ESI‐MS. The ESI‐MS spectra of these compounds revealed characteristic patterns showing distribution envelopes corresponding to 2? and 3? anionic charge states. Also, the isolation of these compounds shows that it may be possible to direct the self‐assembly process of the mixed‐metal systems by controlling the interplay between the cation “shrink‐wrapping” effect, the non‐conventional geometry of the selenite anion and fine adjustment of the experimental variables. Also a detailed IR spectroscopic analysis unveiled a simple way to identify the type of coordination mode of the selenite anions present in POM‐based architectures.     

Thermodynamic investigation of zirconium diselenite

Summary On the basis of calorimetric research of selenium dioxide, zirconium dioxide and zirconium diselenite dissolution reactions in the hydrofluoric acid solution under 298 K a standard enthalpy of Zr(SeO₃)₂ formation reaction from ZrO₂ and SeO₂ and a standard enthalpy of zirconium diselenite formation have been obtained. The value of enthalpy has been equal to -58.1±3.43 kJ mol^-1 in ZrO_2(solid)+2SeO_2(solid) Zr(SeO₃)_2(solid) reaction. The standard enthalpy of zirconium diselenite formation is equal to H_f,298⁰Zr(SeO₃)_2(solid)= -1603.2±3.8 kJ mol^-1. The H_f,298⁰ Zr(SeO₃)_2(solid) value has been determined for the first time.     

Syntheses and structures of three f-element selenite/hydroselenite compounds

The selenite/hydroselenite compounds Ce(SeO₃)(HSeO₃), Tb(SeO₃)(HSeO₃)·2H₂O, and Cs[U(SeO₃)(HSeO₃)]·3H₂O were synthesized by hydrothermal means at 453 K from the reaction of CeO₂ or Tb₄O₇ or UO₂ with SeO₂ and CsCl (as a mineralizer). Ce(SeO₃)(HSeO₃) crystallizes in the non-centrosymmetric orthorhombic space group Pca2₁. The structure comprises a two-dimensional network of interconnected CeO₁₀ bicapped distorted square antiprisms and SeO₃ trigonal pyramids. Tb(SeO₃)(HSeO₃)·2H₂O crystallizes in the non-centrosymmetric orthorhombic space group P2₁2₁2₁. The structure features a two-dimensional layer of interconnected TbO₈ distorted square antiprisms and SeO₃ trigonal pyramids. Cs[U(SeO₃)(HSeO₃)]·3H₂O crystallizes in the centrosymmetric monoclinic space group P2₁/n. The structure consists of two-dimensional layers of interconnected UO₇ pentagonal bipyramids and SeO₃ trigonal pyramids. The layers in all three structures are held together by hydrogen-bonding networks.