Second Dissociation Constant of Bis-[(2-hydroxyethyl)amino]acetic Acid (BICINE) and pH of Its Buffer Solutions from 5 to 55 °C

The values of the second dissociation constant, K ₂, and related thermodynamic quantities of the ampholyte bis[(2-hydroxyethyl)amino]acetic acid (BICINE) have been determined at temperatures from 5 to 55 ^∘C. The pH values of six equimolal buffer solutions, and four buffer solutions having ionic strengths (I = 0.16 mol⋅kg⁻¹) similar to those in blood plasma, have been evaluated at 12 temperatures from 5 to 55 ^∘C using the Bates–Guggenheim convention. The liquid junction potentials (E _j ) between the buffer solutions of BICINE and saturated KCl solution of the calomel electrode at 25 and 37 ^∘C have been estimated by measurement with a flowing junction cell. These values of E _j have been used to ascertain the operational pH values at 25 and 37 ^∘C. The pK ₂ values at 25 and 37 ^∘C are 8.333 and 8.156, respectively. The thermodynamic quantities associated with the second acid dissociation have been calculated from the values of pK ₂ as a function of temperature. The zwitterionic buffer BICINE was shown to be useful as a pH standard in the region close to that of blood serum.     

On the Dissociation of Methyl Orange: Spectrophotometric Investigation in Aqueous Solutions from 10 to 90 ∘C and Theoretical Evidence for Intramolecular Dihydrogen Bonding

The dissociation of methyl orange was investigated by spectrophotometry in aqueous solutions from 10 to 90 ^∘C and by quantum chemical calculations. Combined chemometric and thermodynamic analyses of the spectrophotometric data were used to simultaneously extract the thermodynamic stabilities and the spectrophotometric attributes of the dominant methyl orange species in solutions containing less than 20.00 mmol-kg⁻¹ perchloric acid and submillimolal concentrations of methyl orange. The analyses revealed the presence of only one monomeric deprotonated and one monomeric protonated species. The spectra did not reveal any evidence for the presence of tautomeric equilibria between the protonated azo and ammonium species in the experimental range studied. Thermodynamic analyses of the temperature-dependent dissociation constants showed the reactions to be endothermic and enthalpy driven with increasing acidity and increasing temperature. All molar absorption coefficients in the 275–375 nm range can be adequately reproduced in the 10–90 ^∘C range with a set of Gauss–Lorentz parameters and used to predict the absorption spectra for any desired condition. The dominant features of the spectrophotometric attributes of the methyl orange species could also be retrieved in Time Dependent–Density Functional Theory (TD–DFT) calculations. Topological analyses of the electron density also revealed the formation of a dihydrogen bond between the azo proton and an adjacent phenyl ring hydridic hydrogen which increases the stability of the azo molecules relative to the ammonium molecule.     

Effect of Processing Temperature on Gelation and Physical Properties of Low Density TEOS Based Silica Aerogels

In the present paper the experimental results of the effect of sol-gel processing temperature on the physical properties of the TEOS based silica aerogels are reported and discussed. The aerogels were produced by the two step sol-gel process at various temperatures in the range of 26–70^∘;C followed by supercritical drying using methanol solvent extraction. A remarkable reduction in the gelation time was observed from three and a half days at room temperature to a mere 18 hours at 50^∘;C. The best quality aerogels in terms of low density and high optical transmission were obtained for 6 hours hydrolysis time. The aerogels were characterized by the measurements of bulk density, volume shrinkage, porosity, refractive index and optical transmission. Monolithic aerogels with ultra low density (∼0.018 g/cm³), extremely high porosity (∼99%) and optimum optical transmission at 700 nm (∼75%) were obtained for the molar ratio of TEOS:MeOH:acidic water:basic water at 1:99:10.42:14.58 respectively.     

Thermodynamic Properties of Peptide Solutions. Part 18. Partial Molar Isentropic Compressibilities of Gly-X-Gly Tripeptides (X = Tyr,Pro, Gln,Asp and Glu), and the Peptide Salts K[GlyAspGly], Na[GlyGluGly] and GlyLysGly Acetate in Aqueous Solution at 25 <Superscript>∘</Superscript>C

The partial molar isentropic compressibilities at infinite dilution, K∘_S,2, have been determined for several tripeptides of the sequence glycyl-X-glycine, where X is one of the amino acids tyrosine, proline, glutamine, aspartic acid, glutamic acid and lysine in aqueous solution at 25 ^∘C. These results, along with those for triglycine, were used to estimate the contributions of the amino acid side-chains to the partial molar isentropic compressibilities of polypeptides. Values for K∘_S,2 have also been determined for aqueous solutions of the two peptide salts K[glyaspgly] and Na[glyglugly]. The K∘_S,2 results for the peptides and their salts have been combined with literature data for electrolytes to calculate the changes in isentropic compressibility upon ionization of the acidic side-chains. The results are compared with those for other carboxylic acid systems.     

Low-Temperature Sol-Gel Synthesis of Single-Phase ZrTiO4 Nanoparticles

Ceramic ZrTiO₄ powders were prepared by a sol-gel method using zirconium oxychloride and titanium tetraisopropoxide. In situ high temperature X-ray diffraction results show that crystallization of the amorphous gel starts at 400 ^∘C. Single-phase ZrTiO₄ nanoparticles were obtained after heat treatment at 450 ^∘C for one hour. An average particle size of 46 nm has been determined by nitrogen adsorption analysis. After pressing these sinteractive powders, pellets with controlled pore size distribution were obtained by sintering at temperatures as low as 400 ^∘C. The analysis of pores by mercury porosimetry gives an average porosity of 45%. The electrical resistivity, determined by impedance spectroscopy measurements at 24 ^∘C under different humidity environments, shows the ability of these pellets to adsorb water vapor in the porous surfaces. Pellets fabricated with the nanosized powders prepared by the sol-gel technique are proposed as good candidates to be used in humidity sensing devices.     

Thermodynamics of Electrolyte Mixtures: HCl + NdCl<Subscript>3</Subscript> + H<Subscript>2</Subscript>O from 5 to 55 <Superscript>∘</Superscript>C

Electromotive-force (emf) measurements of cells containing solutions of hydrochloric acid and neodymium chloride were reported at constant total ionic strengths (I) of 0.01, 0.025, 0.05, 0.1, 0.25, 0.5, 1.0, and 1.5 mol-kg⁻¹ at 11 temperatures ranging from 5 to 55 ^∘C, and at I = 2.0 mol-kg⁻¹ at 25 ^∘C. Hydrogen and silver–silver chloride electrodes were used in these cells. Results from the emf measurements, the mean molal activity coefficients of HCl in HCl + NdCl₃ + H₂O mixtures, as well as the Harned interaction coefficients using Harned's rule are reported in the preceding article in this issue. The ion-interaction model of Pitzer is applied here for the evaluation of the Pitzer mixing coefficients, ^Sθ_H,Nd and ψ_H,Cl,Nd, as well as the linear representation of the temperature derivatives of ∂^Sθ_H,Nd /∂ T and ∂ψ_H,Nd,Cl/∂T. The activity coefficients at several ionic strength fractions y of NdCl₃ are given at 25 ^∘C. The results are interpreted in terms of ionic interactions.     

Potentiometric Determination of the Stability Constants of Trimethyltin(IV) Chloride Complexes with Imino-bis(Methylphosphonic Acid) in Water and Dioxane–Water Mixtures

The interaction of trimethyltin(IV) (TMT) with imino-bis(methylphosphonic acid) (IDP), abbreviated as H₄L, was investigated at 25 °C and at ionic strength 0.1 mol⋅dm⁻³ (NaNO₃) using a potentiometric technique. The formation constants of the complexes formed in solution were calculated using the nonlinear least-squares program MINIQUAD-75. The stoichiometry and stability constants are reported for the complexes formed. The results show the formation of 110, 111, 112 and 11-1 complexes for the TMT–IDP system. The concentration distribution of the various complex species was evaluated. The effect of dioxane as a solvent, on both the protonation constants and the formation constants of trimethyltin(IV) complexes with IDP, is discussed. The thermodynamic parameters ΔH ^∘ and ΔS ^∘ calculated from the temperature dependence of the equilibrium constants were evaluated. The effect of ionic strength on the protonation constants of IDP is also discussed.     

Solubility of Litharge (α-PbO) in Alkaline Media at Elevated Temperatures

An inert, flowing autoclave facility was used to investigate the solubility behavior of α-PbO (litharge, tetragonal) in aqueous solutions of morpholine, ammonia and sodium hydroxide between 38 and 260 ^∘C. Lead solubilities increased from about 0.4 mmol-kg⁻¹ at 38 ^∘C to about 4.5 mmol-kg⁻¹ at 260 ^∘C and were relatively insensitive to the concentration and identity of the reagent used to control the pH. The measured lead solubilities were interpreted using a Pb(II) ion hydroxocomplexing model and thermodynamic functions for these equilibria were obtained from a least-squares analysis of the data. A consistent set of thermodynamic properties for the species Pb(OH)⁺, Pb(OH)₂(aq) and Pb(OH)₃⁻ was obtained that will permit accurate lead oxide solubility calculations to be made over broad ranges of temperature and alkalinity.     

Dissociation of Fumaric Acid: Spectrophotometric Investigation in Aqueous Solutions from 10 to 90 <Superscript>∘</Superscript>C and Theoretical Considerations

The dissociation constants of fumaric acid were extracted from UV-vis spectra in the 10–90 ^∘C range. These values were used to extract thermodynamic parameters that showed the temperature effects on the dissociations reactions to be dominantly driven by the solvent. The molar absorption coefficients for the fumaric acid, the bifumarate and fumarate species, can be accurately reproduced with the two-term Gauss–Lorentz equation. Deconvolution of these bands showed strong π–π^∗ transitions for all species and weaker charge-transfer-to-solvent transitions for the charged species. TD-DFT calculations were used to identify the most important molecular orbitals involved in the vertical excitations of the fumaric acid species. The electron population and their states of delocalization were also estimated with topological analyses of the electron density and of the Becke–Edgecombe Electron Localization Function.     

The electrophoresis of poly(N-isopropylacrylamide) microgel particles

 The electrophoretic mobility of a poly(N-isopropylacrylamide) microgel containing carboxylic groups has been measured as a function of the ionic strength, between 0.1 and 100 mM NaCl, over the temperature range 2545 ^∘C. The mobility data obtained have been evaluated using different models, including the porous-sphere, the soft-plate and the soft-sphere models as well as the hard-sphere model developed by Henry and later refined by O'Brien and White. The “porous” or “soft” behaviour is evident at lower temperatures, whereas at higher temperatures none of the models can fully explain the observed behaviour. It is suggested that the discrepancies at higher temperatures can be partly ascribed to the neglect of the relaxation effect in the “soft” models. Received: 30 June 1999/Accepted in revised form: 12 October 1999     

Boiling Temperature and Reversed Deliquescence Relative Humidity Measurements for Mineral Assemblages in the NaCl + NaNO3 + KNO3 + Ca(NO3)2 + H2O System

Boiling temperature measurements have been made at ambient pressure for saturated ternary solutions of NaCl + KNO₃ + H₂O, NaNO₃ + KNO₃ + H₂O, and NaCl + Ca(NO₃)₂ + H₂O over the full composition range, along with those of the single salt systems. Boiling temperatures were also measured for the four component NaCl + NaNO₃ + KNO₃ + H₂O and five component NaCl + NaNO₃ + KNO₃ + Ca(NO₃)₂ + H₂O mixtures, where the solute mole fraction of Ca(NO₃)₂, x{Ca(NO₃)₂}, was varied between 0 and 0.25. The maximum boiling temperature found for the NaCl + KNO₃ + H₂O system is ≈134.9 ^∘C; for the NaNO₃ + KNO₃ + H₂O system is ≈165.1 ^∘C at x(NaNO₃) ≈ 0.46 and x(KNO₃) ≈ 0.54; and for the NaCl + Ca(NO₃)₂ + H₂O system is 164.7 ± 0.6 ^∘C at x{NaCl} ≈ 0.25 and x{Ca(NO₃)₂} ≈ 0.75. The NaCl + NaNO₃ + KNO₃ + Ca(NO₃)₂ + H₂O system forms molten salts below their maximum boiling temperatures and the temperatures corresponding to the cessation of boiling (dry-out temperatures) of these liquid mixtures were determined. These dry-out temperatures range from ≈300 ^∘C when x{Ca(NO₃)₂} = 0 to ≥ 400 ^∘C when x{Ca(NO₃)₂} = 0.20 and 0.25. Mutual deliquescence/efflorescence relative humidity (MDRH/MERH) measurements were also made for the NaNO₃ + KNO₃ and NaCl + NaNO₃ + KNO₃ salt mixture from 120 to 180 ^∘C at ambient pressure. The NaNO₃ + KNO₃ salt mixture has a MDRH of 26.4% at 120 ^∘C and 20.0% at 150 ^∘C. This salt mixture also absorbs water at 180 ^∘C, which is higher than expected from the boiling temperature experiments. The NaCl + NaNO₃ + KNO₃ salt mixture was found to have a MDRH of 25.9% at 120 ^∘C and 10.5% at 180 ^∘C. The investigated mixture compositions correspond to some of the major mineral assemblages that are predicted to control brine composition due to the deliquescence of salts formed in dust deposited on waste canisters in the proposed nuclear repository at Yucca Mountain, Nevada.     

Thermodynamics of a seventh generation carbosilane dendrimer with phenylic substituent on the initial branching center and terminal butyl groups

The temperature dependence of the heat capacity of carbosilane dendrimer of the seventh generation of series 3 × 3 with phenylic substituent on the initial branching center and terminal butyl groups was studied by the methods of precision adiabatic vacuum calorimetry and differential scanning calorimetry over the temperature range T = 7–580 K for the first time. Physical transformations in the above temperature range were detected and their standard thermodynamic characteristics estimated and analyzed. The experimental results were used to calculate standard thermodynamic functions C _p ^∘(T), H ^∘(T)-H ^∘(0), S ^∘(T)-S ^∘(0), and G ^∘(T)-H ^∘(0) (0) over the range T → 0–580 K and standard entropy of formation of dendrimer at T = 298.15 K. The thermodynamic properties of carbosilane dendrimers of the seventh generation of series 4 × 3 with terminal butyl groups and the samples studied in this work were compared.     

Adsorption of Phenolic Compounds from Water on Activated Carbon: Prediction of Multicomponent Equilibrium Isotherms using Single-Component Data

Batch-type experiments were carried out to obtain equilibrium isotherms for the adsorption of phenol and m-cresol in aqueous solutions on activated carbon. Single solute systems, at 20 and 40^∘C, were tested for Langmuir, Freundlich and Sips adsorption isotherms in the range of concentrations up to 200 mg/L. Equilibrium data were more closely followed by the Freundlich and Sips equations for all cases. Adsorption isotherms for bisolute systems at 20^∘C, with two different initial concentrations of phenol and m-cresol, were predicted solely on the basis of single solute equilibrium parameters by using the equations of Butler and Ockrent and the IAS theory. The best agreement with the experimental loading values was afforded with the IAS theory based on Sips isotherm for pure compounds. However, this theory is found to be not able to predict with success the binary isotherms in this work where significant displacement of one solute by the other is observed. Chemical interactions in the adsorbed phase, estimated by a modified Butler–Ockrent model, can be responsible for this lack of success of the conventional IAS theory. The predictions based on the IAS theory are compared with the results of some empirical models.     

Thermodynamics of Nicotinic Acid Interactions with Some Saccharides

The interactions of nicotinic acid with α-D-glucose and maltose, and with α-, β-, hydroxypropyl-α- and hydroxypropyl-β-cyclodextrins were studied by using solution calorimetry at T = 298.15 K and pH = 3.4. The thermodynamic parameters (log₁₀ K, Δ G∘_c, Δ H∘_c and Δ S∘_c) were calculated for the systems in which complex formation was observed. Systems with weak interparticle interactions lacking complex formation were characterized by enthalpic virial coefficients calculated on the basis of the McMillan–Mayer theory. It was found that the complexation affinity of α-cyclodextrin to nicotinic acid is stronger in comparison to β-cyclodextrin and the mono- and disaccharides. The influence of different factors, such as the availability of the macrocyclic hydrophobic cavity, the relationship of the sizes of guest molecule to the host cavity, the presence of bulky hydroxypropyl substitutes and their structure, and the solvation of guest molecules on the stability of complexes and their thermodynamic parameters of interaction is discussed.     

Critical Evaluation of Protonation Constants. Literature Analysis and Experimental Potentiometric and Calorimetric Data for the Thermodynamics of Phthalate Protonation in Different Ionic Media

The protonation constants of phthalate were determined in aqueous NaCl (0.1 ≤ I ≤ 5,mol⋅L⁻¹) and in aqueous Me₄NCl (0.1 mol⋅L⁻¹ ≤ I ≤ 3,mol⋅L⁻¹) at t = 25,^∘C. Experimental data were employed in conjunction with literature data from studies in different ionic media (Et₄NI: 0 ≤ I ≤ 1,mol⋅L⁻¹; NaClO₄: 0.05 mol⋅L⁻¹ ≤ I ≤ 2,mol⋅L⁻¹)to study the dependence on ionic strength using different models, such as the SIT and Pitzer equations, and an Extended Debye-Hückel type equation. Experimental calorimetric data in NaCl and protonation constants at different temperatures in Et₄NI (5 ≤ t ≤ 45^∘C) and in NaClO₄ (15 ≤ t ≤ 35 ^∘C) were also used to study their dependence on temperature. Recommended equilibrium data are reported together with a short discussion of a prospective protocol for drawing these data.     

Mo<Subscript>6</Subscript>Cl<Subscript>12</Subscript>-Incorporated Sol-Gel for Oxygen Sensing Applications

Excitation of hexanuclear molybdenum complexes such as Mo₆Cl₁₂ and its derivatives in the ultraviolet results in a strongly red-shifted luminescence centered at 750nm. Since oxygen efficiently quenches the luminescence, these thermally stable inorganic complexes are candidate lumophores for real-time, high temperature optical fiber based sensing of oxygen. Sol-gel films containing the acetonitrile complex of Mo₆Cl₁₂ were deposited on quartz substrates by dip coating. After drying, the films were heated at 200^∘C for 1 h. The luminescence lineshapes of films before and after heating were unchanged, indicating that heating did not adversely affect the cluster photophysics. Compared to solutions of the acetonitrile complex, quenching by oxygen was smaller in the as-prepared films, but heating at 200^∘C for 1 h increased the quenching, apparently due to increased oxygen permeability resulting from the loss of water or other small molecules from the matrix. These results confirm the potential of hexanuclear molybdenum complexes such as Mo₆Cl₁₂⋅2CH₃CN as the lumophores in fiber optic oxygen sensors that can operate up to 200^∘C.     

The Low Temperature Processing of Titanium Dioxide Films by the Addition of Trimesic Acid

Sol-gel deposited titanium dioxide films are used for a wide range of applications. One of the drawbacks to these films is that they must be heated in excess of 400^∘C in order to obtain durability. We have found that the processing temperature required to prepare sol-gel derived titanium dioxide films can be significantly reduced by the addition of small amounts of trimesic acid. The addition of this material provides a low energy pathway for the growth of titanium dioxide particles. Titanium dioxide films prepared with trimesic acid show significantly enhanced physical properties with respect to cracking and peeling. Scanning Electron Microscopy and X-Ray Diffraction analyses of films are presented.     

Diffusion of Cesium Ions in Agar Gel Containing Alkali Metal Bromides

The present paper gives an account of different aspects of the tracer diffusion of Cs⁺ ions in alkali metal bromides. We have measured the diffusion coefficients, D, of cesium ions in 1% agar gel medium at 25 ^∘C using a zone-diffusion technique over a concentration range of 5 × 10⁻⁵ to 0.1 mol,dm⁻³. The values of the diffusion coefficients were found to deviate from theory, which are explained on the basis of different types of interactions occurring in the ion-gel-water system. The study is also focused on the effect of alkali metal bromides on the obstruction effect and activation energy for the tracer-diffusion of cesium ions in agar gel medium. It is observed that both parameters, extent of obstruction, ∝, and activation energy, E, decrease with increasing charge density of the cation of the supporting electrolyte. The influence of these trends is explained on the basis of competitive hydration between the ions and agar molecules, and the relative distortion in the water structure that is brought about by these different ions and agar molecules.     

Buffer Standards for the Physiological pH of the Zwitterionic Compound, TAPS, From 5 to 55 ∘C

The values of the second dissociation constant, pK ₂, and related thermodynamic quantities of N-[tris(hydroxymethyl)methyl-3-amino]propanesulfonic acid (TAPS) have already been reported at 12 temperatures over the temperature range 5–55 ^∘C, including 37 ^∘C. This paper reports the results for the pH of five equimolal buffer solutions with compositions: (a) TAPS (0.03 mol⋅kg⁻¹) + NaTAPS (0.03 mol⋅kg⁻¹); (b) TAPS (0.04 mol⋅ kg⁻¹) + NaTAPS (0.04 mol⋅kg⁻¹); (c) TAPS (0.05 mol⋅kg⁻¹) + NaTAPS (0.05 mol⋅kg⁻¹); (d) TAPS (0.06 mol⋅kg⁻¹) + NaTAPS (0.06 mol⋅kg⁻¹); and (d) TAPS (0.08 mol⋅kg⁻¹) + NaTAPS (0.08 mol⋅kg⁻¹). The remaining eight buffer solutions consist of saline media of the ionic strength I = 0.16 mol⋅kg⁻¹, matching closely to that of the physiological sample. The compositions are: (f) TAPS (0.04 mol-kg⁻¹) + NaTAPS (0.02 mol-kg⁻¹) + NaCl (0.14 mol⋅kg⁻¹); (g) TAPS (0.05 mol⋅kg⁻¹) + NaTAPS (0.04 mol⋅kg⁻¹) + NaCl (0.12 mol⋅kg⁻¹); (h) TAPS (0.6 mol⋅kg⁻¹) + NaTAPS (0.04 mol⋅kg⁻¹) + NaCl (0.12 mol⋅kg⁻¹); (i) TAPS (0.08 mol⋅kg⁻¹) + NaTAPS (0.06 mol⋅kg⁻¹) + NaCl (0.10 mol⋅kg⁻¹); (j) TAPS (0.04 mol⋅ kg⁻¹) + NaTAPS (0.04 mol⋅kg⁻¹) + NaCl (0.12 mol⋅kg⁻¹); (k) TAPS (0.05 mol⋅kg⁻¹) + NaTAPS (0.05 mol⋅kg⁻¹) + NaCl (0.11 mol⋅kg⁻¹); (l) TAPS (0.06 mol⋅kg⁻¹) + NaTAPS (0.06 mol⋅kg⁻¹) + NaCl (0.10 mol⋅kg⁻¹); and (m) TAPS (0.08 mol⋅kg⁻¹) + NaTAPS (0.08 mol⋅kg⁻¹) + NaCl (0.08 mol⋅kg⁻¹). These buffers are recommended as a pH standard for clinical measurements in the range of physiological application. Conventional pH values, designated as pH(s), for all 13 buffer solutions from 5 to 55 ^∘C have been calculated. The operational pH values with liquid junction corrections, at 25 and 37 ^∘C for buffer solutions, designated above as (b), (c), (d), (e), (j), (l), and (m); have been determined based on the difference in the values of the liquid junction potentials between the accepted phosphate standard and the buffer solutions under investigation.     

Conformation and Thermodynamic Properties of the Binding of Vitamin C to Human Serum Albumin

The binding of vitamin C, L-ascorbic acid (AsA), with human serum albumin (HSA) was investigated by various spectroscopic techniques under simulated physiological conditions. The fluorescence quenching constants (K _SV) at four different temperatures (292, 298, 304, and 310 K) were obtained. The thermodynamic parameters ΔH ^∘ and ΔS ^∘ were calculated to be 6.02 kJ⋅mol⁻¹ and 84.55 J⋅mol⁻¹⋅K⁻¹ using the van’t Hoff equation. Additional experiments to determine the stoichiometry (n) were carried out using isothermal titration calorimetry (ITC) and cyclic voltammetry (CV). The distance, r, between AsA and the tryptophan residues of HSA was calculated to be 3.7 nm according to F?rster’s non-radiation energy transfer theory. The effect of AsA on the conformation of HSA was studied by means of three dimensional fluorescence spectra and CD spectra. The results indicate that the presence of AsA resulted in a slight change of the HSA secondary structure. The effect of common ions on the binding of AsA to HSA was also examined.