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121.
Laurence E. Strong Carter Van Waes Kenneth H. Doolittle II 《Journal of solution chemistry》1982,11(4):237-258
The three monofluorobenzoic acids together with 2,4-difluoro and 2,6-difluorobenzoic acids in aqueous solution are the subject of precision conductance measurements. The experimental data are analyzed to give ionization constants and limiting conductances at temperatures from 0 to 100°C. Walden products for the acid anions are derived from the limiting conductances while the ionization consatants are fitted by statistical methods to the function pK
a
(m)=A+B/T+ C logT+DT. Only the 2,6- acid requires the fourth term of the function to fit the data to a precision of better than 0.03%. Mathematical analysis of the pK function gives the standard changes in enthalpy, entropy, and heat capacity. All the acids studied are more acidic than the parent, benzoic acid, as well as more acidic than the isoelectronic methylbenzoic acids. In general the increased acidity is tied to decreases in enthalpy while entropy changes on ionization differn little from those found for the parent acid. 相似文献
122.
The thermal properties, i.e., heat capacity, enthalpy, entropy, and Gibbs function, and the transition behavior of the copolymer system of 4-hydroxybenzoic acid and 2,6-hydroxynaphthoic acid have been studied based on differential scanning calorimetry. The heat capacities of the glass, crystal, and anisotropic melt are shown to be largely additive on a molar basis. Additivity is lost in the two transition regions, glass transition and disordering transition. Isothermal crystallization experiments on the copolymers revealed the existence of two types of crystals which melt at high temperature (fast-grown crystals) and low temperature (slowly grown crystals). The ATHAS computation method is used to bring heat capacities of the solid state into agreement with approximate frequency spectra. The changes in heat capacity at the glass transitions occur at 434°K for the poly(oxy-1,4-benzoyl) [33.2 J/(K mol)] and at 420°K for poly(oxy-2,6-naphthoyl) [46.5 J/(K mol)]. The copolymers have a transition range of above 100°K. The anisotropic melt is linked to the well-known condis state of poly(oxy-1,4-benzoyl) by a continuous changes in disorder and mobility without an additional first-order transition. 相似文献
123.
Fine particle superacidic sulfated zirconia (SO42−/ZrO2, S-ZrO2) was synthesized by ameliorated method, and composite membranes with different S-ZrO2 contents were prepared by a recasting procedure from a suspension of S-ZrO2 powder and Nafion solution. The physico-chemical properties of the membranes were studied by ion exchange capacity (IEC) and liquid water uptake measurements, scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis, thermogravimetry–mass spectrometry (TG–MS) and Fourier transform infrared (FT-IR) spectroscopy. The results showed that the IEC of composite membrane increased with the content of S-ZrO2, S-ZrO2 was compatible with the Nafion matrix, the incorporation of the S-ZrO2 could increase the crystallinity and also improve the initial degradation temperature of the composite membrane. The performance of single cell was the best when the S-ZrO2 content was 15 wt.%, and achieved 1.35 W/cm2 at 80 °C and 0.99 W/cm2 at 120 °C based on H2/O2 and at a pressure of 2 atm, the performance of the single cell with optimized S-ZrO2 was far more than that of the Nafion at the same condition (e.g. 1.28 W/cm2 at 80 °C, 0.75 W/cm2 at 120 °C). The 15 wt.% S-ZrO2/Nafion composite membrane showed lower fuel cell internal resistance than Nafion membranes at high temperature and low relative humidity (RH). 相似文献
124.
Densities and sound velocities of binary mixtures of cyclohexanone, 2-butanone, 1,4-dioxane and 1,2-dimethoxyethane were measured
at 298.15 K and also the densities at 303.15 K. Excess volumes were determined from densities. Isentropic compressibilities
were determined from densities and sound velocities, and excess thermal expansion factors were determined from excess volumes
of two temperatures. Excess isothermal compressibilities and excess isochoric heat capacities were then estimated using excess
isobaric heat capacities previously reported. Excess volumes and excess isentropic and isothermal compressibilities were negative
except for cyclohexanone+1,4-dioxane system.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
125.
The solubility and enthalpy of solution of benzene, cyclohexane, hexane, and heptane in formanide have been determined from titration microcalorimetric experiments at 25°C. The solution enthalpies are significantly more endothermic than in water but still the solubility is much higher. The entropy changes in formamide are small and positive and do not vary significantly with size. The enthalpies of solution of some 1-alkanols, 1-chloro- and 1,5-dichloropentane and pentane-1,5-diol were measured as functions of concentration. The solution enthalpies for 1-alkanols from methanol to decanol increase linearly with chain length. The enthalpic interaction coefficients hxx are small and negative in formamide while they are large and positive in water. The partial molar heat capacities C
p,2
o
for 1-propanol, 1-pentanol, benzene and cyclohexane in formamide were determined at 25°C from drop heat capacity measurements. Values of C
p,2
o
are only slightly larger than the molar heat capacities of the liquid solutes. 相似文献
126.
We study a neural network model consisting ofN neurons where a dendritic connection between each pair of neurons exists with probabilityp and is absent with probability 1-p. For the Hopfield Hamiltonian on such a network, we prove that ifp c[(lnN)/N]1/2, the model can store at leastm=
cpN patterns, where
c 0.027 ifc 3 and decreases proportional to 1/(–lnc) forc small. This generalizes the results of Newman for the standard Hopfield model. 相似文献
127.
A new iodometric method for quantifying aqueous solutions of iodide-oxidizing and iodine-reducing substances, as well as plain iodine/iodide solutions, is presented. It is based on the redox potential of said solutions after reaction with iodide (or iodine) of known initial concentration. Calibration of the system and calculations of unknown concentrations was performed on the basis of developed algorithms and simple GWBASIC-programs. The method is distinguished by a short analysis time (2–3 min) and a simple instrumentation consisting of pH/mV meter, platinum and reference electrodes. In general the feasible concentration range encompasses 0.1 to 10–6 mol/L, although it goes down to 10–8 mol/L (0.001 mg Cl2/L) for oxidants like active chlorine compounds. The calculated imprecision and inaccuracy of the method were found to be 0.4–0.9% and 0.3–0.8%, respectively, resulting in a total error of 0.5–1.2%. Based on the experiments, average imprecisions of 1.0–1.5% at c(Ox)>10–5 M, 1.5–3% at 10–5 to 10–7 M, and 4–7% at <10–7 M were found. Redox-iodometry is a simple, precise, and time-saving substitute for the more laborious and expensive iodometric titration method, which, like other well-established colorimetric procedures, is clearly outbalanced at low concentrations; this underlines the practical importance of redox-iodometry.
An erratum to this article is available at . 相似文献
An erratum to this article is available at . 相似文献
128.
《Arabian Journal of Chemistry》2020,13(9):7115-7131
The impregnation of magnetite (Mt) nanoparticle (NPs) onto Musa acuminata peel (MApe), to form a novel magnetic combo (MApe-Mt) for the adsorption of anionic bromophenol blue (BPB) was studied. The SEM, EDX, BET, XRD, FTIR and TGA were used to characterize the adsorbents. The FTIR showed that the OH and CO groups were the major sites for BPB uptake onto the adsorbent materials. The average Mt crystalline size on MApe-Mt was 21.13 nm. SEM analysis revealed that Mt NPs were agglomerated on the surface of the MApe biosorbent, with an average Mt diameter of 25.97 nm. After Mt impregnation, a decrease in BET surface area (14.89 to 3.80 m2/g) and an increase in pore diameter (2.25–3.11 nm), pore volume (0.0052–0.01418 cm3/g) and pH point of zero charge (6.4–7.2) was obtained. The presence of Pb(II) ions in solution significantly decreased the uptake of BPB onto both MApe (66.1–43.8%) and MApe-Mt (80.3–59.1%), compared to other competing ions (Zn(II), Cd(II), Ni(II)) in the solution. Isotherm modeling showed that the Freundlich model best fitted the adsorption data (R2 > 0.994 and SSE < 0.0013). In addition, maximum monolayer uptake was enhanced from 6.04 to 8.12 mg/g after Mt impregnation. Kinetics were well described by the pseudo-first order and liquid film diffusion models. Thermodynamics revealed a physical, endothermic adsorption of BPB onto the adsorbents, with ΔHo values of 15.87–16.49 kJ/mol, corroborated by high desorption (over 90%) of BPB from the loaded materials. The viability of the prepared adsorbents was also revealed in its reusability for BPB uptake. 相似文献
129.
The influence of a water vapor admixture in helium, nitrogen, and carbon dioxide on capacity coefficients of C3−C5 alcohols and pyridine during chromatography process in capillary columns with polar (PEG-20M) and nonpolar (SE-30) stationary
phases was studied. The introduction of a water admixture into the carrier gas, increases the capacity coefficient of polar
organic compounds on the capillary column with PEG-20M and has almost no effect on this value in the case of SE-30. The change
in retention of polar organic compounds on the capillary column with the PEG-20M polar phase occurs due to a change in the
properties of the stationary phase when it adsorbs water from the mobile phase.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2258–2261, November, 1998. 相似文献
130.
Jeffrey S. Boyer 《The Journal of chemical thermodynamics》2003,35(12):1917-1928
The standard molar heat capacity C°p,m of adenine(cr) has been measured using adiabatic calorimetry over the range 6<(T/K)<310 and the results used to derive thermodynamic functions for adenine(cr) at smoothed temperatures. At T=298.15 K, C°p,m=(142.67±0.29) J · K−1 · mol−1 and the third law entropy S°m=(145.62±0.29) J · K−1 · mol−1. The standard molar Gibbs free energy of formation ΔfG°m at T=298.15 K for crystalline adenine was calculated, using the standard molar enthalpy of formation for the compound and entropies of the elements from the literature, and found to be ΔfG°m=(301.4±1.0) kJ · mol−1. The results were combined with solution calorimetry and solubility measurements from the literature to yield revised values for the standard molar thermodynamic properties of aqueous adenine at T=298.15 K: ΔfG°m=(313.4±1.0) kJ · mol−1, ΔfH°m=(129.5±1.4) kJ · mol−1, and Sm°=(217.68±0.44) J · K−1 · mol−1. 相似文献