Accurate heat capacity measurements on powdered samples using a Quantum Design physical property measurement system

We have evaluated the accuracy of the heat capacity option of a Quantum Design physical property measurement system (PPMS) by measuring the heat capacity of various types of conducting and insulating samples over the temperature range from (2 to 300) K. In particular, the accuracy of measurements on a copper pellet was determined to be ±2% for 2 K < T < 20 K and ±0.6% for 20 K < T < 300 K, however similar measurements on a powdered sample of benzoic acid had errors as high as 20%. A new method for heat capacity measurements of powdered samples using a PPMS system has been developed that allows us to obtain heat capacity measurements for both insulating and conducting powdered samples with an accuracy of ±1% from (20 to 300) K and ±2% to ±5% for T < 20 K. Since the heat capacity of substances (and corresponding entropy contribution) is small at low temperatures for lattice-only contributions, the accuracy of ±2% to ±5% below 20 K is considered acceptable. As a test of the new method, the heat capacity of powdered bulk hematite has been measured in the temperature range from (2 to 300) K with the PPMS, and its standard entropy at T = 298.15 K was calculated to be (87.33 and 87.27) J · K^?1 · mol^?1, which deviates ?0.08% and ?0.15% from the accepted reference value, respectively. We recommend that this new method become the standard for accurate heat capacity measurements on insulating powdered samples using a PPMS system and the corresponding thermodynamic calculations.     

On‐line electroextraction in capillary electrophoresis: Application on the determination of glutamic acid in soy sauces

We present an on‐line, single step coupling between liquid‐liquid extraction and capillary electrophoresis with capacitively coupled contactless conductivity detection, which allows an efficient analysis of complex food matrices with high sodium content. The sodium depletion was demonstrated using an aqueous two‐phase system. The aqueous two‐phase system enables the electrically driven extraction of the target compounds. The sample was prepared in Dextran‐rich phase (8% w/v 500 kDa Dextran, DEX). The background electrolyte (acetic acid 5.0 mol/L) contained 6% w/v of 6 kDa PEG. As proof of applicability, we employed the developed method for glutamic acid quantification on soy sauces. The peak area of glutamic acid presents no significant difference (α = 0.05), while the peak area of the sodium presented a reduction of 11.7 ± 0.2 and 19 ± 3% for premium and low‐cost soy sauce samples analyzed. The glutamic acid concentration for premium soy sauce sample was 2.7 ± 0.8 and 4.8 ± 0.4 g/L, and for low‐cost soy sauce sample, the concentration was 9.9 ± 0.9 g/L, which agreed with those obtained by other analytical techniques.     

Vapour pressures of isobutane at T = (310 to 407) K

Twenty-eight measurements of the vapour pressure for isobutane have been obtained by means of a metal-bellows variable volumometer at temperatures from 310 K to 407 K. The volume-fraction purity of isobutane used through the measurement was 0.9999. The expanded uncertainties (k = 2) in temperature and pressure measurements have been estimated to be less than ±4 mK and ±1.1 kPa, respectively. The agreement of the present measurements on various volumes of the bellows at the same temperature is almost within the absolute average deviations of ±0.2 kPa. The discrepancies between two series of the present measurement, in which the sample fillings and adopted platinum resistance thermometers are different, have also been confirmed as enough lower than the experimental uncertainty. Throughout the present study, the direct comparisons of the vapour-pressure measurements on the same temperatures with several points data from the literatures were made in order to assess the reliability of the present ones quantitatively. In addition, based on the present measurements as input data, the Wagner-type vapour-pressure correlation was provided, which was also used for the systematic comparisons between the present measurement and the literature data.     

A spinning thermometer to monitor microwave heating and glass transitions in dynamic nuclear polarization

As previously demonstrated by Thurber and Tycko, the peak position of ⁷⁹Br in potassium bromide (KBr) allows one to determine the temperature of a spinning sample. We propose to adapt the original design by using a compact KBr tablet placed at the bottom of the magic angle spinning rotor, separated from the sample under investigation by a thin disk made of polytetrafluoroethylene (or ‘Teflon’®). This design allows spinning the sample up to at least 16 kHz. The KBr tablet can remain in the rotor when changing the sample under investigation. Calibration in the range of 98 < T < 320 K has been carried out in a static rotor by inserting a platinum thermometer. The accuracy is better than ± 0.9 K, even in the presence of microwave irradiation. Irradiation with 5 W microwaves at 263 GHz leads to a small temperature increase of 3.6 ± 1.4 K in either static or spinning samples. The dynamic nuclear polarization enhancement decreases with increasing temperature, in particular when a frozen glassy sample undergoes a glass transition. Copyright © 2011 John Wiley & Sons, Ltd.     

Assembly and performance of a 6.4 T cryogen‐free dynamic nuclear polarization system

We report on the assembly and performance evaluation of a 180‐GHz/6.4 T dynamic nuclear polarization (DNP) system based on a cryogen‐free superconducting magnet. The DNP system utilizes a variable‐field superconducting magnet that can be ramped up to 9 T and equipped with cryocoolers that can cool the sample space with the DNP assembly down to 1.8 K via the Joule–Thomson effect. A homebuilt DNP probe insert with top‐tuned nuclear magnetic resonance coil and microwave port was incorporated into the sample space in which the effective sample temperature is approximately 1.9 K when a 180‐GHz microwave source is on during DNP operation. ¹³C DNP of [1‐¹³C] acetate samples doped with trityl OX063 and 4‐oxo‐TEMPO in this system have resulted in solid‐state ¹³C polarization levels of 58 ± 3% and 18 ± 2%, respectively. The relatively high ¹³C polarization levels achieved in this work have demonstrated that the use of a cryogen‐free superconducting magnet for ¹³C DNP is feasible and in fact, relatively efficient—a major leap to offset the high cost of liquid helium consumption in DNP experiments.     

Calorimetry under Stress A Preliminary Study in Single Crystalline Cu-Zn-Al Shape Memory Alloys

A non-differential calorimetric analyzer was developed for an INSTRON 1123 machine (a stress-strain-temperature analyzer) with a temperature chamber INSTRON 1110. The study was performed using the Joule effect and pseudoelastic martensitic transformations in single crystals of Cu-Zn-Al alloys. The analysis of the system establishes that: the sensitivity of calorimetric measurements after a filter of two poles and two zeros is 166 mV W^-1 (at 297 K), the noise is near 1.5 μV and the drift is close to 30 μV in 6 h. The reproducibility of the sensitivity working with one sample is better than ±0.3%, and the change to a new sample keeps the value below ±0.5%. The uncertainty in reproducibility in the martensitic transformation (including repositioning) does not overcome ±1.6%. The used calorimetric sensors limit the temperature to 373 K. The furnace control originated fluctuations on the base line (near ±20 μV), which by means of an auxiliary signal processing were reduced to 50% (less than ±10 μV). This revised version was published online in July 2006 with corrections to the Cover Date.     

Heat capacity and thermodynamic properties of 2,4-dichlorobenzaldehyde (C7H4Cl2O)

Heat capacities of 2,4-dichlorobenzaldehyde have been measured with a high-precision automatic adiabatic calorimeter over the temperature range from (79 to 371) K. The melting temperature, molar enthalpy, and entropy of fusion were determined by the heat capacity measurements to be (347.24 ± 0.13) K, (20468 ± 19) J · mol⁻¹, and (58.94 ± 0.04) J · K⁻¹ · mol⁻¹, respectively. The melting temperatures for the sample and the absolutely pure compound have been obtained from fractional melting experiments to be (347.230 and 347.619) K, respectively, and the chemical purity of the sample was calculated to be 0.9921 mol fraction according to the Van't Hoff equation. Moreover, the solid-to-liquid phase transition of the substance was further investigated by using DSC technique. The results obtained from the heat capacity measurements were in agreement with those from the DSC analysis.     

Background current elimination in thin layer ion-selective membrane coulometry

A promising method for the elimination of undesired capacitive currents in view of realizing a potentially calibration free coulometric ion detection system is presented. The coulometric cell is composed of a porous polypropylene tube doped with a liquid calcium-selective membrane and a silver/silver chloride wire as an inner electrode, forming a thin layer sample between wire and tubing. The total charge passed through the system during potential controlled electrolysis of the thin layer sample is indeed found to be proportional to the amount of calcium present, but non-Faradaic processes do contribute to the obtained signal. We introduce here a multi-pulse procedure that allows one to perform a second excitation pulse at the same excitation potential after exhaustive ion transfer voltammetry of calcium has taken place. The intercept of the calibration curve after background subtraction is found as 20.6 ± 0.6 μC, significantly lower than the value of 54.1 ± 0.8 μC for the uncorrected curve. Changes in sample temperature (from 23 °C to 38 °C) did equally not affect the background corrected coulometric readings, supporting that the procedure renders the readout principle more robust.     

Heat capacity and enthalpy of fusion of pyrimethanil laurate (C24H37N3O2)

Low-temperature heat capacities of pyrimethanil laurate (C₂₄H₃₇N₃O₂) were precisely measured with an automated adiabatic calorimeter over the temperature range between T = 78 K and T = 340 K. The sample was observed to melt at (321.52 ± 0.04) K. The molar enthalpy and entropy of fusion as well as the chemical purity of the compound were determined to be (67244 ± 11) J · mol⁻¹, (209.28 ± 0.02) J · mol⁻¹ · K⁻¹, (0.9943 ± 0.0004) mass fraction, respectively. The extrapolated melting temperature for the absolutely pure compound obtained from fractional melting experiments was (322.264 ± 0.006) K.     

Potentiometric study of acid–base equilibria in the {KCl + LiCl} eutectic melt at temperatures in the range (873 to 1073) K

Some heterogeneous reactions of oxide ion exchange (carbonate ion dissociation and magnesium oxide dissolution) in the molten {KCl + LiCl} eutectic at temperatures of (873, 973 and 1073) K were studied using an electrochemical cell with an oxygen membrane electrode Pt(O₂)|ZrO₂(Y₂O₃). The dissociation constant of the CO₃²⁻ was found to increase with increasing temperature: pK (873 K)=(2.39 ± 0.05); pK (973 K)=(1.81 ± 0.09); pK (1073 K)=(1.53 ± 0.08). Removal of CO₂ from the gas above the melt allows the complete transformation of CO₃²⁻ to O²⁻. pP_MgO values decrease more from (6.99 ± 0.08) to (5.41 ± 0.04). The oxobasicity indices, pI_(KCl+LiCl), were calculated from the solubility data to be 3.2 at 873 K, 3.4 at 973 K, and 3.6 at 1073 K. This trend suggests an increase in acidity with increasing temperature of {KCl + LiCl}.     

New Knudsen effusion apparatus with simultaneous gravimetric and quartz crystal microbalance mass loss detection

A new Knudsen effusion apparatus, enabling simultaneous gravimetric and quartz crystal microbalance mass loss detection, is described. This device allows the measurement of vapour pressures of small sample mass (50 to 100) mg over a wide temperature range (350 to 650) K using very short effusion time intervals. The performance of the apparatus was checked by measuring the vapour pressures of anthracene, benzanthrone, and 1,3,5-triphenylbenzene, between (0.1 and 1) Pa, over temperature intervals of 20 K. The derived standard molar enthalpies of sublimation and vapour pressures are in excellent agreement with the mean of the available literature values and with the recommended values. The new working methodology and design of this apparatus allows the measurement of high quality vapour pressure data due to: accurate temperature measurement and control; improvement in vacuum thermal contact between the effusion cell and the oven metal block; optimisation of the quartz crystal sensor head microbalance position; efficient temperature control of the quartz crystal microbalance head; accurate measurement of the resonance crystal frequency using an impedance circuit analyser methodology.     

Low temperature study of nickel hydroxide electrode in frozen electrolyte

The electrochemical behaviour of nickel hydroxide layer obtained in situ on nickel substrate was studied in frozen electrolyte, solid tetrabutylammonium hydroxide hydrate at temperatures down to 170 K by cyclic voltammetry and chronoamperometry. The decrease of temperature causes substantial decrease of the current and the increase of the difference between peak potentials. The temperature dependence of proton diffusion through nickel hydroxide film was also estimated from chronoamperometric experiments. The activation energy of this process in 170–298 K is equal to 0.25 ± 0.03 eV.     

An improved technique for accurate heat capacity measurements on powdered samples using a commercial relaxation calorimeter

We have modified our earlier technique for accurate PPMS heat capacity measurements on powdered samples by means of applying Wakefield grease or small copper strips in the sample preparation instead of using Apiezon N high-vacuum grease. For the Wakefield grease measurements, we put a small amount of Wakefield thermal compound in a copper cup instead of potting with Apiezon N, and the accuracy of measurements on powdered benzoic acid was determined to be ±1% and ±4% in the temperature ranges of 10 K < T < 280 K and 280 K < T < 300 K, respectively. The Wakefield grease was found to improve the accuracy somewhat but overall there was no noticeable improvement in the “grease region” above T = 220 K. To overcome the known shortcomings of using Apiezon N grease above 220 K, we have replaced the Apiezon N grease with small copper strips in the sample preparation to aid thermal conductivity, which results in a less time-intensive two-step technique for the PPMS heat capacity measurement but with an accuracy, based on measurements of benzoic acid, that is ±1% from T = (10 to 300) K and, more importantly, the elimination of the “grease problem”. As an additional test of the new technique, the heat capacity of powdered bulk rutile has been measured twice within the temperature range from (2 to 300) K using the PPMS, and its standard entropy at T = 298.15 K was calculated to be (50.39 ± 0.50) and (50.31 ± 0.50) J · K^?1 · mol^?1, which deviates 0.08% and ?0.08% from the measurement results of our low-temperature adiabatic and semi-adiabatic calorimeters, respectively. We recommend that this technique become the standard for accurate heat capacity measurements on insulating powdered samples using a PPMS system and the corresponding thermodynamic calculations.     

(p, Vm, T) and phase equilibrium measurements for a natural gas-like mixture using an automated isochoric apparatus

We have developed an automatic apparatus for measuring phase equilibrium and (p, V_m, T) properties of gas mixtures in our laboratory. Based upon the isochoric method, the apparatus can operate at temperatures ranging from 100 K to 500 K at pressures up to 35 MPa, and yield absolute results in fully automated operation. Temperature measurements are accurate to 0.01 K and pressure measurements are accurate to 0.002 MPa. The isochoric method utilizes pressure versus temperature measurements along an isomole (near isochore) and detects phase boundaries by locating the change in the slope of the isomoles.We also have developed a strategy that allows us, when using the above isochoric method together with a second apparatus capable of isothermal density measurements, to collect derived densities that are competitive in accuracy with those of the densimeter, but with a procedure and design that is easy to automate. We present data on a natural gas-like mixture. The experimental data indicate that prediction of the dew point curve with current equations of state is unreliable.     

Temperature-dependent rate constants for the reactions of chlorine atom with methanol and Br2

Kinetics of the reaction of Cl atoms with methanol has been investigated at 2 Torr total pressure of helium and over a wide temperature range 225-950 K, using a discharge flow reactor combined with an electron impact ionization quadrupole mass spectrometer. The rate constant of the reaction Cl + CH₃OH → products (1) was determined using both absolute measurements under pseudo-first order conditions, monitoring the kinetics of Cl-atom consumption in excess of methanol and relative rate method, k₁ = (5.1 ± 0.8) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, and was found to be temperature independent over the range T = 225-950 K. The rate constant of the reaction Cl + Br₂ → BrCl + Br (3) was measured in an absolute way monitoring Cl-atom decays in excess of Br₂: k₃ = 1.64 × 10⁻¹⁰ exp(34/T) cm³ molecule⁻¹ s⁻¹ at T = 225-960 K (with conservative 15% uncertainty). The experimental data for k₃ can also be adequately represented by the temperature independent value of k₃ = (1.8 ± 0.3) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹. The kinetic data from the present study are compared with previous measurements.     

Comparative studies of oxygenated fuel synthesis with diesel from the measurements of density,speed of sound and refractive index

An experimental investigation on the feasibility and relevance of the tri fuel blends of ethanol and dibutyl ether with diesel was studied to replace pure diesel. The solubility of the ethanol and dibutyl ether with a percentage of 25% and 75% resulted with no phase separation, found miscible and stable with diesel at any percentage. However, the properties such as densities and refractive index experimentally verified for different blend ratios. A density of test samples with various compositions was tested. High precise equipment is engaged to analyze the density, speed of sound, refractive index for various fuel compositions. The temperature ranges between 298 K and 343 K show a greater impact on variation in the fuel properties. Density, speed of sound, refractive indices measured as a function of the temperature with an accuracy of?±?0.001 and?±?0.0001. Further, the validation of experimental method has been tested using Lorentz–Lorenz (L–L) analysis with a deviation of 0.4%. The uncertainty for fluid velocity is?±?0.3 m s^?1, and the experimental estimated excess molar volume uncertainty is 2?×?10^?3 cm³ mol^?1. The substantiation of intermolecular interactions between the liquids is found to be significant in both experimental and prediction analysis of each sample. The exergy destruction specifies with 46% which includes the air flow and chemical heat energy transfer losses.

     

(Vapor + liquid) phase equilibrium measurements for {trifluoroiodomethane (R13I1) + propane (R290)} from T = (258.150 to 283.150) K

In this paper, the (vapor + liquid) equilibrium data for (R13I1 + R290) were measured by a vapor-recirculation apparatus at temperatures from (258.150 to 283.150) K. The VLE data were correlated by the Peng–Robinson equation of state with two different models, the van der Waals mixing rule, and the Huron–Vidal mixing rule involving the NRTL activity coefficient model. Good agreements were found between the calculated data and the experimental data. The maximum average absolute relative deviation of pressure (AARD p) was 0.48%, while the maximum average absolute deviation of composition (AAD y) was 0.0040. Meanwhile, zeotropic behavior can be found for the measured system in this study. The total combined standard uncertainties for temperature, pressure and composition measurements were ±5 mK, ±0.0005 MPa and ±0.005, respectively.     

Synthesis and in vitro biological evaluations of novel tetrapeptide as therapeutic agent for wound treatment

We report a new small peptide containing four amino acid residues (glycine-aspartic acid-proline-histidine) conjugated with palmitic acid (Palmitoyl-GDPH) that was synthesized, characterized and evaluated for its biological activities. The Palmitoyl-GDPH was prepared by solid phase peptide synthesis (SPPS) with high percentage purity of 98.6%. The results of circular dichroism (CD) demonstrated the feasibility and stability of Palmitoyl-GDPH secondary structure at many temperatures up to 60 °C. Palmitoyl-GDPH showed its greatest collagenase inhibition activity and nitric oxide (NO) scavenging effect of 80.00 ± 2.22% at 1.0 mg/ml and 83.40 ± 8.08% at 2.5 mg/ml, respectively. In addition, in-vitro cell based study revealed that Palmitoyl-GDPH was not toxic and possessed strong proliferative activity towards normal human dermal fibroblast (NHDF) cell line. Wound scratch assay method showed that Palmitoyl-GDPH significantly promoted the cell migration which progressed faster compared to tetracycline-treated group (positive control) by about 98.39 ± 2.79% and 95.79 ± 3.83%, respectively. Collectively, these results suggested that newly synthesized Palmitoyl-GDPH possessed a strong candidate for use as therapeutic agent and can be a novel approach in the treatment of cutaneous wounds.     

Molar heat capacity and thermodynamic properties of 1-cyclohexene-1,2-dicarboxylic anhydride [C8H8O3]

The molar heat capacity C_p,m of 1-cyclohexene-1,2-dicarboxylic anhydride was measured in the temperature range from T=(80 to 360) K with a small sample automated adiabatic calorimeter. The melting point T_m, the molar enthalpy Δ_fusH_m and the entropy Δ_fusS_m of fusion for the compound were determined to be (343.46 ± 0.24) K, (11.88 ± 0.02) kJ · mol⁻¹ and (34.60 ± 0.06) J · K⁻¹ · mol⁻¹, respectively. The thermodynamic functions [H_(T)−H_(298.15)] and [S_(T)−S_(298.15)] were derived in the temperature range from T=(80 to 360) K with temperature interval of 5 K. The mass fraction purity of the sample used in the adiabatic calorimetric study was determined to be 0.9928 by using the fractional melting technique. The thermal stability of the compound was investigated by differential scanning calorimeter (DSC) and thermogravimetric (TG) technique, and the process of the mass-loss of the sample was due to the evaporation, instead of its thermal decomposition.     

Thermally induced transformations of calcium carbonate polymorphs precipitated selectively in ethanol/water solutions

For the precipitation of calcium carbonate polymorphs in ethanol/water solutions of calcium chloride by the diffusion of the gases produced by sublimation–decomposition of solid ammonium carbonate, polymorph selection and morphology control of the precipitates were demonstrated by the effect of ethanol/water ratio in the mother liquor. The precipitated phases change systematically from gel-like aggregates of hydrated amorphous calcium carbonate in the absolute ethanol solution to well-shaped rhombohedral particles of calcite in the absolute aqueous solution via almost pure phase of vaterite with dendrite structure in 75%-ethanol/25%-aqueous and 50%-ethanol/50%-aqueous solutions. On heating the precipitated sample in flowing dry nitrogen, all the samples transformed to calcite before the thermal decomposition, where the thermal decomposition temperature shifts to higher temperatures with increasing the water content in the mother liquor due to the systematic increase in the particle size of calcite. Accordingly, the present method of controlled precipitation of calcium carbonate polymorphs is also useful to control the particle size and reactivity of calcite produced by heating the precipitates. Selecting vaterite with dendrite structure from the present series of precipitated samples, the structural phase transition to calcite was characterized as the three-dimensional growth of rhombohedral particles of calcite with the enthalpy change ΔH = ? 2.8 ± 0.1 kJ mol^?1 and the apparent activation energy E_a = 289.9 ± 5.8 kJ mol^?1.