Combined Experimental Uncertainties Research Articles

Energetic and exergetic study of a flat plate collector based solar water heater - investigation of the absorber size

Small sized absorber in a flat plate solar collector is beneficial in terms of cost and minimum heat losses. However, its detailed thermal performance compared to standard size collector is still not fully understood. There is a paucity of research to appreciate thermal performance of solar water heating collector with consideration of a small absorber size (below 1m2) and a standard absorber size (2 m2). The present study attempts to investigate the energy and exergy efficiencies of flat plate solar water heating collector with two absorber plate areas (2 m2 and 0.74 m2) to enumerate size of the absorber required for improved first and second law thermal efficiencies of the collector. The efficiencies of these two collector designs are experimentally compared with the help of a test facility available in the site for given operating temperatures and rate of flow. The combined experimental uncertainty due to the measuring instruments and the measured parameters is also ascertained. The obtained results highlight the significance of the larger absorber size (2m2) for higher thermal efficiency, and lower absorber size (0.74m2) for higher exergetic efficiency. The highest thermal efficiency obtained is 77.38% for larger absorber size, and the highest exergy efficiency of 13.21% is obtained for lower absorber size collector. It is demonstrated that larger and lower absorber size of the collector have higher thermal efficiency and higher exergy efficiency, respectively, than some of the published works.

Laminar burning velocities, CO, and NOx emissions of premixed polyoxymethylene dimethyl ether flames

Polyoxymethylene dimethyl ethers (OMEn) with the chemical structure of CH3O(CH2O)nCH3 are a promising synthetic e-fuel class that can be produced from renewable electricity and improves the emission characteristics of internal combustion engines. However, knowledge of their fundamental combustion properties, such as the laminar burning velocity, is limited in the literature, preventing the full assessment of their application potential in combustion devices. Thus, a heat flux burner is applied in this study to measure the laminar burning velocities of OME1 and OME2 over a wide range of equivalence ratios of 0.6–1.9 and at initial temperatures of 383–401 K under atmospheric pressure for the first time. The obtained data are compared with computed results using a recently developed reaction mechanism for OME1–4. Good agreement is observed. The experimental results are also discussed in the context of a comparison to literature data of dimethyl ether and OME3, demonstrating the minor impact of chain length of (OMEn) on their burning velocities, which is further analyzed in terms of sensitivity analysis on reaction kinetics. Furthermore, the exhaust gas composition of NOx and CO is determined experimentally for flames of OME1 and OME2 using a probe method and compared to the numerical results, observing mostly good agreement, where the remaining differences could be explained on the basis of combined experimental and model uncertainties. This provides unique knowledge on the emission performance of the combustion of (OMEn).

Measurements of solidification kinetics for benzene in methane at high pressures and cryogenic temperatures

A stirred, high-pressure, visual microscopy-based apparatus was used to investigate solid-formation kinetics in LNG-relevant binary mixtures. The apparatus can detect solid crystals as small as 20 µm in a 3.5 mm field of view and is capable of measurements at temperatures as low as 90 K and pressures up to 20 MPa. The apparatus is described in detail and is presented alongside experimental results for two benzene + methane mixtures. Measurements of the solid–fluid equilibrium temperatures for the benzene + methane mixtures at 10 MPa were determined by raising the temperature of the mixture in (0.5 to 1)K steps and observing whether the crystals were still present after 2 h. The equilibrium temperatures were consistent with those predicted using the software package ThermoFAST within the combined experimental and model uncertainties. Formation measurements were carried out using a 100 ppm benzene-in-methane sample. Isobaric constant-cooling experiments at pressures of 8 and 10 MPa were used to construct subcooling formation-probability distributions for this mixture by identifying the temperature at which solid crystals were first observed to form on a copper substrate. The subcooling values at formation ranged from (4.4 to 11.0) K and were used to generate a cumulative probability of formation. The measured formation-probability distribution was fit to a model based on Classical Nucleation Theory to yield an estimate of 5 mJ/m2 for the effective surface free energy of solid benzene in liquid methane on the copper substrate. These results pave the way to probabilistic estimates of risk for solids formation in cryogenic heat exchangers.

A new method to predict the response of thermoluminescent detectors exposed at different positions within a clinical proton beam

The proton depth dose profile measured by luminescent detectors differs from the one measured with reference dosimeters (i.e. ionization chambers) because of several effects including efficiency quenching and changes in the attenuation of the emitted light in case of partial detector irradiation. Using the Microdosimetric d(z) Model in combination with the Monte Carlo radiation transport code PHITS, a methodology was developed to tackle all these factors and calculate the response of luminescent when exposed at different positions along a proton Bragg peak. The results were compared against experimental data gathered with 7LiF:Mg,Ti (MTS-7) and 7LiF:Mg,Cu,P (MCP-7) thermoluminescent detectors, showing a very good agreement (average relative deviation ~ 3% for both detector types, smaller than the combined experimental uncertainty).

Assessment of the reliability of elastic geobarometry with quartz inclusions

Elastic geobarometry makes use of the contrast in elastic proprieties between host and inclusion crystals to determine the entrapment conditions of the inclusions from the residual stress and strain measured in the inclusion when its host is at ambient conditions. The theoretical basis has been developed extensively in the past few years, but an experimental validation of the method is still required. We performed two syntheses experiments with quartz inclusions in pure almandine garnet at eclogitic conditions. Experiment labelled Alm-1 with synthesis performed at P = 3.0 GPa and T = 775 °C and Alm-2 at P = 2.5 GPa and T = 800 °C. All the experiments have been carried out in a piston-cylinder press. Isolated, fully-enclosed quartz inclusions in the recovered garnets have been then measured using micro-Raman spectroscopy. All fully-buried inclusions exhibit Raman peaks at higher frequencies and wavenumbers than those obtained from quartz crystals at ambient pressure. If these peak shifts are interpreted as a remnant pressures by use of hydrostatic calibrations of the Raman shifts of quartz with pressure, the remnant pressures show a large spread in values and lead to significant errors in back-calculated entrapment pressures, of up to 1.4 GPa for inclusions synthesised at 3.0 GPa. These results confirm that quartz inclusions trapped inside garnet are not subject to hydrostatic pressure.We therefore used the phonon-mode Grüneisen tensors of quartz to calculate the full strain state of each inclusion, from which the full anisotropic stress state can be calculated by using the elastic properties of quartz. The mean residual remnant stress of the inclusions determined in this way show a much smaller spread in values. Entrapment pressures calculated from this mean stress with the isotropic model for host-inclusion systems differ from the known experimental values by <0.2 GPa, which is of the order of the combined experimental uncertainties. These results show that the most significant effect of the elastic anisotropy of quartz is on the Raman shifts of the inclusion, and not on the subsequent calculation of entrapment conditions.

Triborate Formation Constants and Polyborate Speciation under Hydrothermal Conditions by Raman Spectroscopy using a Titanium/Sapphire Flow Cell

Solvent-corrected reduced isotropic Raman spectra of aqueous boric acid + sodium borate solutions have been obtained from perpendicular and parallel polarization measurements in a novel custom-made titanium flow cell with sapphire windows over the temperature range 25 to 300 °C at 20 MPa using the perchlorate anion, ClO4-, as an internal standard. The reduced isotropic spectra of solutions yielded the first reported quantitative speciation results for polyborate ions in equilibrium with boric acid and borate in high-temperature aqueous solutions above 200 °C. The spectra obtained from solutions at low sodium/boron ratios, 0 < mNaST/ mBST < 0.254, displayed well-defined bands at 880, 747, 615, and 532 cm-1, corresponding to the species B(OH)3 > [B(OH)4]- > [B3O3(OH)4]- ≫ [B5O6(OH)4]-, respectively. The triborate ion, [B3O3(OH)4]-, was found to be the major polyborate species in these boric acid-rich solutions in the range 25 to 300 °C. Thermodynamic formation constants for the triborate species, [B3O3(OH)4]-, calculated from the peak areas, are in agreement with the literature values reported by Mesmer et al at 50, 100, and 200 °C to within the combined experimental uncertainties. At 300 °C, the value for the formation constant, log K31, m b= 2.259 ± 0.060, is larger than the value extrapolated from the results of Mesmer et al. by a factor of ∼3.

Ab Initio Prediction of Proton Exchange Barriers for Alkanes at Brønsted Sites of Zeolite H-MFI.

A hybrid of high level and low level quantum mechanics (QM) methods has been employed to predict intrinsic and apparent energy barriers for the direct proton exchange mechanism of methane, ethane, propane, n-butane, and i-butane on Brønsted sites of H-MFI. The specific hybrid MP2:PBE+D2 + ΔCC implementation used is known to yield the so-called "chemical accuracy" (±4 kJ/mol). Whereas the apparent enthalpy barriers decrease with increasing C number from 104 to 63 kJ/mol, in line with the decreasing heat of adsorption, the intrinsic enthalpy barriers are constant within 124-127 kJ/mol at 500 K. For methane, ethane, propane, and n-butane, we find the expected agreement of apparent barriers with activation energies from batch recirculation reactor experiments. The activation energies derived from NMR experiments (103-113 kJ/mol) are similarly constant as the predicted intrinsic barriers but systematically lower. For i-butane the predicted intrinsic and apparent barriers for the direct proton exchange step are the same as for n-butane with deviations of 2-5 kJ/mol, while the experiments yield values that are 50-60 kJ/mol lower, far outside the estimated range of combined experimental and computational uncertainty (±14 kJ/mol). A change to the indirect proton exchange mechanism, in which a hydride ion is transferred between the alkane and a tert-butyl carbenium ion can be excluded, because we confirm previous findings that the barrier for dehydrogenation that would create a tert-butyl cation from i-butane is much too high, 188 and 132 kJ/mol for the intrinsic and apparent enthalpy barriers, respectively, at 500 K. The possible role of extraframework- and framework-bound alumina species is discussed.

High-precision measurements of n=2→n=1 transition energies and level widths in He- and Be-like argon ions

We performed a reference-free measurement of the transition energies of the $1s 2p\,^1P\_1\to 1s^2 \,^1S\_0$ line in He-like argon, and of the $1s 2s^2 2p\,^1P\_1\to 1s^2 2s^2\,^1S\_0$ line in Be-like argon ions. The highly-charged ions were produced in the plasma of an Electron-Cyclotron Resonance Ion Source. Both energy measurements were performed with an accuracy better than 3 parts in $10^6$, using a double flat-crystal spectrometer, without reference to any theoretical or experimental energy. The $1s 2s^2 2p\,^1P\_1\to 1s^2 2s^2\,^1S\_0$ transition measurement is the first reference-free measurement for this core-excited transition. The $1s 2p\,^1P\_1\to 1s^2 \,^1S\_0$ transition measurement confirms recent measurement performed at the Heidelberg Electron-Beam Ion Trap (EBIT). The width measurement in the He-like transition provides test of a purely radiative decay calculations. In the case of the Be-like argon transition, the width results from the sum of a radiative channel and three main Auger channels. We also performed Multiconfiguration Dirac-Fock (MCDF) calculations of transition energies and rates and have done an extensive comparison with theory and other experimental data. For both measurements reported here, we find agreement with the most recent theoretical calculations within the combined theoretical and experimental uncertainties.

Sensitivity of process design to phase equilibrium uncertainty: Study of the isopropanol + DIPE + 2-methoxyethanol system

Process simulators depend on models for estimating basic thermodynamic properties. The parameters of these models are subject to uncertainties due to measurement errors in the experimental data and the uncertainties are typically assumed to be insignificant in process simulation. This work shows the effect of model parameter uncertainty in the process simulation of extractive distillation for the separation of a diisopropyl ether – isopropyl alcohol minimum boiling azeotrope with entrainer 2-methoxyethanol. This is achieved by extending the application of the uncertainty quantification framework used by Hajipour and Satyro to an extractive distillation process. The approach used was to obtain the best fit of experimentally available literature data and then perform uncertainty quantification based on the combined experimental and model uncertainties using a Monte Carlo simulation for the uncertainty propagation and Aspen Plus® for process simulation. This systematic analysis of model uncertainty provides quantitative insight into the propagation of thermodynamic model errors to the various unit operations. It was found that while the propagated model parameters are able to predict the phase behaviour with a 95% certainty, significant deviations may occur for the process simulation. The approach can be used to improve the confidence with which appropriate safe design margins are assigned to columns and identify the risk areas in the design.

Ion-Pair Formation Constants of Lithium Borate and Lithium Hydroxide under Pressurized Water Nuclear Reactor Coolant Conditions

A custom-made high-precision flow AC conductance instrument was used to measure the frequency-dependent molar electrical conductivities of aqueous solutions of lithium borate from T = 298 K to T = 548 K at a constant pressure, p = 20 MPa. Ion-pair formation constants of lithium borate, KA,m[LiB(OH)40], were derived from these measurements using the Turq–Blum–Bernard–Kunz (TBBK) conductivity model. Our results are consistent with previous low-temperature studies and are the first to be reported at temperatures above 318 K. Under ambient conditions, the degree of association of LiB(OH)40 is half an order of magnitude higher than that of NaB(OH)40 and KB(OH)40, but at T ≥ 448 K, the association constants of all three ion-pairs are equal within the combined experimental uncertainties. The results have been used to derive new equations to represent the temperature dependence of the limiting conductivity of lithium, and the ion-pair formation constant of lithium borate under PWR operating conditions. A new mode...

Impact of the Normalization Condition and Model Information on Evaluated Prompt Fission Neutron Spectra and Associated Uncertainties

Low evaluated uncertainties compared to experimental information and a strong model impact were observed in some prompt fission neutron spectrum (PFNS) evaluations that include mean values and covariances stemming from a rigid model. Here, we show by studying the 239Pu PFNS ENDF/B-VII.1 evaluation via generalized least-squares analyses that strong model correlations in combination with the normalization condition on the estimated PFNS and its covariances result in surprisingly low evaluated uncertainties. Furthermore, the model changes the evaluated results by >1σ of combined experimental uncertainties near the average outgoing neutron energy (˜2 MeV). We show both analytically and by means of representative numerical examples that the normalization condition on the spectrum and its covariances naturally leads to uncertainties reduced by a fully positively correlated scaling uncertainty.

Thermal conductivity of Ionic Liquids: An estimation approach

Knowledge about the thermal conductivity of Ionic Liquids can be essential for research and process development. To compensate missing experimental data an estimation approach has been developed. Independency of other, also often unknown, substance properties was a major claim to this work. A structure interpolating approach was chosen assigning contributions to thermal conductivity to the ions. 375 Experimental data points for 39 Ionic Liquids have been collected and used for the development. The data for substances not used for the fitting could be estimated with an average absolute error of 4.99% (the error for substances used for parameter fitting was only 1.21%). A detailed error analysis resulted in a combined experimental and predictive uncertainty of 7.96% for unknown Ionic Liquids. Based on the proposed correlation a systematic screening of Ionic Liquids concerning thermal conduction is possible.

Standard partial molar heat capacities and enthalpies of formation of aqueous aluminate under hydrothermal conditions from integral heat of solution measurements

Heats of solution of sodium aluminum oxide, NaAlO2(s), were measured in aqueous sodium hydroxide solutions using a Tian–Calvet heat-flow calorimeter (Setaram, Model C80) with high pressure “batch cells” made of hastelloy C-276, at five temperatures from (373.15 to 523.15)K, steam saturation pressure, and concentrations from (0.02 to 0.09)mol·kg−1. Standard molar enthalpies of solution, ΔsolnH∘, and relative standard molar enthalpies, [H∘(T)−H∘(298.15K)], of NaAl(OH)4(aq) were determined from the measured heats of solution. The results were fitted with the “density” model. The temperature dependence of ΔsolnH∘ from the model yielded the standard molar heat capacities of reaction, ΔsolnCp∘, from which standard partial molar heat capacities for aqueous aluminate, Cp∘[A1(OH)4−,aq], were calculated. Standard partial molar enthalpies of formation, ΔfH∘, and entropies, S∘, of A1(OH)4−(aq) were also determined. The values for Cp∘[A1(OH)4−,aq] agree with literature data determined up to T=413K from enthalpy of solution and heat capacity measurements to within the combined experimental uncertainties. They are consistent with differential heat capacity measurements up to T=573K from Schrödle et al. (2010) [29] using the same calorimeter, but this method has the advantage that measurements could be made at much lower concentrations in the presence of an excess concentration of ligand. To our knowledge, these are the first standard partial molar heat capacities measured under hydrothermal conditions by the integral heat of solution method in a commercial calorimeter to be reported in the literature.

Monitor backscatter factors for the Varian 21EX and TrueBeam linear accelerators: measurements and Monte Carlo modelling

Linac backscattered radiation (BSR) into the monitor chamber affects the chamber's signal and has to be accounted for in radiotherapy dose calculations. In Monte Carlo (MC) calculations, the BSR can be modelled explicitly and accounted for in absolute dose. However, explicit modelling of the BSR becomes impossible if treatment head geometry is not available. In this study, monitor backscatter factors (MBSFs), defined as the ratio of the charge collected in the monitor chamber for a reference field to that of a given field, have been evaluated experimentally and incorporated into MC modelling of linacs with either known or unknown treatment head geometry. A telescopic technique similar to that by Kubo (1989 Med. Phys. 16 295–98) was used. However, instead of lead slits, a 1.8 mm diameter collimator and a small (2 mm diameter) detector positioned at extended source to detector distance were used. This setup provided a field of view to the source of less than 3.1 mm and allowed for MBSF measurements of open fields from 1 × 1 to 40 × 40 cm2. For the fields with both X and Y dimensions exceeding 15 cm, a diode detector was used. A pinpoint ionization chamber was used for smaller fields. MBSFs were also explicitly modelled in MC calculations using BEAMnrc and DOSXYZnrc codes for 6 and 18 MV beams of a Varian 21EX linac. A method for deriving the values that are used in MC absolute dose calculations was demonstrated. These values were derived from measured MBSFs for two 21EX and four TrueBeam energies. MBSFs were measured for 6 and 18 MV beams from Varian 21EX, and for 6 MV, 10 MV-FFF, 10 MV, and 15 MV beams from Varian TrueBeam linacs. For the open field sizes modelled in this study for the 21EX, the measured MBSFs agreed with MC calculated values within combined statistical (0.4%) and experimental (0.2%) uncertainties. Variation of MBSFs across field sizes was about a factor of two smaller for the TrueBeam compared to 21EX Varian linacs. Measured MBSFs and the derived factors allow for the incorporation of the BSR effect into accurate radiotherapy dose calculations without explicit backscatter modelling.

Limiting Conductivities and Ion Association Constants of Aqueous NaCl under Hydrothermal Conditions: Experimental Data and Correlations

Frequency-dependent electrical conductivities of solutions of aqueous sodium chloride have been measured from T = 298 K to T = 623 K at p = 20 MPa, over a very wide range of ionic strength (2·10–5 to 0.17 mol·kg–1) using a unique high-precision flow-through alternating current (AC) electrical conductance instrument. Experimental values for the equivalent conductivity, Λ, were used to calculate molar conductivities at infinite dilution, Λ°, using the Fuoss–Hsia–Fernandez–Prini (FHFP) and Turq–Blum–Bernard–Kunz (TBBK) ionic conductivity models. The resulting values for the limiting conductivity Λ° and the ion association constant of NaCl, from this work and critically evaluated literature data above 277 K, were represented to within the combined experimental uncertainties, as functions of viscosity and solvent density, respectively. New values and new correlations are reported for the limiting equivalent conductivities of the sodium ion, λ°(Na+), and the chloride ion, λ°(Cl–) from 277 K and 100 kPa to 1073 ...

Does the presence of heterotrophic bacterium Pseudomonas reactans affect basaltic glass dissolution rates?

Far-from-equilibrium steady-state basaltic glass dissolution rates were measured using newly developed Bacterial Mixed-Flow Reactors (BMFR) at 25°C. Experiments were performed in aqueous pH 4, 6, 8, and 10 buffer solutions with and without nutrients and in: 1) the absence of bacteria, 2) the presence of 0.1–0.4gwet/L dead Pseudomonas reactans, and 3) in the presence of 0.9–19.0gwet/L live P. reactans extracted from a deep subsurface oxygen-bearing basaltic aquifer. The BMFR allows steady-state rate measurements at constant concentrations of live or dead bacteria. Experiments ran for 30–60days on a single basaltic glass powder, initially at bacteria- and nutrient-free conditions until Si steady-state glass dissolution was achieved. Then, in 2–4 steps, either live or dead P. reactans were added via nutrient-rich or nutrient-free inlet solutions, and the flux of Si and other mineral constituents from the basaltic glass were measured until a new chemical steady state was attained. Scanning Electron Microscope and X-ray Photoelectron Spectroscopic techniques verified the presence of live bacteria on the basaltic glass surfaces through biofilm formation, bacterial dissolution imprints, and enrichment of surface layers in C and N. The presence of either live or dead P. reactans lowers constant pH steady-state basaltic glass dissolution rates by no more than ~0.5log units which is close to combined analytical and experimental uncertainties of the experiments. The presence of bacteria did not cause any significant modification in trace element release rates from dissolving basaltic glass within the uncertainty of the measurements. Experiments in nutrient-rich solutions yielded close to stoichiometric release of all major elements from basaltic glass at pH 6–8 likely as a result of organic ligand complexation with aqueous Al and Fe, which would otherwise form hydroxy-precipitates. As the results of this work suggest, at most, a small inhibiting effect of live or dead P. reactans on basaltic glass dissolution rates, geochemical modeling of mineral reactivity in basaltic aquifers likely does not require explicit provision for the presence of heterotrophic bacteria on basaltic glass reactivity.

Study ofB→πlνandB→ρlνdecays and determination of|Vub|

We present an analysis of exclusive charmless semileptonic B-meson decays based on 377 million BBbar pairs recorded with the BABAR detector at the Upsilon(4S) resonance. We select four event samples corresponding to the decay modes B0 --> pi- l+ nu, B+ --> pi0 l+ nu, B0 --> rho- l+ nu, and B+ -> rho0 l+ nu, and find the measured branching fractions to be consistent with isospin symmetry. Assuming isospin symmetry, we combine the two B --> pi l nu samples, and similarly the two B --> rho l nu samples, and measure the branching fractions BF(B0 --> pi- l+ nu) = (1.41 +- 0.05 +- 0.07) x 10^-4 and BF(B0 --> rho- l+ nu) = (1.75 +- 0.15 +- 0.27) x 10^-4, where the errors are statistical and systematic. We compare the measured distribution in q^2, the momentum transfer squared, with predictions for the form factors from QCD calculations and determine the CKM matrix element |V_ub|. Based on the measured partial branching fraction for B --> pi l nu in the range q^2 < 12 GeV^2 and the most recent LCSR calculations we obtain |V_ub| = (3.78 +- 0.13 +0.55-0.40) x 10^-3, where the errors refer to the experimental and theoretical uncertainties. From a simultaneous fit to the data over the full q^2 range and the FNAL/MILC lattice QCD results, we obtain |V_ub| = (2.95 +- 0.31) x 10^-3 from B --> pi l nu, where the error is the combined experimental and theoretical uncertainty.

ChemInform Abstract: Trajectory Studies of Collisional Relaxation of Highly Excited CS2 by H2, CO, HCl, CS2, and CH4

The collisional relaxation of vibrationally highly excited CS2 by thermal ensembles of H2, CO, HCl, CS2, and CH4 is investigated theoretically using classical trajectory calculations with empirical potentials that include van der Waals and electrostatic interactions. The successive collisions method suggested by Bruehl and Schatz is shown to be an effective way to study energy transfer from vibrationally hot polyatomic molecules to molecules of a thermal bath. The average energy transferred per collision to H2 and CO matches experimental measurements within a factor of 2, while that to CH4 and CS2 matches experiment within a factor of 4. These differences are consistent with combined theoretical and experimental uncertainties. Energy transfer to HCl is, however, off by as much as a factor of 7, despite our use of a potential which is realistic at long range. The energy transferred is partitioned mostly to relative translation when the collider is H2 or CO and partly to rotation for CH4. The energy dependence of the energy transfer and the interrelations among energy‐transfer moments, including 〈ΔEup〉, 〈ΔEdown〉, and 〈ΔE2〉, are also reported.

Rational approaches for combining redundant, independent measurements to minimize combined experimental uncertainty

General methods for combining multiple, independent measurements for minimized uncertainty are presented. Although the methods are general, the presentation is motivated by the reduction of heat exchanger performance data. It is demonstrated that the prevalent practice of using the arithmetic mean of two measured heat transfer rates for the evaluation of heat exchanger performance, which is widely adopted in the literature and used in engineering standards, does not always lead to reduced experimental uncertainty and very rarely—if ever—leads to a minimized experimental uncertainty. For the case of averaging two redundant measurements, simple criteria are developed to determine whether using the arithmetic mean or a single measurement yields a smaller absolute or relative uncertainty. Then, more general methods are proposed in the form of weighted-linear averages, in which the weight factors are determined to minimize the combined uncertainty. The methods are generally applicable to averaging any number of redundant measurements with varying uncertainties.

Hypothetical Thermodynamic Properties. Subcooled Vaporization Enthalpies and Vapor Pressures of Polyaromatic Heterocycles and Related Compounds

The vaporization enthalpies and vapor pressures of the liqiud phase from T = 298.15 K to T = 500 K of a series of polyaromatic nitrogen heterocyles (PANH) have been derived using correlation gas chromatography. The vaporization enthalpies at T = 298.15 K measured in kilojoules per mole include: tri-n-butylamine (62.7 ± 1.3), 2-phenylpyridine (68.4 ± 1.9), 3-phenylpyridine (68.4 ± 1.6), quinazoline (58.9 ± 2.0), (−)-nicotine (63.9 ± 2.1), 4-phenylpyrimidine (68.8 ± 2.5), 2,2-dipyridine (67.0 ± 2.3), 2,4-dipyridine (70.9 ± 1.6), 4,4-dipyridine·1.7H2O (70.7 ± 2.4), 4,4-dipyridine (anhydrous, 71.1 ± 2.6), 1,7-phenanthroline (79.4 ± 4.7), 4,7-phenanthroline (80.8 ± 4.8), 2-benzylpyridine (69.8 ± 2.8), and 9-methylcarbazole (79.4 ± 3.2). Fusion enthalpies (kJ·mol−1, Tfus/K) are also reported for 2,4-bipyridine (17.4 ± 0.4, 332.8), 4,4-dipyridine (16.1 ± 1.0, 377.5), 4,4-dipyridine·1.7H2O (35.5 ± 0.5, 341.3), 4-phenylpyrimidine (18.8 ± 0.3, 334.1), 1,7-phenanthroline (18.2 ± 0.3, 350.3), and 4,7-phenanthroline (21.8 ± 0.5, 445.5). Vaporization enthalpy comparisons are made with available literature values by means of a thermochemical cycle; agreement is within the combined experimental uncertainties for all compounds except 4,4-dipyridine, for which evidence is provided that suggests that the current sublimation enthalpy value in the literature was measured on a hydrated form. Vapor pressures and normal boiling temperatures for the liquid phase are compared with available experimental literature values at elevated temperatures. Good agreement is found for most compounds.