Literatura académica sobre el tema "Tracers (Chemistry) Enthalpy"

In the fields of Security and Defense, explosive traces must be analyzed at the sites of the terrorist events. The persistence on surfaces of these traces depends on the sublimation processes and the interactions with the surfaces. This study presents evidence that the sublimation process of these traces on stainless steel (SS) surfaces is very different than in bulk quantities. The enthalpies of sublimation of traces of four highly energetic materials: triacetone triperoxide (TATP), 2,4-dinitrotoluene (DNT), 2,4,6-trinitrotoluene (TNT), and 1,3,5- trinitrohexahydro-s-triazine (RDX) deposited on SS substrates were determined by optical fiber coupled-grazing angle probe Fourier Transform Infrared (FTIR) Spectroscopy. These were compared with enthalpies of sublimation determined by thermal gravimetric analysis for bulk amounts and differences between them were found. The sublimation enthalpy of RDX was very different for traces than for bulk quantities, attributed to two main factors. First, the beta-RDX phase was present at trace levels, unlike the case of bulk amounts which consisted only of the alpha-RDX phase. Second, an interaction between the RDX and SS was found. This interaction energy was determined using grazing angle FTIR microscopy. In the case of DNT and TNT, bulk and traces enthalpies were statistically similar, but it is evidenced that at the level of traces a metastable phase was observed. Finally, for TATP the enthalpies were statistically identical, but a non-linear behavior and a change of heat capacity values different from zero was found for both trace and bulk phases.

A potentiometric and UV spectrophotometric investigation on Mn2+-ampicillin and Mn2+-amoxicillin systems in NaCl aqueous solution is reported. The potentiometric measurements were carried out under different conditions of temperature (15 ≤ t/°C ≤ 37). The obtained speciation pattern includes two species for both the investigated systems. More in detail, for system containing ampicillin MLH and ML species, for that containing amoxicillin, MLH2 and MLH ones. The spectrophotometric findings have fully confirmed the results obtained by potentiometry for both the systems, in terms of speciation models as well as the stability constants of the formed species. Enthalpy change values were calculated via the dependence of formation constants of the species on temperature. The sequestering ability of ampicillin and amoxicillin towards Mn2+ was also evaluated under different conditions of pH and temperature via pL0.5 empirical parameter (i.e., cologarithm of the ligand concentration required to sequester 50% of the metal ion present in traces).

Environmental context Sesquiterpenes, chemicals emitted by terrestrial vegetation, are oxidised in the ambient atmosphere leading to the formation of secondary organic aerosol. Although secondary organic aerosol can have significant effects on air quality from local to global scales, considerable gaps remain in our understanding of their various sources and formation mechanisms. We report studies on the oxidation of sesquiterpenes aimed at improving aerosol parameterisation for these reactions for incorporation into future air quality models. Abstract A series of sesquiterpenes (SQT) were individually oxidised under a range of conditions, including irradiation in the presence of NOx, reactions with O3 or reactions with NO3 radicals. Experiments were conducted in either static mode to observe temporal evolution of reactants and products or in dynamic mode to ensure adequate collection of aerosol at reasonably low reactant concentrations. Although some measurements of gas-phase products have been made, the focus of this work has been particle phase analysis. To identify individual products, filter samples were extracted, derivatised and analysed using gas chromatography mass spectrometry techniques. The results indicate that secondary organic aerosol (SOA) is readily formed from SQT oxidation. The high reactivity of these systems and generally high conversion into SOA products gives rise to high SOA levels. SOA yields (ratio of SOA formed to hydrocarbon reacted) averaged 0.53 for ozonolysis, 0.55 for photooxidation and 1.19 for NO3 reactions. In select experiments, SOA was also analysed for the organic matter/organic carbon (OM/OC) ratio, and the effective enthalpy of vaporisation (ΔHvapeff). The OM/OC ranged from 1.8 for ozonolysis and photooxidation reactions to 1.6 for NO3 reactions, similar to that from SOA generated in monoterpene systems. ΔHvapeff was measured for β-caryophyllene–NOx, β-caryophyllene–O3, β-caryophyllene–NO3, α-humulene–NOx and α-farnesene–NOx systems and found to be 43.9, 41.1, 44.9, 48.2 and 27.7 kJ mol–1. Aerosol yields and products identified in this study are generally in good agreement with results from several studies. A detailed examination of the chamber aerosol for the presence of chemical tracer compounds was undertaken. Only β-caryophyllinic acid, observed mainly under β-caryophyllene photooxidation and ozonolysis experiments, was detected in ambient aerosol. Chemical analysis yielded compounds having oxygen and nitrogen moieties present, which indicates continued evolution of the particles over time and presents high dependence on the SQT–oxidant system studied. This study suggests that SOA from laboratory ozonolysis experiments may adequately represent ambient aerosol in areas with SQT emissions.

Abstract The compound LiNi0.5Mn1.5O4 is used as novel cathode material for Li-ion batteries and represents a variant to replace conventional LiMn2O4. For a further improvement of battery materials it is necessary to understand kinetic processes at and in electrodes and the underlying diffusion of lithium that directly influences charging/discharging times, maximum capacities, and possible side reactions. In the present study Li tracer self-diffusion is investigated in polycrystalline sintered bulk samples of near stoichiometric LiNi0.5Mn1.5O4 with an average grain size of about 50–70 nm in the temperature range between 250 and 600 °C. For analysis, stable 6Li tracers are used in combination with secondary ion mass spectrometry (SIMS). The tracer diffusivities can be described by the Arrhenius law with an activation enthalpy of (0.97 ± 0.05) eV, which is interpreted as the sum of the formation and migration energy of a thermally activated Li vacancy.

All nuclear power plants are licensed to operate at a specific thermal power. Verification of this power level is done by measuring the change in the enthalpy of the feedwater times the mass flow of the feedwater through each steam generator for pressurized water reactor or through the reactor vessel for boiling water reactors. When performing this measurement, it is necessary to determine the enthalpy of the steam exiting each steam generator or the reactor vessel. Since the exit enthalpy of most units is saturated steam, the state of the steam must be defined in terms of the enthalpy of dry stream minus the fraction of moisture being carried over with the steam times the heat of vaporization. The quality of the steam can be measured using a throttling calorimeter, but a much more accurate means of measurement is with the use of a chemical tracer salt that is only carried over with the main steam by droplets of water. Thus, the fraction of moisture in the steam can be defined as the ratio of concentration of the tracer in the steam divided by concentration of the tracer in the riser of the generator or reactor vessel. If the moisture content of the steam is not measured, one must assume that it is zero, which is conservative, since it provides an upper limit for the thermal power calculation. However, from an operational standpoint, if moisture carryover is present but is not accounted for, the thermal power of the unit will be below its licensed power level causing a loss in electrical output due both a lower thermal power and a reduction in turbine cycle efficiency. Moreover, if there is significant carryover, there is the potential for additional damage to the turbine blading and housing from the impingement of the water droplets. This paper describes how a chemical tracer can be used to measure the moisture content of the steam, considerations for the injection and sample points, plant alignment recommendations and the support required from a utility, when performing this test. The paper will also discuss the potential for long term degradation, the effects of extended power uprates on carryover and the limitations of chemistry guidelines.