DOCSLIB.ORG

Supercritical Fluid Extraction of Positron-Emitting Radioisotopes from Solid Target Matrices

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Supercritical Fluid Extraction of Positron-Emitting Radioisotopes from Solid Target Matrices XA0101188 11. United States of America Supercritical Fluid Extraction of Positron-Emitting Radioisotopes From Solid Target Matrices D. Schlyer, Brookhaven National Laboratory, Chemistry Department, Upton, Bldg. 901, New York 11973-5000, USA Project Description Supercritical fluids are attractive as media for both chemical reactions, as well as process extraction since their physical properties can be manipulated by small changes in pressure and temperature near the critical point of the fluid. What is a supercritical fluid? Above a certain temperature, a vapor can no longer be liquefied regardless of pressure critical temperature - Tc supercritical fluid r«gi on solid a u & temperature Fig. 1. Phase diagram depicting regions of solid, liquid, gas and supercritical fluid behavior. The critical point is defined by a critical pressure (Pc) and critical temperature (Tc) for a particular substance. Such changes can result in drastic effects on density-dependent properties such as solubility, refractive index, dielectric constant, viscosity and diffusivity of the fluid[l,2,3]. This suggests that pressure tuning of a pure supercritical fluid may be a useful means to manipulate chemical reactions on the basis of a thermodynamic solvent effect. It also means that the solvation properties of the fluid can be precisely controlled to enable selective component extraction from a matrix. In recent years there has been a growing interest in applying supercritical fluid extraction to the selective removal of trace metals from solid samples [4-10]. Much of the work has been done on simple systems comprised of inert matrices such as silica or cellulose. Recently, this process as been expanded to environmental samples as well [11,12]. However, very little is understood about the exact mechanism of the extraction process. Of course, the widespread application of this technology is highly dependent on the ability of scientists to model and predict accurate phase equilibria in complex systems [13]. In this project, we plan to explore the feasibility of utilizing supercritical fluids as solvents for reaction and extraction of radioisotopes produced from solid enriched targets. The reason for this work is that many of these enriched target materials used for radioisotope production are expensive. Quite often, post-irradiation recovery of the radioisotope from the target matrix renders that material unusable, or necessitates performing a time-consuming recovery step. The use of supercritical fluids could allow us to extract the desired radioisotope in a significantly shorter time, and in some useable chemical form, while not altering the chemical state of the enriched target matrix. This project will involve three levels of development. Within the first level, we will plan to assemble and test a supercritical fluid extraction station. A prototype system is already in place for this phase. Supercritical Fluid Pump Extraction cell — Fig. 2. Supercritical CO2 fluid extraction station The first test case would be on iodine extraction from tellurium oxide. One radioisotope of clinical importance in Nuclear Imaging is iodine-124 (4.2 day half-life) produced by the 124Te(p,n)124I and the 124Te(d,2n)124I nuclear reactions [14,15] from enriched tellurium-124. One way that is used to separate the iodine radioisotope is by dry distillation at high temperatures. [16] This process typically takes many hours and most certainly affects the morphology of tellurium, which can impact, on subsequent irradiations using the same target material. Supercritical fluid extractions will be performed using tracer amounts of the radioisotope produced at the BNL cyclotron facility, and various pure fluids. Carbon dioxide fluid will be used in the first trials, but other fluids may prove better suited for the process. In the second phase, we plan to construct an optical cell [17] that will withstand the high system pressures of the supercritical fluid. We already have experience in this area because there is an ongoing project in the PET group exploring the utility of solvent clusters within supercritical fluids for conducting microscale tracer chemistry. Fig. 3. High pressure UV-Vis cell utilizes HPLC stainless cross with fiber optics serving as the cell windows. We plan to utilize this optical cell in conjunction with the radioisotope extraction project for the purpose of performing direct solubility measurements by UV-Vis spectroscopy. This would not only be extremely useful for fine-tuning process parameters, but provide us with a detailed understanding of the molecular process in supercritical fluid mixtures that could allow us to model the phase equilibria and better predict general efficacy of the method. In the third phase of development, we need to explore whether the radioisotope extract exists in some usable chemical form. That is, can the extract be used in synthesis to render the radioisotope as part of a larger biomolecule of interest either for therapeutic reasons or diagnostic imaging? Initially, we plan to use atomic absorption spectroscopy and radio-HPLC to help elucidate chemical state of the extracted iodine. Once elucidated, we plan to try to alter the chemical state of the radioisotope by controlling post- irradiation chemistry that may occur as a consequence of the extraction methodology. This can be accomplished by manipulating the operating parameters of the fluid (ie. temperature and pressure), changing the nature of the fluid, and by introducing certain reactive additives to the fluid. References Clifford, A.A. Supercritical Fluids, Fundamentals for Applications; NATO ASI Ser.Ser E, 273, Kluwer Academic Publishers: 1994; 449-479. Savage, P.E.; Gopalan, S.; Martino, L; Brock, E.E. AIChEJ. 1995, 41(7), 1723-1778. Phelps, C.L.; Smart, N.G.; Wai, CM. Past, Present and Possible Future Applications of Supercritical Fluid Extraction Technology, J. Chem. Edu., 1996, 12, 1163-1168. Wai, CM. Metal Extraction with Supercritical Fluids, in Emerging Separation Technologies for Metals II, R.G. Bautista (Ed.), TMS, the Minerals, Metals and Materials Society, Warrendale, PA., 1996, 233-248. Toews, K.L.; Smart, N.G.; Wai, CM., Radiochim. Ada, 1996, 75, 179. Furton, K.G.; Chen, L.; Jaffe, R., Anal. Chim. Ada. 1996, 304, 203. Wai, CM.; Wang, S.; Liu, Y.; Lopez-Avila, V.; Beckert, W.F. Talanta, 1996, 43, 2083. Lin, Y.; Wai, CM., Anal. Chem., 1994, 66, 1971. Wai, CM.; Lin, Y.; Brauer, R.D.; Wang, S.; Beckert, W.F., Talanta, 1993, 40, 1325. Laintz, K.E.; Wai, CM., Yonker, C.R.; Smith, R.D., Anal. Chem., 1992, 64, 2875. Lin, Y.; Wai, CM.; Jean, F.M.; Brauer, R.D., Environ. Sci. Technol., 1994, 28, 1190. Bertsch, P.M.; hunter, D.B.; Sutton, S.R., Bajt, S.; Rivers, M.L., Environ. Sci. Technol., 1994, 28, 980. Brennecke, J.F.; Eckert, C.A., Phase Equilibria for Supercritical Fluid Process Design, AIChEJ., 1989, 35(9), 1409-1427. Kondo, K.; Lambrecht, R.M.; Wolf, A.P., Appl. Radiat. hot., 1977, 28, 395-401. Firouzbahkt, MX.; Schlyer, D.J.; Finn, R.D.; Laguzzi, G.; Wolf, A.P., Nucl. Inst. Meth. 1993, B79, 909-910. Weinreich, R.; Knust, E.J., J. Radioanal. Nucl. Chem., Letters, 1996, 213(4), 253-261. Carrot, M.; Wai, CM., Anal. Chem., 1998, 70, 2421-2425..
Load more

Recommended publications
  • Lecture Notes: BCS Theory of Superconductivity
    Lecture Notes: BCS theory of superconductivity Prof. Rafael M. Fernandes Here we will discuss a new ground state of the interacting electron gas: the superconducting state. In this macroscopic quantum state, the electrons form coherent bound states called Cooper pairs, which dramatically change the macroscopic properties of the system, giving rise to perfect conductivity and perfect diamagnetism. We will mostly focus on conventional superconductors, where the Cooper pairs originate from a small attractive electron-electron interaction mediated by phonons. However, in the so- called unconventional superconductors - a topic of intense research in current solid state physics - the pairing can originate even from purely repulsive interactions. 1 Phenomenology Superconductivity was discovered by Kamerlingh-Onnes in 1911, when he was studying the transport properties of Hg (mercury) at low temperatures. He found that below the liquifying temperature of helium, at around 4:2 K, the resistivity of Hg would suddenly drop to zero. Although at the time there was not a well established model for the low-temperature behavior of transport in metals, the result was quite surprising, as the expectations were that the resistivity would either go to zero or diverge at T = 0, but not vanish at a finite temperature. In a metal the resistivity at low temperatures has a constant contribution from impurity scattering, a T 2 contribution from electron-electron scattering, and a T 5 contribution from phonon scattering. Thus, the vanishing of the resistivity at low temperatures is a clear indication of a new ground state. Another key property of the superconductor was discovered in 1933 by Meissner.
    [Show full text]
  • Equation of State and Phase Transitions in the Nuclear
    National Academy of Sciences of Ukraine Bogolyubov Institute for Theoretical Physics Has the rights of a manuscript Bugaev Kyrill Alekseevich UDC: 532.51; 533.77; 539.125/126; 544.586.6 Equation of State and Phase Transitions in the Nuclear and Hadronic Systems Speciality 01.04.02 - theoretical physics DISSERTATION to receive a scientific degree of the Doctor of Science in physics and mathematics arXiv:1012.3400v1 [nucl-th] 15 Dec 2010 Kiev - 2009 2 Abstract An investigation of strongly interacting matter equation of state remains one of the major tasks of modern high energy nuclear physics for almost a quarter of century. The present work is my doctor of science thesis which contains my contribution (42 works) to this field made between 1993 and 2008. Inhere I mainly discuss the common physical and mathematical features of several exactly solvable statistical models which describe the nuclear liquid-gas phase transition and the deconfinement phase transition. Luckily, in some cases it was possible to rigorously extend the solutions found in thermodynamic limit to finite volumes and to formulate the finite volume analogs of phases directly from the grand canonical partition. It turns out that finite volume (surface) of a system generates also the temporal constraints, i.e. the finite formation/decay time of possible states in this finite system. Among other results I would like to mention the calculation of upper and lower bounds for the surface entropy of physical clusters within the Hills and Dales model; evaluation of the second virial coefficient which accounts for the Lorentz contraction of the hard core repulsing potential between hadrons; inclusion of large width of heavy quark-gluon bags into statistical description.
    [Show full text]
  • Liquid-Liquid Transition in Supercooled Aqueous Solution Involving a Low-Temperature Phase Similar to Low-Density Amorphous Water
    Liquid-liquid transition in supercooled aqueous solution involving a low-temperature phase similar to low-density amorphous water * * # # Sander Woutersen , Michiel Hilbers , Zuofeng Zhao and C. Austen Angell * Van 't Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904,1098 XH Amsterdam, The Netherlands # School of Molecular Sciences, Arizona State University, Tempe, AZ, 85287-1604, USA The striking anomalies in physical properties of supercooled water that were discovered in the 1960-70s, remain incompletely understood and so provide both a source of controversy amongst theoreticians1-5, and a stimulus to experimentalists and simulators to find new ways of penetrating the "crystallization curtain" that effectively shields the problem from solution6,7. Recently a new door on the problem was opened by showing that, in ideal solutions, made using ionic liquid solutes, water anomalies are not destroyed as earlier found for common salt and most molecular solutes, but instead are enhanced to the point of precipitating an apparently first order liquid-liquid transition (LLT)8. The evidence was a spike in apparent heat capacity during cooling that could be fully reversed during reheating before any sign of ice crystallization appeared. Here, we use decoupled-oscillator infrared spectroscopy to define the structural character of this phenomenon using similar down and upscan rates as in the calorimetric study. Thin-film samples also permit slow scans (1 Kmin-1) in which the transition has a width of less than 1 K, and is fully reversible. The OH spectrum changes discontinuously at the phase- transition temperature, indicating a discrete change in hydrogen-bond structure.
    [Show full text]
  • Application of Supercritical Fluids Review Yoshiaki Fukushima
    1 Application of Supercritical Fluids Review Yoshiaki Fukushima Abstract Many advantages of supercritical fluids come Supercritical water is expected to be useful in from their interesting or unusual properties which waste treatment. Although they show high liquid solvents and gas carriers do not possess. solubility solutes and molecular catalyses, solvent Such properties and possible applications of molecules under supercritical conditions gently supercritical fluids are reviewed. As these fluids solvate solute molecules and have little influence never condense at above their critical on the activities of the solutes and catalysts. This temperatures, supercritical drying is useful to property would be attributed to the local density prepare dry-gel. The solubility and other fluctuations around each molecule due to high important parameters as a solvent can be adjusted molecular mobility. The fluctuations in the continuously. Supercritical fluids show supercritical fluids would produce heterogeneity advantages as solvents for extraction, coating or that would provide novel chemical reactions with chemical reactions thanks to these properties. molecular catalyses, heterogenous solid catalyses, Supercritical water shows a high organic matter enzymes or solid adsorbents. solubility and a strong hydrolyzing ability. Supercritical fluid, Supercritical water, Solubility, Solvation, Waste treatment, Keywords Coating, Organic reaction applications development reached the initial peak 1. Introduction during the period from the second half of the 1960s There has been rising concern in recent years over to the 1970s followed by the secondary peak about supercritical fluids for organic waste treatment and 15 years later. The initial peak was for the other applications. The discovery of the presence of separation and extraction technique as represented 1) critical point dates back to 1822.
    [Show full text]
  • Using Supercritical Fluid Technology As a Green Alternative During the Preparation of Drug Delivery Systems
    pharmaceutics Review Using Supercritical Fluid Technology as a Green Alternative During the Preparation of Drug Delivery Systems Paroma Chakravarty 1, Amin Famili 2, Karthik Nagapudi 1 and Mohammad A. Al-Sayah 2,* 1 Small Molecule Pharmaceutics, Genentech, Inc. So. San Francisco, CA 94080, USA; [email protected] (P.C.); [email protected] (K.N.) 2 Small Molecule Analytical Chemistry, Genentech, Inc. So. San Francisco, CA 94080, USA; [email protected] * Correspondence: [email protected]; Tel.: +650-467-3810 Received: 3 October 2019; Accepted: 18 November 2019; Published: 25 November 2019 Abstract: Micro- and nano-carrier formulations have been developed as drug delivery systems for active pharmaceutical ingredients (APIs) that suffer from poor physico-chemical, pharmacokinetic, and pharmacodynamic properties. Encapsulating the APIs in such systems can help improve their stability by protecting them from harsh conditions such as light, oxygen, temperature, pH, enzymes, and others. Consequently, the API’s dissolution rate and bioavailability are tremendously improved. Conventional techniques used in the production of these drug carrier formulations have several drawbacks, including thermal and chemical stability of the APIs, excessive use of organic solvents, high residual solvent levels, difficult particle size control and distributions, drug loading-related challenges, and time and energy consumption. This review illustrates how supercritical fluid (SCF) technologies can be superior in controlling the morphology of API particles and in the production of drug carriers due to SCF’s non-toxic, inert, economical, and environmentally friendly properties. The SCF’s advantages, benefits, and various preparation methods are discussed. Drug carrier formulations discussed in this review include microparticles, nanoparticles, polymeric membranes, aerogels, microporous foams, solid lipid nanoparticles, and liposomes.
    [Show full text]
  • Lecture 5 Non-Aqueous Phase Liquid (NAPL) Fate and Transport
    Lecture 5 Non-Aqueous Phase Liquid (NAPL) Fate and Transport Factors affecting NAPL movement Fluid properties: Porous medium: 9 Density Permeability 9 Interfacial tension Pore size 9 Residual saturation Structure Partitioning properties Solubility Ground water: Volatility and vapor density Water content 9 Velocity Partitioning processes NAPL can partition between four phases: NAPL Gas Solid (vapor) Aqueous solution Water to gas partitioning (volatilization) Aqueous ↔ gaseous Henry’s Law (for dilute solutions) 3 Dimensionless (CG, CW in moles/m ) C G = H′ CW Dimensional (P = partial pressure in atm) P = H CW Henry’s Law Constant H has dimensions: atm m3 / mol H’ is dimensionless H’ = H/RT R = gas constant = 8.20575 x 10-5 atm m3/mol °K T = temperature in °K NAPL to gas partitioning (volatilization) NAPL ↔ gaseous Raoult’s Law: CG = Xt (P°/RT) Xt = mole fraction of compound in NAPL [-] P° = pure compound vapor pressure [atm] R = universal gas constant [m3-atm/mole/°K] T = temperature [°K] Volatility Vapor pressure P° is measure of volatility P° > 1.3 x 10-3 atm → compound is “volatile” 1.3 x 10-3 > P° > 1.3 x 10-13 atm → compound is “semi-volatile” Example: equilibrium with benzene P° = 76 mm Hg at 20°C = 0.1 atm R = 8.205 x 10-5 m3-atm/mol/°K T = 20°C (assumed) = 293°K Assume 100% benzene, mole fraction Xt = 1 3 CG = Xt P°/(RT) = 4.16 mol/m Molecular weight of benzene, C6H6 = 78 g/mol 3 3 CG = 4.16 mol/m × 78 g/mol = 324 g/m = 0.32 g/L 6 CG = 0.32 g/L x 24 L/mol / (78 g/mol) x 10 = 99,000 ppmv One mole of ideal gas = 22.4 L at STP (1 atm, 0 C), Corrected to 20 C: 293/273*22.4 = 24.0 L/mol Gas concentration in equilibrium with pure benzene NAPL Example: equilibrium with gasoline Gasoline is complex mixture – mole fraction is difficult to determine and varies Benzene = 1 to several percent (Cline et al., 1991) Based on analysis reported by Johnson et al.
    [Show full text]
  • A Review of Supercritical Fluid Extraction
    NAT'L INST. Of, 3'«™ 1 lY, 1?f, Reference NBS PubJi- AlllDb 33TA55 cations /' \ al/l * \ *"»e A U O* * NBS TECHNICAL NOTE 1070 U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards 100 LI5753 No, 1070 1933 NATIONAL BUREAU OF STANDARDS The National Bureau of Standards' was established by an act of Congress on March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology and facilitate their effective application for public benefit. To this end, the Bureau conducts research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau's technical work is per- formed by the National Measurement Laboratory, the National Engineering Laboratory, and the Institute for Computer Sciences and Technology. THE NATIONAL MEASUREMENT LABORATORY provides the national system ot physical and chemical and materials measurement; coordinates the system with measurement systems of other nations and furnishes essential services leading to accurate and uniform physical and chemical measurement throughout the Nation's scientific community, industry, and commerce; conducts materials research leading to improved methods of measurement, standards, and data on the properties of materials needed by industry, commerce, educational institutions, and Government; provides advisory and research services to other Government agencies; develops,
    [Show full text]
  • Introduction to Unconventional Superconductivity Manfred Sigrist
    Introduction to Unconventional Superconductivity Manfred Sigrist Theoretische Physik, ETH-Hönggerberg, 8093 Zürich, Switzerland Abstract. This lecture gives a basic introduction into some aspects of the unconventionalsupercon- ductivity. First we analyze the conditions to realized unconventional superconductivity in strongly correlated electron systems. Then an introduction of the generalized BCS theory is given and sev- eral key properties of unconventional pairing states are discussed. The phenomenological treatment based on the Ginzburg-Landau formulations provides a view on unconventional superconductivity based on the conceptof symmetry breaking.Finally some aspects of two examples will be discussed: high-temperature superconductivity and spin-triplet superconductivity in Sr2RuO4. Keywords: Unconventional superconductivity, high-temperature superconductivity, Sr2RuO4 INTRODUCTION Superconductivity remains to be one of the most fascinating and intriguing phases of matter even nearly hundred years after its first observation. Owing to the breakthrough in 1957 by Bardeen, Cooper and Schrieffer we understand superconductivity as a conden- sate of electron pairs, so-called Cooper pairs, which form due to an attractive interaction among electrons. In the superconducting materials known until the mid-seventies this interaction is mediated by electron-phonon coupling which gises rise to Cooper pairs in the most symmetric form, i.e. vanishing relative orbital angular momentum and spin sin- glet configuration (nowadays called s-wave pairing). After the introduction of the BCS concept, also studies of alternative pairing forms started. Early on Anderson and Morel [1] as well as Balian and Werthamer [2] investigated superconducting phases which later would be identified as the A- and the B-phase of superfluid 3He [3]. In contrast to the s-wave superconductors the A- and B-phase are characterized by Cooper pairs with an- gular momentum 1 and spin-triplet configuration.
    [Show full text]
  • Phase Diagrams a Phase Diagram Is Used to Show the Relationship Between Temperature, Pressure and State of Matter
    Phase Diagrams A phase diagram is used to show the relationship between temperature, pressure and state of matter. Before moving ahead, let us review some vocabulary and particle diagrams. States of Matter Solid: rigid, has definite volume and definite shape Liquid: flows, has definite volume, but takes the shape of the container Gas: flows, no definite volume or shape, shape and volume are determined by container Plasma: atoms are separated into nuclei (neutrons and protons) and electrons, no definite volume or shape Changes of States of Matter Freezing start as a liquid, end as a solid, slowing particle motion, forming more intermolecular bonds Melting start as a solid, end as a liquid, increasing particle motion, break some intermolecular bonds Condensation start as a gas, end as a liquid, decreasing particle motion, form intermolecular bonds Evaporation/Boiling/Vaporization start as a liquid, end as a gas, increasing particle motion, break intermolecular bonds Sublimation Starts as a solid, ends as a gas, increases particle speed, breaks intermolecular bonds Deposition Starts as a gas, ends as a solid, decreases particle speed, forms intermolecular bonds http://phet.colorado.edu/en/simulation/states- of-matter The flat sections on the graph are the points where a phase change is occurring. Both states of matter are present at the same time. In the flat sections, heat is being removed by the formation of intermolecular bonds. The flat points are phase changes. The heat added to the system are being used to break intermolecular bonds. PHASE DIAGRAMS Phase diagrams are used to show when a specific substance will change its state of matter (alignment of particles and distance between particles).
    [Show full text]
  • States of Matter
    States of Matter A Museum of Science Traveling Program Description States of Matter is a 60- minute presentation about the characteristics of solids, liquids, gases, and the temperature- dependent transitions between them. It is designed to build on NGSS-based curricula. NGSS: Next Generation Science Standards Needs We bring all materials and equipment, including a video projector and screen. Access to 110- volt electricity is required. Space Requirements The program can be presented in assembly- suitable spaces like gyms, multipurpose rooms, cafeterias, and auditoriums. Goals: Changing State A major goal is to show that all substances will go through phase changes and that every substance has its own temperatures at which it changes state. Goals: Boiling Liquid nitrogen boils when exposed to room temperature. When used with every day objects, it can lead to unexpected results! Goals: Condensing Placing balloons in liquid nitrogen causes the gas inside to condense, shrinking the balloons before the eyes of the audience! Latex-free balloons are available for schools with allergic students. Goals: Freezing Nitrogen can also be used to flash-freeze a variety of liquids. Students are encouraged to predict the results, and may end up surprised! Goals: Behavior Changes When solids encounter liquid nitrogen, they can no longer change state, but their properties may have drastic—sometimes even shattering— changes. Finale Likewise, the properties of gas can change at extreme temperatures. At the end of the program, a blast of steam is superheated… Finale …incinerating a piece of paper! Additional Content In addition to these core goals, other concepts are taught with a variety of additional demonstrations.
    [Show full text]
  • Superconductivity in Pictures
    Superconductivity in pictures Alexey Bezryadin Department of Physics University of Illinois at Urbana-Champaign Superconductivity observation Electrical resistance of some metals drops to zero below a certain temperature which is called "critical temperature“ (H. K. O. 1911) How to observe superconductivity Heike Kamerling Onnes - Take Nb wire - Connect to a voltmeter and a current source - Put into helium Dewar - Measure electrical resistance R (Ohm) A Rs V Nb wire T (K) 10 300 Dewar with liquid Helium (4.2K) Meissner effect – the key signature of superconductivity Magnetic levitation Magnetic levitation train Fundamental property of superconductors: Little-Parks effect (’62) The basic idea: magnetic field induces non-zero vector-potential, which produces non- zero superfluid velocity, thus reducing the Tc. Proves physical reality of the vector- potential Discovery of the proximity Non-superconductor (normal metal, i.e., Ag) effect: A supercurrent can flow through a thin layers of a non- superconducting metals “sandwitched” between two superconducting metals. tin Superconductor (tin) Explanation of the supercurrent in SNS junctions --- Andreev reflection tin A.F. Andreev, 1964 Superconducting vortices carry magnetic field into the superconductor (type-II superconductivity) Amazing fact: unlike real tornadoes, superconducting vortices (also called Abrikosov vortices) are all exactly the same and carry one quantum of the Magnetic flux, h/(2e). By the way, the factor “2e”, not just “e”, proves that superconducting electrons move in pairs.
    [Show full text]
  • 8. Gas-Phase Kinetics
    8. Gas-Phase Kinetics The unimolecular decomposition of gaseous di-tert-butyl peroxide is studied by monitoring the increase in the total pressure that results from the formation of three moles of gaseous products for each mole of reactant. The reaction is studied as a function of initial pressure, in the range 30-80 torr, and at two temperatures — ~150˚C and ~165˚C. General Reading 1. Oxtoby, Nachtrieb, and Freeman, Chemistry: Science of Change (2nd edit.); Sections 15.1-15.3 and 15.5-15.6. 2. Shoemaker, Garland, and Nibler, Experiments in Physical Chemistry (6th edit.); pp. 280-290 (Experiment 24); pp. 630-631 (manometers). [See also especially ref. 15 listed on p. 290.] 3. Levine, Physical Chemistry (4th edit.); Section 17.10 (unimolecular reaction theory). Theory Gas-phase decomposition reactions often exhibit first-order kinetics. Yet it is now understood that the mechanisms for these reactions are actually bimolecular in nature. The reaction is initiated by collisional activation (bimolecular) to an energy that is high enough to undergo decomposition, A + A ® A + A* k1 (1) where A* represents the activated molecule. The observed first-order behavior is a consequence of a competition between the reverse of this process (collisional deactivation), A* + A ® A + A k2 (2) and the decay of A* to products, A* ® products kA (3) When the rate of Process (2) is much greater than that of (3), the concentration of A* is maintained essentially at its equilibrium level. Then Process (3) becomes the rate-determining step, and the reaction exhibits first-order kinetics. This qualitative description can also be reached more quantitatively by employing the steady-state approximation, whereby the rates of change for reaction intermediates are set equal to zero.
    [Show full text]