Inert gas technique

The loading of the activated monomeric catalyst could be decreased to a level of 0.04 mol% for certain substrates [turn over number 2,450], The developed process does not require the use of inert gas techniques and in most cases chromatographic purification was not necessary to obtain analytically pure products as no side products were formed and the catalyst could be separated by filtration. 

This chapter describes the removal of water and atmospheric gases from some common solvents and the purification of some common laboratory reagents which are susceptible to contamination by moisture and oxygen. Equipment designs and procedures are introduced here as needed, but most of this material is described in other sections of this book general inert-gas techniques (1.1, 1.3-1.5), conventional distillation (1.2), desiccants (3.2.B), freeze-pump-thaw degassing (5.3.A), and trap-to-trap distillation (5.3.D,E). 

A limitation of the Grignard reaction lies in the sensitivity of the organomagnesium compounds to moisture and oxygen. Therefore, solvents should be carefully dried, and inert gas techniques are required. Small amounts of water in ethereal solvents sometimes are tolerable, however. 

Diisobutylaluminum hydride (DIBAH) (1 M in hexane) was purchased from Aldrich Chemical Company, Inc., and used without further purification. DIBAH should be handled with caution and all operations should be performed by employing the usual inert gas techniques (cannula, syringe, etc.). 

In contrast, the preparation of hydrogels from prepolymers via cross-linking condensation reaction offers a simple method for the preparation of films/mem-branes and coated surfaces. Here, in addition to the aforementioned laboratory equipment, a spin coater is necessary as well as a cabinet for drying and curing. The use of inert gas techniques is not required, since the underlying reaction is not affected by oxygen. 

Oxygen and nitrogen also are deterrnined by conductivity or chromatographic techniques following a hot vacuum extraction or inert-gas fusion of hafnium with a noble metal (25,26). Nitrogen also may be deterrnined by the Kjeldahl technique (19). Phosphoms is determined by phosphine evolution and flame-emission detection. Chloride is determined indirecdy by atomic absorption or x-ray spectroscopy, or at higher levels by a selective-ion electrode. Fluoride can be determined similarly (27,28). Uranium and U-235 have been determined by inductively coupled plasma mass spectroscopy (29). 

In plasma chromatography, molecular ions of the heavy organic material to be analy2ed are produced in an ionizer and pass by means of a shutter electrode into a drift region. The velocity of drift through an inert gas at approximately 101 kPa (1 atm) under the influence of an appHed electric field depends on the molecular weight of the sample. The various sonic species are separated and collected every few milliseconds on an electrode. The technique has been employed for studying upper atmosphere ion molecule reactions and for chemical analysis (100). 

Techniques for handling sodium in commercial-scale appHcations have improved (5,23,98,101,102). Contamination by sodium oxide is kept at a minimum by completely welded constmction and inert gas-pressured transfers. Residual oxide is removed by cold traps or micrometallic filters. Special mechanical pumps or leak-free electromagnetic pumps and meters work well with clean Hquid sodium. Corrosion of stainless or carbon steel equipment is minimi2ed by keeping the oxide content low. The 8-h TWA PEL and ceiling TLV for sodium or sodium oxide or hydroxide smoke exposure is 2 mg/m. There is no defined AID for pure sodium, as even the smallest quantity ingested could potentially cause fatal injury. 

As in the case of many metal—ahoy systems, weld ductihty is not as good as that of the base metal. Satisfactory welds can be made in vanadium ahoys provided the fusion zone and the heat-affected zone (HAZ) are protected from contamination during welding. Satisfactory welds can be made by a variety of weld methods, including electron-beam and tungsten-inert-gas (TIG) methods. It is also likely that satisfactory welds can be made by advanced methods, eg, laser and plasma techniques (see Lasers Plasma technology). 

The methods of choice for beryUium oxide in beryUium metal are inert gas fusion and fast neutron activation. In the inert gas fusion technique, the sample is fused with nickel metal in a graphite cmcible under a stream of helium or argon. BeryUium oxide is reduced, and the evolved carbon monoxide is measured by infrared absorption spectrometry. BeryUium nitride decomposes under the same fusion conditions and may be determined by measurement of the evolved nitrogen. Oxygen may also be determined by activation with 14 MeV neutrons (20). The only significant interferents in the neutron activation technique are fluorine and boron, which are seldom encountered in beryUium metal samples. 

Aluminum and Alloys Aluminum and its alloys are made in practically all the forms in which metals are produced, including castings. Thermal conductivity of aluminum is 60 percent of that of pure copper, and unalloyed aluminum is used in many heat-transfer applications. Its high electrical conductivity makes aluminum popular in electrical apphcations. Aluminum is one of the most workable of metals, and it is usually joined by inert-gas-shielded arc-welding techniques. 

Prevention of a flammable atmosphere may be accomplished using any of the alternatives presented in NEPA 69. in cases where fuel concentration cannot be limited, the most common technique (inerting) is to add a suitable inert gas such as nitrogen, so that the residual oxygen concentration is insufficient to support a flame. A safety factor is then applied. Eor most flammable gases and vapors this typically involves reducing the oxygen concentration to less than 5-8 vol% (see Chapter 2-7 of NEPA 69). 

Free-radical polymerisation techniques involving peroxides or azodi-isobutyronitrile at temperatures up to about 100°C are employed commercially. The presence of oxygen in the system will affect the rate of reaction and the nature of the products, owing to the formation of methacrylate peroxides in a side reaction. It is therefore common practice to polymerise in the absence of oxygen, either by bulk polymerisation in a full cell or chamber or by blanketing the monomer with an inert gas. 

Either UV-VIS or IR spectroscopy can be combined with the technique of matrix isolation to detect and identify highly unstable intermediates. In this method, the intomediate is trapped in a solid inert matrix, usually one of the inert gases, at very low temperatures. Because each molecule is surrounded by inert gas atoms, there is no possiblity for intermolecular reactions and the rates of intramolecular reactions are slowed by the low temperature. Matrix isolation is a very useful method for characterizing intermediates in photochemical reactions. The method can also be used for gas-phase reactions which can be conducted in such a way that the intermediates can be rapidly condensed into the matrix. 

However, the spatial resolution of AES is mueh greater than that of XPS and ean approaeh approximately 25 nm. This makes AES a powerful technique for constructing high-resolution maps showing the distribution of chemical species across a surface. Because of the small analysis area, it is an easy matter to combine AES with inert gas sputtering to construct depth profiles showing the distribution of chemical species as a function of distance away from the surface and into the bulk of the solid. Quantitative analysis can be done using sensitivity factors and an equation similar to Eq. 17. 

Monomeric neutral SO4 can be obtained by reaction of SO3 and atomic oxygen photolysis of S03/ozone mixtures also yields monomeric SO4, which can be isolated by inert-gas matrix techniques at low temperatures (15-78 K). Vibration spectroscopy indicates either an open peroxo Cj structure or a closed peroxo C2v structure, the former being preferred by the most recent study, on the basis of agreement between observed and calculated frequencies and reasonable values for the force constants ... 

Figure 10-158B. Platecoil double- and single-embossing designs for standard units. The Platecoil is fabricated using resistance, spot, seam, and Tungsten Inert Gas (TIG) and/or Metal Inert Gas (MIG) welding techniques in order to hold and seal the two plates together. (Used by permission Cat. 5-63, 1994. Tranter , Inc.)...

It can be welded by resistance, tungsten-inert gas (TIG), plasma arc and electron beam techniques. To protect the metal from attack by air, resistance welding is carried out under water and the TIG method is best performed in a chamber of argon. The latter three methods produce ductile welds that equal the base metal in most of its characteristics. 

Changes in free energy and the equilibrium constants for Reactions 1, 2, 3, and 4 are quite sensitive to temperature (Figures 2 and 3). These equilibrium constants were used to calculate the composition of the exit gas from the methanator by solving the coupled equilibrium relationships of Reactions 1 and 2 and mass conservation relationships by a Newton-Raphson technique it was assumed that carbon was not formed. Features of the computer program used were as follows (a) any pressure and temperature may be specified (b) an inert gas may be present (c) after... 

In general, the flow rate F(t) consists of the following additive components the controlled flow rate Fd of the entering gas, the flow rate Fi which is due to parasitic leaks and/or diffusion, and the flow rate Fw resulting from possible adsorption-desorption processes on the system walls (in Section I, references are given to papers dealing with the elimination or control of the wall effects in the flash filament technique). In each of these flow rate components a particular ratio of the investigated adsorbate and of the inert gas exists and all these components contribute to the over-all mean values Fh(t) and F (t). 

Semibatch or fully continuous operation with continuous removal of a by-product gas is also common. It is an important technique for relieving an equilibrium limitation, e.g., by-product water in an esterification. The pressure in the vapor space can be reduced or a dry, inert gas can be sparged to increase Ai and lower a, thereby increasing mass transfer and lowering u/ so that the forward reaction can proceed. 

Nitridoborates of lanthanum and the lanthanides were obtained from reactions of lanthanide metal or lanthanide metal nitride with layer-like (a-)BN at elevated temperatures (3>1200°C). These reactions require elaborated techniques in the inert gas sample-handling and the use of efficient heating sources, such as induction heating. Only some compounds remain stable in this high-temperature segment, and the yields of such reactions are often limited due to the competing stability of binary phases, allowing only the most (thermodynamically) stable compounds to exist. 

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