Substances thermally stable

The processes of distillation and sublimation are closely related. There are three ways in which the vaporisation of a thermally stable substance may take place on heating —... 

The main difference between field ionization (FI) and field desorption ionization (FD) lies in the manner in which the sample is examined. For FI, the substance under investigation is heated in a vacuum so as to volatilize it onto an ionization surface. In FD, the substance to be examined is placed directly onto the surface before ionization is implemented. FI is quite satisfactory for volatile, thermally stable compounds, but FD is needed for nonvolatile and/or thermally labile substances. Therefore, most FI sources are arranged to function also as FD sources, and the technique is known as FI/FD mass spectrometry. 

Thermal Stability. The saturated C —C 2 ketones are thermally stable up to pyrolysis temperatures (500—700°C). At these high temperatures, decomposition can be controlled to produce useful ketene derivatives. Ketene itself is produced commercially by pyrolysis of acetone at temperatures just below 550°C (see Ketenes, ketene dil rs, and related substances). 

Methods for removing water from solids depends on the thermal stability of the solids or the time available. The safest way is to dry in a vacuum desiccator over concentrated sulfuric acid, phosphorus pentoxide, silica gel, calcium chloride, or some other desiccant. Where substances are stable in air and melt above 100°, drying in an air oven may be adequate. In other cases, use of an Abderhalden pistol may be satisfactory. 

Evaporation can be performed directly from reactors or kettles provided that substances are thermally stable. Such evaporation is time consuming because of the low heat-transfer surface area per unit volume. In the case of temperature sensitive materials, the residence time in the evaporator must be short and the temperature should be as low as possible. Consequently, continuous vacuum evaporators with a short residence time should be used to treat such materials. Falling-film (thin-film) evaporators are suitable to perform such operations. A typical falling-film evaporators is shown in Fig. 7.2-14. Centrifugal evaporators are also commonly used. 

How do we decide whether to separate a mixture by gc or hplc In gc, mixtures are examined in the vapour phase, so that we have to be able to form a stable vapour from our mixture, or convert the substances in it to derivatives that are thermally stable. Only about 20% of chemical compounds are suitable for gc without some form of sample modification the remainder are thermally unstable or involatile. In addition, substances with highly polar or ionisable functional groups often show poor chromatographic behaviour by gc, being very prone to tailing. Thus hplc is the better technique for macromolecules, inorganic or other ionic species, labile natural products, pharmaceutical compounds and biochemicals. 

Polymerizing, Decomposing, and Rearranging Substances Most of these substances are stable under normal conditions or with an added inhibitor, but can energetically self-react with the input of thermal, mechanical, or other form of energy sufficient to overcome its activation energy barrier (see Sec. 4, Reaction Kinetics, Reactor Design, and Thermodynamics). The rate of self-reaction can vary from imperceptibly slow to violently explosive, and is likely to accelerate if the reaction is exothermic or self-catalytic. 

These methods require that the sample is either a gas or, at least, a volatile substance which can be easily converted into a gas (this explains the utility of mass spectrometry in the field of organic chemistry). In inorganic chemistry it is often more difficult to obtain a gaseous sample, and so other ionization sources have been developed. If the sample is thermally stable, it may be volatilized by depositing it on a filament and heating the filament (thermal ionization mass spectrometry - see below). In restricted cases (e.g., organometallic chemistry), chemical treatment of the sample may give a more volatile sample. 

Mixed metal oxide pigments are thermally stable, water-insoluble materials. They are not classified as hazardous substances, and are therefore not subject to international transport regulations. When stored under dry conditions their pigment properties do not deteriorate. 

Switching also implies molecular and supramolecular bistability since it resides in the reversible interconversion of a molecular species or supramolecular system between two thermally stable states by sweeping a given external stimulus or field. Bistability in isolated molecules or supermolecules is, for instance, found in optical systems such as photochromic [8.229] or thermochromic substances or devices, in electron transfer or magnetic processes [8.239], in the internal transfer of a bound substrate between the two binding sites of a ditopic receptor (see Section 4.1 see also Fig. 33) [6.77]. Bistability of polymolecular systems is of a supramolecular nature as in a phase transition or a spin transition, both of which involve an assembly of interacting species. 

As a rule, recrystallization cannot be used for the purification of organic metals. Recrystallization is usually performed under definite thermal influence and leads to dirtied, imperfect crystals. Ion radical salts are not thermally stable in solution. The direct donor-to-acceptor interaction is the best way to limit chemical impurities. In this case, the reaction mixture contains minimal amounts of substances that are not included in the structure of a given ion radical salt. The oxidation of donors in the presence of anions or ion exchange usually results in the formation of less pure crystals. 

Often safety data or material safety data sheets mention the thermal stability as an intrinsic property of a substance or mixture. In fact, this is an oversimplification of a concept that must be defined in a more comprehensive way. Basically, a substance or a mixture is thermally stable in a situation where the heat released can be removed in such a way that no temperature increase occurs. This definition... 

For the unambiguous determination of the thermal stability of a substance or mixture of substances, it is generally necessary to determine the heat flow rate P (in pW g ) The substance can be considered to be thermally stable at a certain... 

If a sample does not pass test 3c, but is shown to be thermally stable using a DSC, test 4a must additionally be performed. If the substance is thermally unstable,... 

A search for new efficient techniques of rare earth element separation and purification from calcium is a current problem, as production of high-purity rare earths is of great importance in advanced technology and material science. This problem may be solved by vacuum sublimation of volatile compounds when the difference in vapor pressure of the components present is used. This technique of purification was tested for Nd and Ca separation in vacuum. The well-known volatile and thermally stable dipivaloylmethanates were taken as starting substances. It was found that the addition of pivalic acid to the Nd(dpm)3 + Ca(dpm)2 mixture caused an increase in the separation efficiency and led to pure Nd(dpm)3 in the sublimate . ... 

A new deep violet compound, (02CIF3), was discovered by reacting O2F2 with GIF at 119° to 140° K. It is thermally stable up to 195° K. for over a year. The stability of its concentrated deep violet solution in anhydrous HF depends on the oxygen pressure. This solution has practically the same conductivity as HF and thus the compound is not an electrolyte. The compound is a very strong oxidizing substance, and reacts with NH3, various hydrocarbons, and even ice at low temperatures. It seems to surpass even ozone as an oxidizer. 

Interference arises when the intensity of the signal from the required substance is modified by another substance, although the signals of the two substances are adequately resolved. Thus, the presence of phosphate reduces the flame emission of calcium by forming thermally stable calcium phosphate. The instrumental conditions can sometimes be altered to reduce the effect, or the interfering material removed by pretreatment of the sample. Standard solutions are usually chosen to resemble samples as closely as possible in the hope that any interference will occur equally in both. 

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