Unstable substance/material

Unstable substance/material substance or material that decomposes, whether violently or not, in the pure state or in the state as normally produced. 

Chloropropionaldehyde is a very unstable substance which polymerizes rapidly especially in the presence of traces of hydrochloric acid. The crude material must be oxidized without delay as standing before oxidation will cause a lowering of the yield. 

Dilution or simple mixing with a stable compound is sufficient to stabilise an unstable substance. In the case of a simple mixture with a neutral substance, this stabilisation process is called desensitisation . Thus hardeners such as benzoyl peroxide are nomially in the form of suspensions in heavy esters or oils. This peroxide is mixed with 30% of water by weight. Dynamite is nitroglycerine stabilised with the help of a neutral material. In all these cases, heat that is produced by the potential beginning of decomposition is absorbed by the inert substance. 

Hydrazine is an unstable substance because of its positive enthalpy of formation. It decomposes when heated. The decomposition can cause an inflammation even in the absence of air. It can also combust spontaneously in the presence of various materials from clothes to soil (see tables in Part Three the self-ignition temperatures vary according to the materials in contact with hydrazine). Also, violent decomposition of hydrazine in a steel reactor occurs when in a carbon dioxide atmosphere. 

Another example of scale-up effects relates to the storage of chemically unstable substances. Well-established procedures can be followed on a small scale. In a commercial unit, the storage of such materials must be reviewed from the standpoint of critical mass. The heat removal capacity of the equipment must be substantially larger than the spontaneous exothermic rate of heat release in the bulk material. Temperature gradients must also be considered. 

These exceedingly unstable substances, which are invoked as the active intermediates in bioluminescence,20 were first prepared and isolated by Adam and Liu.2 The /-butyl system (2a) was the originally synthesized derivative by dehydrative cyclization of the a-hydroperoxy acid (6) by dicyclohexylcarbodiimide (DCC) [Eq. (5)]. It was not possible to isolate the pure material in view of its great thermal instability but its characteristic carbonyl band at 1875 cm-1 in the infrared (IR) served as unequivocal structure identification of these novel cyclic peroxides. 

NOTE 4-Azidophenol is an unstable substance, easily oxidized by air, and producing black crystals. However, this material can be used without difficulty in the next synthetic step. Storage of larger amounts... 

Vacuum distillation. This expression is commonly used to denote a distillation under reduced pressure lower than that of the normal atmosphere. Because the boiling point of a substance depends on the pressure, it is often possible by sufficiently lowering the pressure to distil materials at a temperature low enough to avoid partial or complete decomposition, even if they are unstable when boiled at atmospheric pressure. 

Generally the substances in this class are unstable when heated or exposed to light they should be stored cool and in the dark. However, for liquids with added stabilizer cooling may cause separation of the material from the stabilizer. Similarly, precipitation of a potentially explosive compound from a diluent may occur on cooling. In both cases this can represent a hazardous situation. 

During precipitate ageing, a gradual transformation of an initially precipitated metastable phase into a final crystalline form often occurs. The metastable phase may be an amorphous precipitate, a polymorph of the final material, a hydrated species or some system-contaminated substance (Mullin, 2001). In 1896, Ostwald promulgated his rule of stages which states that an unstable... 

Tliis categor> includes chemicals reacting vitli otlier chemicals or materials and tlie decomposition of unstable chemicals (e.g tlie reaction of some substances witli water or moist air or witli strong oxidizing and reducing agents). 

In connection with colour-producing reagents, it must be recognised that solutions of such reagents are frequently unstable and normally should not be stored for more than a day or so. Even in the solid state, many of these materials tend to deteriorate slowly and it is advisable that, in general, only small quantities should be stored so that fresh supplies are obtained at frequent intervals. A little-used reagent which may have been in stock for some months should be subjected to a trial run with the appropriate substance before commencing the actual determination. 

Raw materials. Most luminous organisms can be stored at —70°C or below under aerobic conditions, or with dry ice, without a significant loss of luminescence activity for a period of several months or more, although a trial is always recommended. Even if a substance already extracted is unstable when stored with dry ice (like the luciferase of Cypridina and the luciferins of euphausiids and dinoflag-ellates), the same substance in the organisms before extraction can be safely stored at — 70° C or with dry ice. The material can also be stored with liquid nitrogen for added safety, but the quantity storable in a laboratory setup (e.g., Dewar flask) is limited. 

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