Mixture unstable

Reactivity This classification includes substances that react with water violently to produce gases or explosive mixtures unstable or explosive substances and substances that contain cyanide or sulfide or generate toxic gases when exposed to a pH in the range of 2 to 12.5. 

In a general manner, diesel engines, jet engines, and domestic or industrial burners operate with lean mixtures and their performance is relatively insensitive to the equivalence ratio. On the other hand, gasoline engines require a fuel-air ratio close to the stoichiometric. Indeed, a too-rich mixture leads to an excessive exhaust pollution from CO emissions and unburned hydrocarbons whereas a too-lean mixture produces unstable combustion (reduced driveability and misfiring). 

Several parameters come into the relation between density and equivalence ratio. Generally, the variations act in the following sense a too-dense motor fuel results in too lean a mixture causing a potential unstable operation a motor fuel that is too light causes a rich mixture that generates greater pollution from unburned material. These problems are usually minimized by the widespread use of closed loop fuel-air ratio control systems installed on new vehicles with catalytic converters. 

Unstable displacement is clearly less preferable, since a mixture of oil and water is produced much earlier than in the stable displacement situation, and some oil may be left unrecovered at the abandonment condition which may be dictated by a maximum water cut. 

Figure A3.3.2 A schematic phase diagram for a typical binary mixture showmg stable, unstable and metastable regions according to a van der Waals mean field description. The coexistence curve (outer curve) and the spinodal curve (iimer curve) meet at the (upper) critical pomt. A critical quench corresponds to a sudden decrease in temperature along a constant order parameter (concentration) path passing through the critical point. Other constant order parameter paths ending within tire coexistence curve are called off-critical quenches.

In general, however, the diacetyl derivatives are unstable in the presence of water, undergoing hydrolysis to the mono-acetyl compound, so that when they (or a mixture of mono- and di-acetyl derivatives) are crystallised from an aqueous solvent, e.g., dilute alcohol, only the mono-acetyl derivative is obtained. A further disadvantage of the use of acetic anhydride in the absence of a solvent is that all the impm-ities in the amine are generally present in the reaction product. Heavily substituted amines, t.g., 2 4 6-tribromoaniline, react extremely slowly with acetic anhydride, but in the presence of a few drops of concentrated sulphuric acid as catalyst acetylation occurs rapidly, for example ... 

Boil a mixture of 10 g. (10 ml.) of o-toluidine and 38 g. (35 ml.) of acetic anhydride in a 75 or 100 ml. Claisen flask fitted with a reflux condenser (Fig. Ill, 28, 1, but with trap replaced by a calcium chloride or cotton wool guard tube) for 1 hour. Arrange the flask for distillation under reduced pressure (compare Fig. II, 20, 1) and distil acetic acid and the excess of acetic anhydride pass over first, followed by the diacetyl derivative at 152-153°/20 mm, some mono-acetyl-o-toluidine (1-2 g.) remains in the flask. The yield of diacetyl-o-toluidine is 14-15 g, it is a colourless, somewhat unstable hquid, which slowly sohdifies to yield crystals, m.p. 18°, To prepare the (mono-) acetyl-o-toluidine, warm a mixture of 5 g. 

Dissolve 0 01 mol of the phenohc ether in 10 ml. of warm chloroform, and also (separately) 0 01 mol of picric acid plus 5 per cent, excess (0 -241 g.) in 10 ml. of chloroform. Stir the picric acid solution and pour in the solution of the phenohc ether. Set the mixture aside in a 100 mb beaker and ahow it to crystallise. Recrystahise the picrate from the minimum volume of chloroform. In most cases equahy satisfactory results may be obtained by conducting the preparation in rectified spirit (95 per cent. CjHgOH). The m.p. should be determined immediately after recrystallisation. It must be pointed out, however, that the picrates of aromatic ethers suflFer from the disadvantage of being comparatively unstable and may undergo decomposition during recrystaUisation. 

Natural neon is a mixture of three isotopes. Six other unstable isotopes are known. 

Natural vanadium is a mixture of two isotopes, 50V (0.24%) and 51V (99.76%). 50V is slightly radioactive, having a half-life of > 3.9 x 10i7 years. Nine other unstable isotopes are recognized. 

Natural rhenium is a mixture of two stable isotopes. Twenty six other unstable isotopes are recognized. 

Twenty one isotopes of samarium exist. Natural samarium is a mixture of several isotopes, three of which are unstable with long half-lives. 

A mixture of 0.40 mol of propargyl chloride and 150ml of dry diethyl ether was cooled at -90°C (liquid nitrogen bath) and a solution of 0.40 mol of ethyl-lithium (note 1) in about 350 ml of diethyl ether (see Exp. 1) was added with vigorous stirring and occasional cooling (note 2). The temperature of the reaction mixture was kept between -70 and -90°C. The formation of the lithium derivative proceeded almost instantaneously, so that the solution obtained could be used directly after the addition of the ethyl 1ithium, which was carried out in 15-20 min. This lithium acetylide solution is very unstable and must be kept below -60°C. 

The upward flow of gas and Hquid in a pipe is subject to an interesting and potentially important instabiHty. As gas flow increases, Hquid holdup decreases and frictional losses rise. At low gas velocity the decrease in Hquid holdup and gravity head more than compensates for the increase in frictional losses. Thus an increase in gas velocity is accompanied by a decrease in pressure drop along the pipe, a potentially unstable situation if the flows of gas and Hquid are sensitive to the pressure drop in the pipe. Such a situation can arise in a thermosyphon reboiler, which depends on the difference in density between the Hquid and a Hquid—vapor mixture to produce circulation. The instabiHty is manifested as cycHc surging of the Hquid flow entering the boiler and of the vapor flow leaving it. 

Xenates and Perxenates. Alkali metal xenates of composition MHXe04-1.5H20, where M is sodium, potassium, mbidium, or cesium, have been prepared by free2e-dryiQg mixtures of xenon trioxide and the corresponding metal hydroxides ia 1 1 molar ratios. The xenates are unstable, explosive solids. 

Chlorine Addition. Chlorine addition and some chlorine substitution occurs at normal or slightly elevated temperatures in the absence of catalysts. The chlorination of molten naphthalene under such conditions yields a mixture of naphthalene tetrachlorides, a monochloronaphthalene tetrachloride, and a dichloronaphthalene tetrachloride, as well as mono- and dichloronaphthalenes (35). Sunlight or uv radiation initiates the addition reaction of chlorine and naphthalene resulting in the production of the di- and tetrachlorides (36). These addition products are relatively unstable and, at ca 40—50°C, they decompose to form the mono- and dichloronaphthalenes. 

Very unstable modifications, like the reddish, chlorine-free a-copper phthalocyanine, can be stabilized with amides or salts of copper phthalocyanine sulfonic acids (59—63). Mixture with other metal phthalocyanines, eg, tin, vanadium, aluminum, or magnesium, also inhibits crystallization change and poor performance in binders and prints (flocculation) due to the hydrophobic character of unsubstituted phthalocyanines. 

Primary aromatic amines react with aldehydes to form Schiff bases. Schiff bases formed from the reaction of lower aUphatic aldehydes, such as formaldehyde and acetaldehyde, with primary aromatic amines are often unstable and polymerize readily. Aniline reacts with formaldehyde in aqueous acid solutions to yield mixtures of a crystalline trimer of the Schiff base, methylenedianilines, and polymers. Reaction of aniline hydrochloride and formaldehyde also yields polymeric products and under certain conditions, the predominant product is 4,4 -methylenedianiline [101 -77-9] (26), an important intermediate for 4,4 -methylenebis(phenyhsocyanate) [101-68-8], or MDI (see Amines, aromatic amines, l thylenedianiline). 

Paraformaldehyde [30525-89-4] is a mixture of polyoxymethylene glycols, H0(CH20) H, with n from 8 to as much as 100. It is commercially available as a powder (95%) and as flake (91%). The remainder is a mixture of water and methanol. Paraformaldehyde is an unstable polymer that easily regenerates formaldehyde in solution. Under alkaline conditions, the chains depolymerize from the ends, whereas in acid solution the chains are randomly cleaved (17). Paraformaldehyde is often used when the presence of a large amount of water should be avoided as in the preparation of alkylated amino resins for coatings. Formaldehyde may also exist in the form of the cycHc trimer trioxane [110-88-3]. This is a fairly stable compound that does not easily release formaldehyde, hence it is not used as a source of formaldehyde for making amino resins. 

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