Contact with Air

When working with flammable hydrocarbons, if it is not possible to remove the flammable materials themselves, then air/oxygen should be removed from the item that is to be worked on in order to avoid the creation of an explosive mixture or a flammable cloud. 

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Before this treatment, the cassiterite content of the ore is increased by removing impurities such as clay, by washing and by roasting which drives off oxides of arsenic and sulphur. The crude tin obtained is often contaminated with iron and other metals. It is, therefore, remelted on an inclined hearth the easily fusible tin melts away, leaving behind the less fusible impurities. The molten tin is finally stirred to bring it into intimate contact with air. Any remaining metal impurities are thereby oxidised to form a scum tin dross ) on the surface and this can be skimmed off Very pure tin can be obtained by zone refining. 

Substances which decompose (or otherwise change) in contact with air must be recrystallised in an indifferent atmosphere, such as carbon dioxide, nitrogen or hydrogen. The apparatus of Fig. 

Although thiosulfate is one of the few reducing titrants not readily oxidized by contact with air, it is subject to a slow decomposition to bisulfite and elemental sulfur. When used over a period of several weeks, a solution of thiosulfate should be restandardized periodically. Several forms of bacteria are able to metabolize thiosulfate, which also can lead to a change in its concentration. This problem can be minimized by adding a preservative such as Hgl2 to the solution. 

The flash point of furfural is 143°F by Tag Closed Cup. Because of its chemical reactivity, furfural should be kept away from strong acids, alkaHes or strong oxidi2ing chemicals. When furfural is stored for long periods in contact with air, there is a gradual darkening of color, increase in acidity, and formation of a soluble polymer. 

In normal practice, inhibitors such as hydroquinone (HQ) [123-31 -9] or the monomethyl ether of hydroquinone (MEHQ) [150-76-5] are added to acrylic monomers to stabilize them during shipment and storage. Uninhibited acrylic monomers should be used prompdy or stored at 10°C or below for no longer than a few weeks. Improperly iahibited monomers have the potential for violent polymerizations. HQ and MEHQ require the presence of oxygen to be effective inhibitors therefore, these monomers should be stored in contact with air and not under inert atmosphere. Because of the low concentration of inhibitors present in most commercial grades of acrylic monomers (generally less than 100 ppm), removal before use is not normally required. However, procedures for removal of inhibitors are available (67). 

The effective saturation depth,, represents the depth of water under which the total pressure (hydrostatic plus atmospheric) would produce a saturation concentration equal to for water ia contact with air at 100% relative humidity. This can be calculated usiag the above equation, based on a spatial average value of T, measured by a clean water test. For design purposes,, can be estimated from clean water test results on similar systems, and it can range from 5 to 50% of tank Hquid depth. Effective depth values for coarse bubble diffused air, fine bubble diffused air, and low speed surface aerators are 26 to 34%, 21 to 44%, and 5 to 7%, of the Hquid depth, respectively. 

Difluorophosphoric Acid. Difluorophosphoric acid (2) is a mobile, colorless Hquid. It fumes on contact with air, probably owing to HE aerosol formation. The mp of anhydrous difluorophosphoric acid has been reported to be —96.5 1°C (63) and —91.3 1°C (64). The density at 25°C is 1.583 g/mL. It partially decomposes on heating above 80—100°C. An extrapolated normal boiling point is 116°C (63) although it boils at 107—111°C... 

Usually, organoboranes are sensitive to oxygen. Simple trialkylboranes are spontaneously flammable in contact with air. Nevertheless, under carefully controlled conditions the reaction of organoboranes with oxygen can be used for the preparation of alcohols or alkyl hydroperoxides (228,229). Aldehydes are produced by oxidation of primary alkylboranes with pyridinium chi orochrom ate (188). Chromic acid at pH < 3 transforms secondary alkyl and cycloalkylboranes into ketones pyridinium chi orochrom ate can also be used (230,231). A convenient procedure for the direct conversion of terminal alkenes into carboxyUc acids employs hydroboration with dibromoborane—dimethyl sulfide and oxidation of the intermediate alkyldibromoborane with chromium trioxide in 90% aqueous acetic acid (232,233). 

Methylene iodide [75-11-6], CH2I2, also known as diio dome thane, mol wt 267.87, 94.76% I, mp 6.0°C, and bp 181°C, is a very heavy colorless Hquid. It has a density of 3.325 g/mL at 20°C and a refractive index of 1.7538 at 4°C. It darkens in contact with air, moisture, and light. Its solubiHty in water is 1.42 g/100 g H2O at 20°C it is soluble in alcohol, chloroform, ben2ene, and ether. Methylene iodide is prepared by reaction of sodium arsenite and iodoform with sodium hydroxide reaction of iodine, sodium ethoxide, and hydroiodic acid on iodoform the oxidation of iodoacetic acid with potassium persulfate and by reaction of potassium iodide and methylene chloride (124,125). Diiodoform is used for determining the density and refractive index of minerals. It is also used as a starting material in the manufacture of x-ray contrast media and other synthetic pharmaceuticals (qv). 

Alkali Treatment. Caustic washing is the treatment of materials, usually products from petroleum refining, with solutions of caustic soda. The process consists of mixing a water solution of lye (sodium hydroxide or caustic soda) with a petroleum fraction. The treatment is carried out as soon as possible after the petroleum fraction is distilled, since contact with air forms free sulfur, which is corrosive and difficult to remove. The lye reacts either with any hydrogen sulfide present to form sodium sulfide, which is soluble in water, or with mercaptans, foUowed by oxidation, to form the less nocuous disulfides. 

Some phosphoms—hydrogen compounds are pyrophoric, eg, diphosphine [13445-50-6] 2 4 common impurity in phosphine. Such contaminated phosphine usually ignites spontaneously on contact with air. 

The alkyls and aryls, R3AI (in monomer form), are colorless Hquids or low melting soHds easily oxidized and hydrolyzed when exposed to the atmosphere. Triethylalurninum (TEA), one of the most commercially important members of this family of chemicals, is so reactive it bursts into flame on contact with air, ie, it is pyrophoric, and it reacts violently with water. This behavior is typical and special techniques are necessary for the safe handling and use of organoalurninum compounds. 

The Lo-Cat process, Hcensed by US Filter Company, and Dow/Shell s SulFerox process are additional Hquid redox processes. These processes have replaced the vanadium oxidizing agents used in the Stretford process with iron. Organic chelating compounds are used to provide water-soluble organometaHic complexes in the solution. As in the case of Stretford units, the solution is regenerated by contact with air. 

Properties. Thallium is grayish white, heavy, and soft. When freshly cut, it has a metallic luster that quickly dulls to a bluish gray tinge like that of lead. A heavy oxide cmst forms on the metal surface when in contact with air for several days. The metal has a close-packed hexagonal lattice below 230°C, at which point it is transformed to a body-centered cubic lattice. At high pressures, thallium transforms to a face-centered cubic form. The triple point between the three phases is at 110°C and 3000 MPa (30 kbar). The physical properties of thallium are summarized in Table 1. 

Zirconium is a highly active metal which, like aluminum, seems quite passive because of its stable, cohesive, protective oxide film which is always present in air or water. Massive zirconium does not bum in air, but oxidizes rapidly above 600°C in air. Clean zirconium plate ignites spontaneously in oxygen of ca 2 MPa (300 psi) the autoignition pressure drops as the metal thickness decreases. Zirconium powder ignites quite easily. Powder (<44 fim or—325 mesh) prepared in an inert atmosphere by the hydride—dehydride process ignites spontaneously upon contact with air unless its surface has been conditioned, ie, preoxidized by slow addition of air to the inert atmosphere. Heated zirconium is readily oxidized by carbon dioxide, sulfur dioxide, or water vapor. 

Many reactions can be carried out between potassium cyanide and organic compounds with the alkalinity of the KCN acting as a catalyst these reactions are analogous to reactions of sodium cyanide. The reactions of potassium cyanide with sulfur and sulfur compounds are also analogous to those of sodium cyanide. Potassium cyanide is reduced to potassium metal and carbon by heating it out of contact with air in the presence of powdered magnesium. Magnesium is converted to the nitride ... 

In diying solids it is important to distinguish between hygroscopic and nonhygroscopic materials. If a hygroscopic material is maintained in contact with air at constant temperature and humidity until equilibrium is reached, the material will attain a definite moisture content. This moisture is termed the equilibrium moisture content for the specified conditions. Equilibrium moisture may be adsorbed as a surface film or condensed in the fine capillaries of the solid at reduced pressure, and its concentration will vaiy with the temperature and humidity of the surrounding air. However, at low temperatures, e.g., 15 to 50°C, a plot of equilibrium moisture content versus percent relative humidity is essentially independent of temperature. At zero humidity the equilibrium moisture content of all materials is zero. 

B. Aminolhymol.—The crude, wet nitrosothymol so obtained is worked up with a mixture of 900 cc. of 28 per cent ammonia water (sp. g. 0.90) and 1600 cc. of water the brown solution is filtered free of a little resinous matter, and hydrogen sulfide is passed into it. The brown color disappears and a white precipitate of aminothymol forms. The passage of hydrogen sulfide is continued for thirty minutes longer (Note 3), when the base is filtered and washed well with cold water, contact with air being avoided as far as possible (Note 4). 

The acetal (b 82.5°) is removed during fractional distn. Traces of benzene, if present, can be removed as the benzene/MeOH azeotrope by distn in the presence of MeOH. Distn from LiAlHa removes aldehydes, peroxides and water. Dioxane can be dried using Linde type 4X molecular sieves. Other purification procedures include distn from excess C2H5MgBr, refluxing with Pb02 to remove peroxides, fractional crystn by partial freezing and the addition of KI to dioxane acidified with aq HCl. Dioxane should be stored out of contact with air, preferably under N2. 

Film or Skin Formation Some liquids form a skin (like hot milk) with heat. Others form a film (like paints) on contact with air ... 

Contact with air Quench and Drain or Double Seal... 

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