Metallic divided state

Perhaps the earliest discussion of this problem was that given in 1857 by Faraday," whose researches on colloidal gold, silver, and other metals led him to conclude that ... the gold is reduced in exceedingly fine particles which, becoming diffused, produce a beautiful fluid. .. the various preparations of gold, whether ruby, green, violet or blue. .. consist of that substance in a metallic divided state ... 

Because serious fires can occur, great care should be taken in handling magnesium metal, especially in the finely divided state. Water should not be used on burning magnesium or on magnesium fires. 

Because transition metals even in a finely-divided state do not readily combine with CO, various metal salts have been used to synthesize metal carbonyls. Metal salts almost always contain the metal in a higher oxidation state than the resulting carbonyl complex. Therefore, most metal carbonyls result from the reduction of the metal in the starting material. Such a process has been referred to as reductive carbonylation. Although detailed mechanistic studies ate lacking, the process probably proceeds through stepwise reduction of the metal with simultaneous coordination of CO (90). 

In the presence of oxygen, aqueous sodium cyanide dissolves most metals in the finely divided state, with the exception of lead and platinum. This is the basis of the MacArthur process for the extraction of gold and silver from their ores that, in the case of gold, may be represented as follows ... 

Direct Metal Reaction. The DMR process is carried out over a catalyst with fatty acids ia a melted state or dissolved ia hydrocarbons. The acid reacts directiy with the metal, suppHed ia a finely divided state, produciag the metal soap and ia some cases hydrogen. Catalysts iaclude water, aUphatic alcohols, and low molecular-weight organic acids. 

Tin finds widespread use beeause of its resistanee to eorrosion, or as foil or to provide proteetive eoats/plates for other metals. Properties of lead whieh make industrial applieation attraetive surround its soft, plastie nature permitting it to be rolled into sheets or extruded through dies. In the finely-divided state lead powder is pyrophorie in bulk form the rapidly-formed proteetive oxide layer inhibits further reaetion. It dissolves slowly in mineral aeids. Industrial uses inelude roofing material, piping, and vessel linings, e.g. for aeid storage. 

Some materials sueh as oil-impregnated eotton and iron pyrites are prone to spontaneous eombustion, whilst seleeted materials sueh as metal alkyls and metals in a finely divided state burn on immediate eontaet with water or air. These are termed pyrophorie . Examples and preeautions for their eontrol are deseribed in Chapter 6. 

Certain metals/alloys - the alkali metals (lidiium, potassium, sodium) and even some metals/ alloys which undergo slow oxidation or are rendered passive in bulk form but which, in the finely divided state, inflame immediately when exposed to oxygen (e.g. aluminium, magnesium, zirconium). 

Most of the salts described below can be prepd by interaction of PA with the corresponding inorganic salt or base. Inasmuch as PA is only sparingly sol in w, it is advisable to dissolve the metallic salt (or base) in w, and then introduce the PA in a finely divided state. For 4 example,the Ksalt (see below) can be prepd by saturating a hot K carbonate soln with powdered PA 2C6H2(N02)30H+K2C03 2C6 H2(NO 2) 30K+H20+C02. When the liq is cooled, pale yel acicularcrysts separate out... 

Zirconium. While almost any metal in the finely divided state exhibits pyrophoric properties, a few metals when abraded emit a shower of sparks of sufficient temp to ignite hydrocarbon vapors. Cerium is the best known metal of this kind for commercial purposes, such as gas lighter flints. For military purposes Zr is the most used. It has found applications in HE and armor-piercing incendiary ammo, the lining of shaped-charge rounds, and in incendiary cluster bombs (Ref 7, pp 100-01)... 

Nearly all unsaturated compounds react rapidly with hydrogen at low temperatures and pressures in presence of finely divided metal catalysts such as nickel, platinum and palladium. For maximum catalytic effect, the metal is taken in the finely divided state. This is achieved for platinum and palladium by reducing the metal oxide with hydrogen. A specially active form of nickel is the Raney nickel . 

Most reactions of bromine are highly exothermic which can cause incandescence or sudden increase in pressure and rupture of reaction flasks. There are a number of cases of explosions documented in the literature. (NFPA. 1986. Fire Protection Guide on Hazardous Materials, 9th ed. Quincy, MA National Fire Protection Association) Reactions of liquid bromine with most metals (or any metal in finely divided state), metal hydrides, carbonyls and nitrides can be explosive. Many oxides and halides of nonmetals, such as nitrogen triiodide or phosphorus trioxide, react explosively or burst into flame in contact with liquid bromine. 

Iodine vapors combine with metals forming their iodides. The rates of such reactions vary with metals, their states, and temperatures. The reaction is very rapid when the metal is in finely divided state, and slow when the metal is in massive form. 

Reactions with metals in a finely-divided state or at elevated temperatures, produce metal chlorides and metal iodides ... 

Iron reacts with nonmetals forming their binary compounds. It combines readily with halogens. Reaction is vigorous with chlorine at moderate temperature. With oxygen, it readily forms iron oxides at moderate temperatures. In a finely divided state, the metal is pyrophoric. Iron combines partially with nitrogen only at elevated temperatures. It reacts with carbon, sulfur, phosphorus, arsenic, and silicon at elevated temperatures in the absence of air, forming their binary compounds. 

The reaction is violent when hthium metal is in finely divided state. Lithium reacts violently with dilute acids, liberating hydrogen ... 

When the metal is in a finely divided state or a thin foil, both the reactions above are rapid. 

At ordinary temperatures, bulk nickel in compact form has no perceptible reactivity with air or water. However, in finely-divided state, the metal reacts... 

The compd is ignitable with an open flame. The hydride is much safer to handle and ship in a dry, finely divided state than the metal itself. This greater inertness also shows itself in a lower ignition sensitivity and a slower burning rate than powdered Zr (Ref 7)... 

The arsenides of the heavy metals are usually black and readily oxidisable. On exposure to moist air they are converted into the metal and arsenious acid. In the dry finely divided state they may ignite spontaneously at ordinary temperatures. At higher temperatures, in absence of air, the arsenides of the noble metals lose nearly all their arsenic, while other heavy metal arsenides form lower arsenides. The arsenides are more stable than the corresponding phosphides. 

Bismuth stands low in the electromotive series, and its salts are easily reduced to the metal which in the finely divided state is intensely black. 

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