Tetrachromates

Zinc yellow is made by a variety of processes, all based on the reaction of 2inc compounds, chromates, and potassium salts in aqueous solution. If products free of chloride and especially sulfate are desired, they are excluded from the system. In one process, for example, 2inc oxide is swollen with potassium hydroxide and the chromates are added as a solution of potassium tetrachromate [12422-53-6] (194). 

Tetrachromate electrolytes The alkaline tetrachromate baths are used to a small extent chiefly for the direct chromium plating of zinc die-castings, brass or aluminium, since the solutions do not attack these metals . The original bath was developed by Bornhauser (German Pat. 608 757) and contained 300 g/1 of chromic acid, 60g/l of sodium hydroxide, 0-6-0-8g/l of sulphuric acid and 1 ml/l of alcohol. 

The essential constituent of the bath is sodium tetrachromate, Na2Cr40,3, which is, however, only stable at temperatures below about 25°C. This temperature should therefore not be exceeded in the operation of the bath. Current densities of 75-150 A/dm are used. The current efficiency of the bath is high (30-35%) so that the metal is deposited at the rate of about 1 tm/min. The deposits are normally matt in appearance, but are comparatively soft and readily polished. 

A proprietary tetrachromate bath has been used in Germany under the name of the D process. By the use of additions of magnesium oxide and sodium tungstate it is claimed that the current efficiency of the bath can be raised to as high as 35-40%. Other additives such as indium sulphate, sodium selenate or sodium hexavanadate enable bright deposits to be obtained. 

In Rb2Cr30io the trichromate ions consist of three Cr04 tetrahedra sharing corners and in RbCr40i3 the tetrachromate ions consist of four such cornersharing tetrahedra. In these anions the terminal and bridging Cr—O bonds... 

Ammonium nitrate, 4522 Ammonium permanganate, 4518 Ammonium tetrachromate(2—), 4253 Ammonium tetranitroplatinate(II), 4578 Ammonium trichromate (2—), 4252 Anilinium nitrate, 2379... 

These compounds are represented by salts containing four chromium atoms in the molecule for instance, the compound produced by replacing ethylenediamine in triethylenediamino-chromic salts by three molecules of dihydroxo-diethylenediamino-chromie salt, giving rise to the kexaethylenediamino-hexol-tetrachromic salts, of formula... 

Hexaethylenediamino-hexol-tetrachromic Sulphate, [Cr4(OH)6en6](SO4)3.10H2O, is prepared from partially dehydrated chromic alum. The bluish-grey powder obtained is heated with ethylene-diamine monohydrate on a water-bath till a red-coloured mass is produced, which consists of a mixture of potassium sulphate, luteo-chromic sulphate, and the sulphate of the hexol compound, and from the mixture the latter compound is obtained by dissolving out the more soluble salts with water. The crude, difficultly soluble sulphate is purified by dissolving in dilute sulphuric acid and reprecipitating with excess of ammonia. It crystallises in small needles which are almost insoluble in water and soluble in dilute acids. 

Dichromates. See under Chromates, Dichro-mates, Trichromates and Tetrachromates in... 

Chromates, dichromates, trichromates and tetrachromates 3 C274—C275... 

TETRACHROMATES are derivs of hypothetical tetrachromic acid, H2Cr4013. They are strong oxidizing agents and for this reason are briefly described here Ref Mellor 11(1931), 351-53... 

List of Tetrachromates Ammonium Tetrachromate, (NH4)2Cr40)3, mw 452.12 brn-red hygroscopic crysts, less stable than Amm trichromate mp - blackens at 160°, melts at 170° and decomp spontaneously giving off nitrous fumes. Can be prepd by evapg, in cold over coned sulfuric acid, a soln of Amm trichromate in nitric acid, d 1.39(Refs 1 2). 

Accdg to Mr. R. Baumann of PicArsn, these compds are not catalysts, but rather "modifiers , or additives . They are incorporated into solid proplnts in order to modify or affect their combustion characteristics The compds which affect burning rate include Prussian blues, Aram dichromate, trichromate or tetrachromate, etc). Some compds may be employed to reduce the variability of burning rate with combustion pressure and temperature (Eg Pb stearate or salicylate). Other compds may be used to reduce the tendency toward oscillatory combustion(Eg A1 powder in small quantity), or to affect flash characteristics of exhaust gases [Eg KaS04, Ba(N03)2,... 

AN 75 catalyst 6 cellulose acetate -glycoldiglycolate mixt 19%. The proplnt becomes even more effective when 0.5 5.0% of a synergist catalyst is added. The synergists include phenothiazine,linseed oil sucrose or Mg stearate(or ricinoleate, or oleate) in the presence of hydrocarbons] 10)R.W. Lawrence USP 2978305(1961) CA 55, 25260(1961) (Combustion catalysts for comp proplnts include Amm trichromate or tetrachromate or riieirmixts, added in quantity 0.1-4% of total proplnt) 11)W.G. Proell, USP 2987388(1961)... 

It is known that trichromates [Cr3OI0]2- and tetrachromates [Cr4Oi3]2- may crystallize from strongly acid solutions as their alkali metal salts. Tetrachromates are believed to be major constituents of a plating bath for which the following composition is typical ... 

Specialty Chromium-Plating Baths. Chromic acid baths using sodium chromate and sodium hydroxide to form a tetrachromate (92) have had limited use. Porous chromium is used in lubricated wear applications, and is made by chemically etching regular chromium plate, sometimes with light grinding after the etch. Black chromium is used on solar collector surfaces (see PHOTOVOLTAIC CELLS Solarenergy). Baths are sulfate-free, and include fluosilicic acid or acetic acid (91). 

Ammonium nitrate, 4517 Ammonium permanganate, 4513 Ammonium tetrachromate(2—), 4247 Ammonium tetranitroplatinate(II), 4573 Ammonium trichromate(2—), 4246 Anilinium nitrate, 2372... 

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