Alkali Dichromates

The alkali dichromates have a much higher chromium content than monochromates and are also easier to produce pure. They are therefore almost exclusively used in practice. 

Sodium (mono)chromate is converted to dichromate with  

Sodium dichromate dihydrate is obtained from the (mono)chromate by reaction with sulfuric acid or carbon dioxide. 

The reaction between sodium chromate and sulfuric acid proceeds as follows  

The discontinuous or continuous addition of sulfuric acid to sodium chromate solution up to a pH of 3 is technically simple. The resulting solution of sodium dichromate dihydrate is then evaporated to 70% by weight, whereupon almost all the sodium sulfate precipitates out and is separated off. The dichromate solution obtained is in part utilized as such. Further evaporation to ca. 1600 g sodium dichromate/L and cooling to 30 to 40°C leads to crystallization of the dichromate. These processes are either carried out stepwise or continuously. Separation is carried out in a. skimmer or shear centrifuge. 

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Dichroma.te(VI) Glasses. Alkali dichromate(VI) glasses exist in systems such as Li2Cr20y—Na2Cr20y. The glass-transition temperatures of the dichromates are very low, about 0°C. The addition of chlorides or nitrates aids in glass formation. 

Medium acid baths, pH 4-5 At this acidity a dichromate solution plus sulphate ion as activator is sufficient to deposit chromate films in 30 min or so at room temperature or in a few minutes at boiling point. Unfortunately, a solution of alkali dichromate and alkali sulphate is quite unbuffered, and other substances must be added to give the bath a useful life over the working pH range. Acetates have been used successfully, but salts of aluminium, chromium, manganese and zinc have been more commonly employed. The pH of the solution rises slowly during use until basic chromates or sulphates begin to precipitate. The solution can then be rejuvenated by the addition of chromic or sulphuric acid or acid salts. 

The Dow No. 7 treatment, popular in the USA, also falls within this class. The process differs from other chromate treatments in that the activator, magnesium fluoride, is formed on the metal surface by immersion in 20% hydrofluoric acid solution, the parts then being immersed in a 10-15% alkali dichromate solution with or without sufficient alkaline earth fluoride to saturate it. A slow action occurs on the surface and the fluoride film is replaced by a chromate or mixed chromate/fluoride film. 

Alkali dichromates are used as starting materials for the production of chromi-um(III) oxide pigments. They are available as bulk industrial products in the required purity. High impurity levels have an unfavorable effect on the hue. 

Reduction of Alkali Dichromates. In industrial processes, solid alkali dichromates are reacted with reducing agents such as sulfur or carbon compounds. The reaction is strongly exothermic, and with sulfur proceeds as follows ... 

Environmental Protection. Since alkali dichromates or chromic acid anhydride are used as starting materials for the production of chromium(III) oxides, occupational health requirements for the handling of hexavalent chromium compounds must be observed [3.59]. The sulfur dioxide formed on reduction with excess sulfur must be removed from the flue gases according to national regulations, e.g., by oxidation to... 

The preparation of tri arylmethane dyes proceeds through several stages formation of the colorless leuco base in acid media, conversion to the colorless carbinol base by using an oxidizing agent, e.g., lead dioxide, manganese dioxide, or alkali dichromates, and formation of the dye by treatment with acid. The oxidation of the leuco base can also be accomplished with atmospheric oxygen in the presence of catalysts. 

Chromic acid or alkali dichromate and sulphuric acid is employed for the simpler alcohols polyhydric alcohols are usually oxidised with moderately dilute nitric acid if the acid be too concentrated the molecule may be attacked as a whole (cf. Preparation 184). 

Trihydrate, dark green, odorless crystals with bronze luster or cryst powder. Absorption max 668, 609 tint. One gram dissolves in about 25 ml water, about 65 ml alcohol sol in chloroform. In sol in ether. In aq soln it is decolorized by Zn dust and dil H2SOt, but color is restored on exposure to air and more rapidly upon addn of NH,OH, Forms double salts with many inorganic salts. Incompat Caustic alkali, dichromates, alkali iodides, reducing agents. 

Data are taken from Technical Data Sheet TD I05 (Foote Mineral Company) and Mellor, especially from Supplement U, Part II, p. 1230 lf where eutectics with alkali dichromates of m.p. 202— 2I6 C, potentially interesting pyrotechnic ally, are also found,... 

Other soluble starting compounds may be used besides the rare earth nitrates. Thus, Komissarova et al. (1970a) have prepared ternary compounds of Sc starting from ScClj and alkali dichromate solutions. 

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