Dichromate salts

Dichromated Resists. The first compositions widely used as photoresists combine a photosensitive dichromate salt (usually ammonium dichromate) with a water-soluble polymer of biologic origin such as gelatin, egg albumin (proteins), or gum arabic (a starch). Later, synthetic polymers such as poly(vinyl alcohol) also were used (11,12). Irradiation with uv light (X in the range of 360—380 nm using, for example, a carbon arc lamp) leads to photoinitiated oxidation of the polymer and reduction of dichromate to Ct(III). The photoinduced chemistry renders exposed areas insoluble in aqueous developing solutions. The photochemical mechanism of dichromate sensitization of PVA (summarized in Fig. 3) has been studied in detail (13). 

The most widely employed transition metal oxidants for alcohols are based on Cr(VI). The specific reagents are generally prepared from chromic trioxide, Cr03, or a dichromate salt, [Cr207]2-. The form of Cr(VI) in aqueous solution depends upon concentration and pH the pKx and pK2 of H2Cr04 are 0.74 and 6.49, respectively. In dilute solution, the monomeric acid chromate ion [HCr03] is the main species present as concentration increases, the dichromate ion dominates. 

See entry dichromate salts of nitrogenous bases See other... 

See Other DICHROMATE SALTS OF NITROGENOUS BASES, OXIDANTS... 

Aromatic and aliphatic thiols are oxidized by quaternary ammonium dichromate salts to disulphides (8, 13]. Yields are very good, even with the oxidative dimerization of diethyl phosphorothiolothionate to the corresponding disulphide (79%). 

Arenes which are susceptible to oxidation are converted into the quinones under relatively mild conditions using a quaternary ammonium dichromate salt in sulphuric acid. The molarity of the acid is crucial for the success of the reaction. In dilute acid, virtually no oxidation takes place. Optimum conditions appear to require 10 M sulphuric acid and an excess of potassium dichromate. 

Many of the salts of nitrogenous bases (particularly of high nitrogen content) with oxoacids are imstable or explosive. There are separate group entries for AMINIUM lODATES AND PERIODATES, AMINIUM PERCHLORATES DIAZONIUM PERCHLORATES, DICHROMATE SALTS OE NITROGENOUS BASES I-(I,3-DISELENONYLIDENE)PIPERIDINIUM PERCHLORATES HYDRAZINIUM SALTS, HYDROXYLAMINIUM SALTS... 

Addition of stoichiometric amounts of caustic soda or caustic potash yields orange dichromate salt which can be crystallized from the solution. 

Poly(vinyl alcohol) is used in films, coatings, fibers, and as a viscosity modifier of a variety of aqueous systems such as certain cosmetics. Its films, formulated with dichromate salts, may be cross-linked by exposure to ultraviolet light. This property has found application in photoengraving and related fields [6],... 

For example, decammino-dichromic salts, of formula [(NH3)5Cr.OH. Cr (NH3)5]R5. The rhodo-chromic salts belong to this class of polynuclear compound. These pass by the action of ammonia into basic salts of formula [(NH3)6Cr.OH.Cr(NH3)5]OH, which latter, on loss of water, pass... 

Tetraethylenediamino-diol-dichromic Salts, [Cr2en4(OH)2]R4. —When cis-hydroxo-aquo-diethylenediamino-chromic salts or the hydroxo-bisaquo-salts are heated they lose water, and form diol-chromium salts according to the equation... 

Octammino-diol-dichromic Salts, [Cr2(NH3)8(OH)2]R4.—These salts correspond to the ethylenediamine derivatives described above, and are prepared in the same manner. Dubsky, in an attempt to ascertain the constitution of the diol salts, prepared a number of salts belonging to the series. Octammino-diol-dichromic sulphate is formed when hydroxo-aquo-tetrammino-chromic sulphate is heated with excess... 

Heating causes orderly decomposition, but under confinement, a violent explosion. See other DICHROMATE SALTS OF NITROGENOUS BASES... 

It is known that preparation of the oxidant salt under anhydrous conditions is explosion-prone [1], but during repetition of a supposedly safe preparative method [2], recommended for large-scale use [3], ignition and a violent explosion occurred. Use of more water to dissolve completely the chromium trioxide, and a reaction temperature below 25°C, are recommended [4], During a study by TGA of the thermal degradation of the salt, too-rapid heating of the samples led to explosions with fire [5]. See other DICHROMATE SALTS OF NITROGENOUS BASES, OXIDANTS... 

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