Ammonium dichromate oxidation

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). 

A number of manufacturers around the world are using the decomposition of ammonium dichromate to produce chrome oxide (eq. 5) (78). Generally, an excess of finely ground ammonium sulfate is mixed with sodium dichromate, and the dry mixture is heated to form chrome oxide and sodium sulfate, evolving nitrogen and steam. 

The procedure described is that of Wille and Saffer. Propiolaldehyde has also been prepared by the oxidation of propargyl alcohol using ammonium dichromate or manganese dioxide in 10% sulfuric acid. Propiolaldehyde has also been prepared by warming the dimethyl or diethyl acetal with dilute sulfuric acid. ... 

The Cr2072- ion can act as an oxidizing agent in the solid state as well as in water solution. In particular, it can oxidize the NH ion to molecular nitrogen. When a pile of ammonium dichromate is ignited, a spectacular reaction occurs (Figure 20.12). 

The ammonium dichromate resembles a tiny volcano as it bums, emitting hot gases, sparks, and a voluminous green dust of chromium(III) oxide. 

The +3 state of chromium is best represented by chromium(III) oxide, Cr2Os, which is a green, inert solid used as a green pigment. It can be made in rather spectacular fashion by heating ammonium dichromate. Once started, the reaction... 

Modification of the burning rates, pressure exponents, and temp coefficients of burning rate of the fluorocarbon composites has been accomplished with copper, lead, tin, sodium, ammonium and potassium fluoborates sodium, potassium, lithium, lead, copper and calcium fluorides potassium and ammonium dichromate lead and zinc stearate cesium carbonate potassium and ammonium sulfate copper chromite oxides of magnesium, copper and manganese boron zinc dust and carbon black (Ref 75)... 

They are strong oxidants especially when they are combined with sulphuric acid. Ammonium dichromate or chromate (less used) have an oxygen balance(C3= 0 g% for the first one) that makes them likely to be very unstable thermodynamically. This is confirmed by the labelling code 1 (explosive in the dry state) provided by the regulations and attested by a number of accidents. 

The same goes for almost all oxidants and especially molten ammonium nitrate, sodium dichromate (temperatures reach 1100 C), ammonium dichromate in concentrated solution, sodium peroxide (at 240 C, if there is friction) and dinitrogen tetroxide. In all these cases iron combusts in contact with these oxidants. 

Methanol is sometimes used to prepare dichromium trioxide by the reduction of chromium oxide (Vi). Since these attempts lead to spontaneous ignition and detonation, it is preferable to use the thermal decomposition of ammonium dichromate, provided it is carried out with caution (see p.200). 

Hydroxylamine is a powerful reducant, particularly when anhydrous, and if exposed to air on a fibrous extended surface (filter paper) it rapidly heats by aerobic oxidation. It explodes in contact with air above 70°C [1]. Barium peroxide will ignite aqueous hydroxylamine, while the solid ignites in dry contact with barium oxide, barium peroxide, lead dioxide and potassium permanganate, but with chlorates, bromates and perchlorates only when moistened with sulfuric acid. Contact of the anhydrous base with potassium dichromate or sodium dichromate is violently explosive, but less so with ammonium dichromate or chromium trioxide. Ignition occurs in gaseous chlorine, and vigorous oxidation occurs with hypochlorites. 

Cr-ZSM-5 catalysts prepared by solid-state reaction from different chromium precursors (acetate, chloride, nitrate, sulphate and ammonium dichromate) were studied in the selective ammoxidation of ethylene to acetonitrile. Cr-ZSM-5 catalysts were characterized by chemical analysis, X-ray powder diffraction, FTIR (1500-400 cm 1), N2 physisorption (BET), 27A1 MAS NMR, UV-Visible spectroscopy, NH3-TPD and H2-TPR. For all samples, UV-Visible spectroscopy and H2-TPR results confirmed that both Cr(VI) ions and Cr(III) oxide coexist. TPD of ammonia showed that from the chromium incorporation, it results strong Lewis acid sites formation at the detriment of the initial Bronsted acid sites. The catalyst issued from chromium chloride showed higher activity and selectivity toward acetonitrile. This activity can be assigned to the nature of chromium species formed using this precursor. In general, C r6+ species seem to play a key role in the ammoxidation reaction but Cr203 oxide enhances the deep oxidation. 

Sodium chromate is produced as part of the process for obtaining Cr203, and it is probably the most important chromium compound. Although there are other oxides of chromium, Cr203 is very important because of its catalytic properties. One way to obtain this oxide is by the decomposition of ammonium dichromate,... 

Redox compounds that contain both reducing and oxidizing groups in their molecules are, for example, tin (II) perchlorate, peroxyformic acid, ammonium dichromate, and the double salt potassium cyanide with potassium nitrite [43]. 

An effective oxidation of a,p-unsaturated primary alcohols and benzylic alcohols, but not saturated alcohols, to the corresponding aldehydes has been achieved using polymer-supported quaternary ammonium dichromate [14],... 

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. 

Spill a small amount of triturated ammonium dichromate into a small porcelain crucible and put the latter into a porcelain triangle on a ring of a stand. Spread out a sheet of paper underneath. Heat the crucible with the flame of a burner. Remove the latter as soon as the ammonium dichromate begins to decompose. Gather the chro-mium(III) oxide formed. Write the equation of the reaction. 

Reduction of Ammonium Dichromate. Chromium(III) oxide can be obtained by thermal decomposition of ammonium dichromate. Above ca. 200 °C, a highly voluminous product is formed with elimination of nitrogen [3.48]. The pigment is obtained after addition of alkali salts (e.g., sodium sulfate) and subsequent calcination [3.49]. 

When heated, orange ammonium dichromate undergoes a chemical change to ammonia, water vapor, and chromium (III) oxide. A return to the original temperature does not restore the orange color because the ammonium dichromate is no longer there. 

Bichromate Ammonium dichromate [CAS 7789-09-5] (NH4>2Cr207, red solid, soluble, upon heating evolves nitrogen gas and leaves a green insoluble residue of clirumic oxide. 

The oxidative potency of dichromates and chlorochromates decreases under less acidic conditions. This is so, for example, when a less acidic ammonium salt is included as counter-ion of a dichromate or chlorochromate anion. Thus, a number of ammonium dichromates and chlorochromates possessing a milder oxidative potency has been described with the specific purpose of allowing very selective oxidations of unsaturated alcohols in the presence of saturated ones. These selective dichromates and chlorochromates include bis(benzyltriethylammonium)dichromate,135 tetramethylethy-lenediammonium dichromate (TMEDADC),136 imidazolium dichromate (IDC),137 N, A -dimeth y I a m i n o py r id i n i u m chlorochromate (DMAPCC),138 l-(benzoylamino)-3-methylimidazolium chlorochromate (BAMICC)139 and butyltriphenylphosphonium chlorochromate (BTPPCC).140... 

A reaction is called exothermic when it is possible to remove the energy of activation after some time because the reaction proceeds with the help of the energy produced in the reaction. A good example of this is the so-called volcano test some ammonium dichromate is shaped into a small volcano crater. A bright flame is held to the crater until the reaction starts the volcano produces a green powder, chromium(III) oxide ... 

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