Chromium VI Oxide Chromic Acid

Chromium(VI) oxide ( chromic acid ) from sodium chromate and sulfuric acid at ca. 200° 

By Reaction of Sodium Dichromate with Sulfuric Acid 

This reaction can be carried out both in concentrated aqueous solution and, preferably continuously, in the molten state. In the latter case, sodium dichromate is mixed with sulfuric acid in a twin screw and the mixture fed into an externally heated rotary tube furnace. The water first evaporates, then the. sodium hydrogen sulfate melts (at I70°C) and finally the chromium(VI) oxide is formed (I98°C). Temperature control is critical, since chromium(Vl) oxide decomposes at temperatures slightly above this temperature. The reacted mixture then separates in a settling tank. Chromium(Vl) oxide is taken off from below and liquid sodium hydrogen sulfate is taken off from above, both being converted into solid material by cooling rollers. 

Since the last edition of this book an electrochemical process for manufacturing chromic acid has been realized industrially. This is carried out in an electrolysis cell with two chambers separated by a cation-exchanger membrane. The anode side is filled with aqueous sodium dichromate solution, the cathode side with sodium hydroxide. Upon applying direct current oxygen is produced at the anode and hydrogen is produced at the cathode. H+-ions are formed on the anode side and OH -ions are formed on the cathode side. 

Since hydrogen ions can diffuse through the membrane in addition to the sodium ions, the efficiency of the above-described process becomes less favorable with increasing chromic acid concentration and thereby increasing hydrogen ion concentration. Practically achievable are solutions on the anode side in which slightly more than half of the chromium(VI) is present as chromic acid. This chromic acid can be recovered by crystallization. The mother liquor left is returned to the electrolysis process. 

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Treatment with concentrated sulfuric acid completes the dehydration process and red chromium(VI) oxide ( chromic acid") precipitates ... 

Less than 15% of the ore is transformed into chromium compounds, principally chromates, dichromates, chromium(VI) oxide, chromium(III) oxide, and so on. Alkaline oxidative roasting of chromite in rotary kilns yields sodium chromate (see equation 1), which is leached out with water and typically converted into sodium dichromate with sulfimc acid (equation 2) or carbon dioxide (equations). Fiuther treatment of sodium dichromate with sulfuric acid yields chromium(VI) oxide ( chromic acid ), while its reduction (with carbon, sulfur, or anuuo-nium salts) produces chromium(III) oxide. Finally, basic chromium(III) salts, for example Cr(0H)S04, which are used as tanning agents for animal hides, also result from reduction of sodium dichromate. Heterogeneous chromium catalysts are used for the polymerization of ethylene. 

Electrochemical Reduction of Chromium(VI) Oxide ( Chromic acid )... 

Chromium trioxide [chromium(VI) oxide, chromic add, or chromic anhydride], Cr03 (dark red crystals, mp 195 °C), is one of the most powerful and universal oxidants. It is applied in solutions in acetic acid, dilute sulfuric acid, a mixture of acetic acid and dilute sulfuric acid, dilute sulfuric acid and acetone (Jones reagent), acetic anhydride and acetic acid (Fieser reagent) [535], water, water and ether [536,537,535], dichloromethane [539],... 

Lead Chromnte(VI) Oxide. Chromic acid lead-(2 + ) salt (1 2) basic lead chromate red lead chromate chrome red chromium lead oxide Persian red Austrian cinnahar CrPb2Os mol wt 546.40. C 9.52%, Ph 75.84%, O 14.64%, PBCrOj PbO See Colour Index vol. 4, (3rd ed., 1971) p 4677. 

Maintenance of chromium plating baths causes ecological problems that can be solved by electrochemical methods, especially by using membrane electrolysers (i.e. electrodialysis) [39-41]. In these processes, chromium(III) is oxidized at the anode, usually made of lead, to chromium(VI), whereby chromic acid is regenerated. The chromic acid consumed in the plating process is then replaced by adding CrO in the solid form. 

Chromimn Irioxide (Chromic anhydride chromic acid chromium(VI) oxide chromic trioxide chromium oxide)... 

Chromium(VI) oxide is very soluble in water initially, chromic acid , H2Cr04, may be formed, but this has not been isolated. If it dissociates thus ... 

ChromsMure, /. chromic acid, -anhydrid, n. chromic anhydride, chromium(VI) oxide, emisch, n. chromic acid mixture. -Ibsung. /. chromic acid solution, -salz, n. salt of chromic add, chromate. 

Chromium(VI) oxide, C1O3, is the anhydride of chromic acid, H2Cr04, the parent acid of the chromates. In this highly oxidized state, chromium is electron poor and the oxygen atoms attached to it are less likely to share electrons with a proton. 

Chromic acid is usually prepared by adding chromium(VI) oxide (CrCb) or sodium dichromate (Na2Cr207) to aqueous sulfuric acid. 

Chromium(VI) oxide, Cr03, is the acid anhydride of chromic acid, H2Cr04, and dichromic acid, H2Cr207. Recall that there is no change in oxidation state when an acid anhydride is converted to the corresponding acid and so the oxidation state of Cr is +6 in both acids. 

Ethyl sulfate Flammable liquids Fluorine Formamide Freon 113 Glycerol Oxidizing materials, water Ammonium nitrate, chromic acid, the halogens, hydrogen peroxide, nitric acid Isolate from everything only lead and nickel resist prolonged attack Iodine, pyridine, sulfur trioxide Aluminum, barium, lithium, samarium, NaK alloy, titanium Acetic anhydride, hypochlorites, chromium(VI) oxide, perchlorates, alkali peroxides, sodium hydride... 

Trivalent chromium compounds, with the exception of acetate, hexahydrate of chloride, and nitrate salts, are generally insoluble in water (Table 3-2). Some hexavalent compounds, such as chromium(VI) oxide (or chromic acid), and the ammonium and alkali metal salts (e.g., sodium and potassium) of chromic acid are readily soluble in water. The alkaline metal salts (e.g., calcium, strontium) of chromic acid are less soluble in water. The zinc and lead salts of chromic acid are practically insoluble in cold water (Table 3-2). The hexavalent chromium compounds are reduced to the trivalent form in the presence of oxidizable organic matter. However, in natural waters where there is a low concentration of reducing materials, hexavalent chromium compounds are more stable (EPA 1984a). 

SYNS ANHYDRIDE CHROMIQUE (FRENCH) ANIDRIDE CROMICA (ITALIAN) CHROME (TRIOXYDE de) (FRENCH) CHROMIC ACID CHROMIC(VI) ACID CHROMIC ACID, soKd (NA 1463) (DOT) CHROMIC ACID, solution (UN 1755) pOT) CHROMIC ANHYDRIDE CHROMIC TRIOXIDE CHROMIUxM OXIDE CHROMIUM(VI) OXIDE CHROMIUM TRIOXIDE CHROMIUM(6+) TRIOXIDE CHROMIUM TRIOXIDE, anhydrous pOT) CHROMIUM TRIOXIDE, anhydrous pN 1463) pOp CHROMO (TRIOSSIDO di) (ITALIAN) CHROMS-AEUREANHYDRID (GERMAN) CHROMTRIOXID (GERMAN) CHROOMTRIOXYDE pUTCH) CHROOMZUURANHYDRIDE pUTCH) MONO-CHROAQUM OXIDE D MONOCHROMIUM TRIOXIDE O PURATRONIC CHROMIUM TRIOXIDE... 

Chromium(VI) oxide nitrate is a dark red liquid which boils at 63 to 65° at 0.7 mm. Hg pressure. It is soluble in carbon tetrachloride. In water, it reacts immediately to form chromic and nitric acids. It is a more powerful oxidizing agent than vanadium (V) oxide nitrate, and care must be taken to avoid contact with hydrocarbons. It is corrosive to most metallic surfaces, except aluminum, and reacts in the same manner as vanadium (V) oxide nitrate does toward paper, wood, and rubber. It cannot be stored for so long a time as vanadium (V) oxide nitrate but is relatively stable in a sealed ampul in the absence of light and moisture. It can be purified by distillation in vacuum over lead(IV) oxide. 

Walker, B. H. Effect of manganese on the chromic acid oxidation of secondary-tertiaryvicinal glycols. J. Org. Chem. 1967, 32, 1098-1103. Collins, J. C., Hess, W. W., Frank, F. J. Dipyridine-chromium(VI) oxide oxidation of alcohols in dichloromethane. Tetrahedron Lett. 1968, 3363-3366. 

Aqueous chromic acid solutions are used, for instance, as pickling and chromium plating baths in metal and plastic processing. They can be produced by dissolving chromium(VI) oxide in water or from sodium dichromate(VI) and sulfuric acid. 

SYNONYMS Synonyms of chromates vary depending upon the specific compound. Synonyms of chromic acid (CrOs) are as follows chromic anhydride, chromic oxide, chromium (VI) oxide (1 3) chromium trioxide, monochromium trioxide. 

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