Chromic acid electrolysis

Chromic Acid Electrolysis. Alternatively, as shown in Figure 1, chromium metal may be produced electrolyticaUy or pyrometaUurgicaUy from chromic acid, CrO, obtained from sodium dichromate by any of several processes. Small amounts of an ionic catalyst, specifically sulfate, chloride, or fluoride, are essential to the electrolytic production of chromium. Fluoride and complex fluoride catalyzed baths have become especially important in recent years. The cell conditions for the chromic acid process are given in Table 7. 

Finally, chromium metal can be obtained by electrowinning from chromium-rich liquors by two electrochemical processes chromium-alum electrolysis and chromic acid electrolysis. In chromium-alum electrolysis, the chromium-rich liquor is obtained by leaching high-carbon ferrochrome with recycled spent catholyte containing chromium alum [(NHJCr(S0J.12H30] and makeup sulfuric acid. It contains ammonium chromium alum. In chromic acid electrolysis, chromium trioxide is dissolved in deionized water acidified with sulfuric acid. The performance of each electrowinning process is presented in Table 4.85. 

Operating parameters Chrome-alum electrolysis Chromic acid electrolysis... 

When a potential is appHed across the ceU, the sodum and other cations are transported across the membrane to the catholyte compartment. Sodium hydroxide is formed in the catholyte compartment, because of the rise in pH caused by the reduction of water. Any polyvalent cations are precipitated and removed. The purified NaOH may be combined with the sodium bicarbonate from the sodium dichromate process to produce soda ash for the roasting operation. In the anolyte compartment, the pH falls because of the oxidation of water. The increase in acidity results in the formation of chromic acid. When an appropriate concentration of the acid is obtained, the Hquid from the anolyte is sent to the crystallizer, the crystals are removed, and the mother Hquor is recycled to the anolyte compartment of the ceU. The electrolysis is not allowed to completely convert sodium dichromate to chromic acid (76). Patents have been granted for more electrolytic membrane processes for chromic acid and dichromates manufacture (86). 

In the most recent plants, the electrolysis is performed in a membrane cell while the chemical step is carried out by allowing the chromic acid to trickle through a column of solid anthracene. The product - anthraquinone - is also insoluble in the aqueous acid so that the organic conversion is effectively completed in the solid state. The reaction goes to completion provided the particle size of the anthracene falls within a suitable range. The spent redox reagent is then passed through an activated carbon bed to remove traces of... 

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. 

The revival of spent chromic acid is closely connected with these oxidations, since it has been the practice, for some time, to regenerate the chromic acid after use by electrolysis —... 

According to E. Muller and P. Ekwall, in the electrolysis of a soln. of chromic acid using a carbon cathode, a film of chromic chromate begins to form at a... 

The data of L. ScherbakoflE and 0. Essin favour the view that in the electrolysis of cone, sulphuric acid soln. of chromic acid chromium is deposited by the discharge of chromic ions, but, according to E. Liebreich, with soln. in dil. acids, the data for current efficiencies favour the view that the metal is deposited by the discharge of chromous ions. 8. Takegami studied the anodic oxidation of cathodically reduced chromic acid. 

Semiconducting nanowires, nanorods, nanodots, nanocones, nanopins, etc. are interesting due to their broad range of applications. Electrochemically, the most easily fabricated semiconductors are Il-Vl semiconductors, for example, CdS, CdSe. There are three approaches for electrodeposition of semiconductors. The first method [131] is deposition of metal in alumina nanopores, followed by etching of alumina surface by phosphoric/chromic acid to access metallic surface for sulphur or arsenic vapour to attain metal sulphide or arsenide nanostructures. The second method deals with electrolysis of sulphuric acid, causing the sulphide atoms to be deposited in pores. 

Chromium (Cr) is a silvery hard metal used in alloys and refractories. Chrome salts are used in dyeing, photography, pigment manufacture and cements. Electroplating tanks contain solutions of chromic acid which forms a mist during the electrolysis process. 

One example of an indirect electrolysis-the methoxylation of the substituted furan-has already been described. The anode reaction was the oxidation of bromide ion to bromine which subsequently reacted with the furan. Here, two further examples will be presented. The first is the conversion of anthracene to anthraquinone via eicctrogenerated chromic add. A concentrated solution of chromiurnfm) in aqueous sulphuric acid is partly oxidized to chromic acid at a lead dioxide anode ... 

A three compartment electrolytic cell with two fritted disk separators was used. An auxiliary platinum electrode was placed in each of the two end compartments while an aluminium strip, to which a surface treatment based on chromic acid etching had been applied, was placed in the middle compartment. Ihe monomer solution contained methacrylic acid (0.436 M) and -methylenebisacrylamide (0.145 M) in water. The pH of the above solution was adjusted to desired values by adding either sulfuric acid or aqueous sodium hydroxide. The monomer solution (400 mi) was placed in the middle compartment while two end compartments contained water adjusted to the same pH as the monomer solution. While bubbling nitrogen through the monomers solution, the electrolysis was conducted at 10 vdts for 6 hours such that the cathodic reaction would occur in the center compartment. The polymer coating obtained on the aluminium cathode was washed with fresh water, dried in vacuum oven at room temperature, and weighed. The copolymer was then scraped off from aluminium and its composition determined by elemental analysis. 

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