Oxygen Evolving Anodes

Ozone is formed in certain chemical reactions, including the action of fluorine on water (p. 323) and the thermal decomposition ofiodic(VII) (periodic) acid. It is also formed when dilute (about 1 M) sulphuric acid is electrolysed at high current density at low temperatures the oxygen evolved at the anode can contain as much as 30% ozone. 

Oxygen-Evolving Anode. Research efforts to iacorporate the coated metal anode for oxygen-evolving appHcations such as specialty electrochemical synthesis, electro winning, impressed current, electrodialysis, and metal recovery found only limited appHcations for many years. 

The commercial status of metal anodes for oxygen-evolving appHcations maybe summarized as foUows ... 

In electrogalvanizing, copper foil, and other oxygen-evolving appHcations, the greatest environmental contribution has been the elimination of lead-contaminated waste streams through replacement of the lead anode. In addition, the dimensionally stable characteristic of the metal anode iatroduces greater consistency and simplification of the process, thus creating a measure of predictabiUty, and a resultant iacreased level of safety. 

Oxygen evolved from the anodes as well as some hydrogen from the cathodes produces a mist which is trapped by a froth maintained by adding cresyhc acid, sodium siUcate, and gum arabic, or glue plus cresol. Alkaline-earth carbonates prevent lead contamination of the cathode ziac. Most of the lead is deposited ia the cell sludge as iasoluble carbonate—sulfate. 

Ruthenium will be dissolved by oxygen evolution (volatile RUO4). Coatings based on iridium/tantalum oxides are stabile for oxygen evolving anodes, even with simultaneous chlorine evolution in the presence of chlorides. 

Overvoltage associated with the evolution of oxygen at the anode is even less reproducible than the hydrogen overvoltage because of its greater variability with time and because of the formation of metal oxides on the surface of the anode. Approximate values of minimum overvoltage as measured by Caspari on oxygen evolved from a normal NaOH solution at 25 °C at different metals are contained in Table 14. 

The oxygen evolved at the anode rapidly oxidizes the arising chlorous acid into ehloric acid so that the overall reaction at the anode may he expressed by the equation ... 

Arikawa, T., Murakami, Y. and Takasu, Y. (1998) Simultaneous determination of chlorine and oxygen evolving at Ru02/Ti and Ru02-Ti02/Ti anodes by different electrochemical mass spectroscopy. J. Appl. Electrochem. 28, 511-516. 

Lead Dichromate, PbCr.20, containing 2 molecules of water of crystallisation, was described by Preis and Rayman, but its existence was subsequently denied. When a very concentrated solution of chromic acid (130 grams CrOg per 100 c.c.) is electrolysed with a lead anode, a solution is obtained which evolves oxygen when kept, and deposits reddish-brown needles of lead dichromate, PbCr O,. From the volume of oxygen evolved it appears that plumbic dichromate is formed and decomposes thus ... 

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