Cuprous oxide rate determination

At medium and high temperatures copper ultimately follows the parabolic law " . It has been shown " using radioactive tracers that the diffusion of copper ions in cuprous oxide is the rate-determining step at 8(X)-1 000°C, and there is considerable evidence in favour of the view that metal moves outwards through the film by means of vacant sites in the oxide lattice . 

Lewin and Cohen (1967) determined the products of dediazoniation of ben-zophenone-2-diazonium salt (10.42, Scheme 10-77) in five different aqueous systems (Table 10-7). About one-third of the yield is 2-hydroxybenzophenone (10.46) and two-thirds is fluorenone (10.45, run 1) copper has no effect (run 2). On the other hand, addition of cuprous oxide (run 3) has a striking effect on product ratio and rate. The reaction occurs practically instantaneously and yields predominantly fluorenone. As shown in Scheme 10-77, the authors propose that, after primary dediazoniation and electron transfer from Cu1 to 10.43 the sigma-complex radical 10.44 yields fluorenone by retro-electron-transfer to Cu11 and deprotonation. In the presence of the external hydrogen atom source dioxane (run 12) the reaction yields benzophenone cleanly (10.47) after hydrogen atom abstraction from dioxane by the radical 10.43. 

The amount of biocides needed in an antifouling paint will be determined by actual tests performed in real life. A rule of thumb says that the minimum level of protection with copper is a leaching rate of 10 pg/(cm2 day) (WHOI, 1952). The amount of cuprous oxide can be up to 40 wt % of an antifouling paint and it is normally assisted by booster biocides (5 to 10 wt %) to provide full protection against both animals and algae. The amount of biocides will be based on practical performance and, at the same time, be restricted to given legislation. 

For some catalysts, the contrary situation occurs and reoxidation is the rate-determining step. A typical example is cuprous oxide. The observed rates are, in this case, dependent on the oxygen instead of the propene pressure. 

The mechanism in Scheme 8 was proposed for the oxidation reaction. In the first step, the Cu(II) salt, which is formed in the autooxidation of cuprous chloride, forms a complex with the amine. This is followed by a rate-determining electron transfer from the amine to the Cu(II) species giving Cu(I) and an aminium radical. The subsequent steps were considered to be fast. The authors accounted for the secondary hydrogen-deuterium kinetic isotope effect by suggesting that there was hyperconjugative electron release to the aminium ion nitrogen that forms in the slow step of the reaction. 

Since the tetrachloride complex is more stable, CuClj converts to CuCl " in the reaction layer at the electrode surface (Step 3). The complex ion transfers to Ae bulk of the solution through its diffusion layer (Step 4), and dissolved oxygen in the bulk oxidizes the cuprous form to cupric (Step 5). Since the chemical reactions of Steps 2, 3, and 5 are fast, the two diffusion processes. Steps 1 and 4, determine the rate of the process. At low current densities. Step 4 is rate-determining. Both diffusion steps contribute to the resistance at high current densities. 

In solutions containing chloride there is a tendency for the establishment of the Cu/CuCl/cr electrode potential, so that the activity of chloride ions is an important factor in determining the electrode behaviour. From a knowledge of the solubility products of cuprous chloride and cuprous oxide it is possible to predict under what conditions chloride or hydroxyl ions are the potential-determining ions. According to Catty and Spooner, chloride determines the potential if Ooh- < 10 x Oa- and hydroxyl if Oqh- > 10 X <7ci-. This will not hold in concentrated solutions, however, since complex [CuClj] ions as well as simple ions will be present. A further factor to be considered is the ready formation of insoluble basic compounds. In solutions not containing chloride (e.g. sulphate or nitrate solutions), corrosion rates are usually lower and the electrode potential is more steady over... 

As already stated, the pH of the solution alters the potential difference between the anodic and cathodic reactions and this is reflected in the plating rate which increases with pH. This is in agreement with the theoretical principles outlined earlier. The limit to which the pH can increase is determined by the value at which precipitation occurs. Large variations in pH may cause fine precipitates to form (e.g. nickel phosphite, cuprous oxide) and these may act as seed crystals for the complete reduction of the plating solution. 

Barfoed s test is similar to Benedict s test, but determines if a carbohydrate is a monosaccharide or a disaccharide. Barfoed s reagent reacts with monosaccharides to produce cuprous oxide at a faster rate than that of disaccharides ... 

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