Anode lead dioxide

The standard potential for the anodic reaction is 1.19 V, close to that of 1.228 V for water oxidation. In order to minimize the oxygen production from water oxidation, the cell is operated at a high potential that requires either platinum-coated or lead dioxide anodes. Various mechanisms have been proposed for the formation of perchlorates at the anode, including the discharge of chlorate ion to chlorate radical (87—89), the formation of active oxygen and subsequent formation of perchlorate (90), and the mass-transfer-controUed reaction of chlorate with adsorbed oxygen at the anode (91—93). Sodium dichromate is added to the electrolyte ia platinum anode cells to inhibit the reduction of perchlorates at the cathode. Sodium fluoride is used in the lead dioxide anode cells to improve current efficiency. 

Conversion of toluenes to the benzoic acid is also accomplished by anodic oxidation in acetic acid containing some nitric acid. It is not clear if this reaction involves the aromatic radical-cation or if the oxidising agents are nitrogen oxide radicals generated by electron transfer from nitrate ions [66, 67]. Oxidation of 4-fluorotoluene at a lead dioxide anode in dilute sulphuric acid gives 4-fluorobenzoic acid in a reaction which involves loss of a proton from the aromatic radical-cation and them in further oxidation of the benzyl radical formed [68]. 

Scheme 6.8. Oxidation of anisole to quinone at a lead dioxide anode in dilute sulphuric acid.

Reactions of phenols are frequently carried out at a lead dioxide anode in dilute aqueous sulphuric acid. A phenoxonium ion is the reactive intermediate. In dilute solution, the major reaction is para-hydroxylation of the phenol [100]. Phenols... 

Oxidation of methylpyridines in 60-80 % sulphuric acid at a lead dioxide anode leads to the pyridinecarboxylic acid [213]. The sulphuric acid concentration is critical and little of the product is formed in dilute sulphuric acid [214]. In these reactions, electron loss from the n-system is driven by concerted cleavage of a carbon-hydrogen bond in the methyl substituent. This leaves a pyridylmethyl radical, which is then further oxidised to the acid, fhe procedure is run on a technical scale in a divided cell to give the pyridinecarboxylic acid in 80 % yields [215]. Oxida-tionof quinoline under the same conditions leads to pyridine-2,3-dicarboxylic acid [214, 216]. 3-HaIoquino ines afford the 5-halopyridine-2,3-dicarboxylic acid [217]. Quinoxaline is converted to pyrazine-2,3-dicarboxylic acid by oxidation at a copper anode in aqueous sodium hydroxide containing potassium permanganate [218]. 

Early electrochemical processes for the oxidation of alcohols to ketones or carboxylic acids used platinum or lead dioxide anodes, usually with dilute sulphuric acid as electrolyte. A divided cell is only necessary in the oxidation of primary alcohols to carboxylic acids if (he substrate possesses an unsaturated function, which could be reduced at the cathode [1,2]. Lead dioxide is the better anode material and satisfactory yields of the carboxylic acid have been obtained from oxidation of primary alcohols up to hexanol [3]. Aldehydes are intermediates in these reactions. Volatile aldehydes can be removed from the electrochemical cell in a... 

The oxidation of propargyl alcohol to the acid and of but-2-yne-l,4-diol to acetylene dicarboxylic acid is carried out on a technical scale at a lead dioxide anode in sulphuric acid [4, 5]. Electrochemical oxidation of acetylenic secondary alcohols to the ketone at lead dioxide in aqueous sulphuric acid [4], gives better results than the cliromic acid based process of Jones [6], Oxidation of aminoalkan-1-ols to the amino acid at a lead dioxide anode in sulphuric acid is achieved in 31 -73 % 5delds [7]. This route is applied to the technical scale production of (l-alanine from 3-aminopropanol in an undivided cell [8]. 

Aqueous periodic acid can be used to achieve glycol cleavage, combined with anodic oxidation of the iodate, which is formed, back to periodate [70]. Oxidation of iodate is catalysed at a lead dioxide anode [71] but at the potentials required, aldehydes are oxidised to the corresponding acids. Due to this further reaction, the redox-mediated cleavage of diols to form an aldehyde may be difficult to achieve witli a catalytic amount of periodic acid. Cleavage using a stoichiometric amount of periodic acid, followed by recovery of the iodic acid and then its electochemical oxidation, has been achieved [72]. 

Amines are stable to electrochemical oxidation in acid solution because the nitrogen lone pair is protonated and inaccessible for reaction. This is not the case for N-acetylamines, which are oxidisable at a lead dioxide anode in aqueous sulphuric acid [99]. The primary electron tiansfer step involves the amide function and leads to a radical-cation, which loses a proton from the carbon atom adjacent to nitrogen. Subsequent steps lead to an acylimmonium ion, which is trapped by water. N-acetylated primaiy amines are converted to the corresponding carboxylic acid. 

Anodic methoxylation of cyclohexyl isocyanide yields a mixture of six products whose formation is thought to be initiated by methoxy radicals 194). Cholesteryl acetate dibromide has been specifically hydroxylated at C25 in 85 to 93% yield at a lead dioxide anode 19 s. Silyl ethers have been prepared in 60 to 95 % yield by electrolysis of trialkylsilanes at a platinum electrode in ROH- Me4NI,... 

Mohd, Y. and Pletcher, D. (2005), The influence of deposition conditions and dopant ions on the structure, activity, and stability of lead dioxide anode coatings. J. Electrochem. Soc., 152(6) D97-D102. 

Weiss, E., Groenen-Serrano, K. and Savall, A. (2006), Electrochemical degradation of sodium dodecylbenzene sulfonate on boron doped diamond and lead dioxide anodes. J. New Mater. Electrochem. Sys., 9(3) 249-256. 

Zhou, M., Dai, Q., Lei, L., Ma, C. and Wang, D. (2005), Long life modified lead dioxide anode for organic wastewater treatment Electrochemical characteristics and degradation mechanism. Environ. Sci. Technol., 39(1) 363-370. 

The anodic oxidation of phenol (dissolved in sodium sulfate) was studied by De Sucre and Watkinson using two types of lead dioxide anodes [26]. The first was made of 2 mm lead shot that was oxidized for 12 h at 526mA/cm in 20% sulfuric acid while the second anode, electrodeposited lead dioxide flakes, was supplied by a commercial manufacturer. The phenol oxidized faster on the electrodeposited Pb02, which also turned out to be more corrosion-resistant than the oxidized lead shot. While all of the phenol oxidized rapidly (1.5 h) on the electrodeposited anode, not all of it formed CO2. It was found that 80% of the total organic carbon remained in solution after the phenol was completely oxidized. Phenol destruction increased with an increase in current density, and decreased as electrolyte flowrate, pH, and anode particle size were increased. 

The anodic destruction of low-concentration cyanide solutions was studied using nine different lead dioxide anodes which were plated from lead nitrate solutions con-... 

Ho and Chan used a Pb02-coated titanium anode for the electroflotation of palm oil mill effluent [141]. The electroflotation of suspended particles and anodic destruction of soluble substances such as sugars and phenolics occurred simultaneously. At 0.5 A and after 20 hours of operation, 86% of the suspended matter was found to be removed by flotation while 50% of the dissolved substances were destroyed. The lead dioxide anode was not corroded during the course of operation. 

Alkyl derivatives of 7r-electron-deficient heterocyclic compounds may be oxidized anodi-cally to carboxylic acids alkyl and arylalkyl pyridines can be oxidized to the corresponding acids in good yields at a lead dioxide anode in aqueous sulfuric acid [229, 230, 376-378]. 

This section covers the union of two aryl moieties, one of which is a fully alkylated phenol, and the other is either a free phenol or a phenol ether. Clearly, radical dimerization is not operative in such reactions. An early example is to be found in the work of Pitcher and Dietrich (1924) ° who showed that 3,3, 4,4 -tetramethoxybiphenyl was among the products of electrolytic oxidation of veratrole in sulfuric acid using a lead dioxide anode the biaryl was formed in about 20% yield based on reacted veratrole. 

Cathodes are made from different steel materials electrolyte additive is sodium chromate like in the case of the chlorate electrolysis. This cannot be done in the case of lead dioxide anodes, because the oxygen evolution would be catalyzed in this way. The operation temperature is 35-50°C. 

For the electrolytic oxidation of soln. of chromic salts to chromic acid or the chromates, vide supra. According to E. Muller and M. Seller, a soln. of chrome-alum in i i-H2804 is not appreciably oxidized to chromic acid with a smooth platinum anode, but when traces of a lead salt are present in the soln., lead dioxide is deposited on the anode, and oxidation then occurs. About one-third of the oxidation which occurs with a lead dioxide anode occurs when an anode of platinized platinum is used. With a lead dioxide anode, the oxidation occurs quantitatively in fairly cone. soln. of chrome-alum, when the current density is not too high—about 0-005 amp. per... 

The electrolysis was carried out in stacks of 24 cells in parallel in a filter press (Fig. 6.3). Each cell (Fig. 6.4) consisted of a cathode spacer, lead cathode, catholyte distribution block, membrane, anolyte distribution block and lead dioxide anode and will have its own catholyte and anolyte streams with compositions shown in Fig. 6.2. The catholyte distribution block includes turbulence promoters and the cell was designed to minimize the cathode—membrane gap (0.8—3.2 mm) because of the poor conductivity of the catholyte required to obtain the high selectivity for adiponitrile. The electrical connection was bipolar with 300 V appUed across the stack, i.e. about 12 V per cell, which gives a cathode current density in the range 0.4-0.6 A cm . The total cell current was 2870 A. 

Kuhn, A.T. (1976) The electrochemical evolution of oxygen on lead dioxide anodes. - Chemistry Industry, 20,867-871. 

Graves, J.E. Fletcher, D. Clarke, R.L. Walsh, F.C. (1992) The electrochemistry of Magneli phase titanium oxide ceramic electrodes, n. Ozone generation at Ebonex and Ebonex/lead dioxide anodes. J. Appl. Elec-trochem., 22(3), 200-203. 

Since flue gases not only contain SO2 but to a certain extent also NOx, the processes for the simultaneous removal of both components have been developed in many studies. The lead diox-ide-dithionite process has combined direct and indirect conversion of SO2 and NOx, respectively. In the first step dithionite was used as homogeneous redox mediator for the indirect reduction of NOx. SO2 has been led to pass the NO absorption column without reaction and entered an electrochemical cell where it was oxidized to sulfric acid at the lead dioxide anode. A pilot plant for the treatment of 100 Nm h of flue gas having the NO concentration of 600 ppm has been tested on an industrial site [22]. 

The early attempts to use an undivided ceil were based on the concept of a sacrificia] anode reaction A lead dioxide anode was used and isopropanol was added as an anode depolarizer. The reaction ... 

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 ... 

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