Acid electrolysis

Electrolysis of hydrochloric acid yields hydrogen at the cathode and oxygen at the anode from the dilute acid, but chlorine at the anode (of carbon) from the concentrated acid. Electrolysis of the concentrated acid is used on the large scale to recover chlorine. 

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. 

Ziegelbauer JM, Guild AF, O Laoire C, Urgeghe C, Allen RJ, Mukerjee S (2007) Chalcogenide electrocatalysts for oxygen-depolarized aqueous hydrochloric acid electrolysis. Electrochim Acta 52 6282-6294... 

Stereoselective conversion of a thiane 57 to the corresponding trans-thiane-1-oxide 58 was achieved by bromonium ion mediated electrooxidation while a preferential formation of the cis-sulphoxide 58 was observed under acidic electrolysis (equation 38). 

H2, hydrogen, is a colorless, odorless, tasteless, nonpolar, diamagnetic, diatomic gas with the lowest atomic weight and density of any known substance. It has low solubility in water and is very flammable. Hydrogen is prepared by reactions of metals with water, steam or various acids, electrolysis of water, the water gas reaction and thermal cracking of hydrocarbons. It combines with metals and nonmetals to form hydrides. 

Encouraging laboratory experiments since 1994 with oxygen-depolarised cathodes (ODC) in chlor-alkali as well as hydrochloric acid electrolysis motivated the development of this technique up to the industrial scale. Based on the predictions of the theory, the reduction of cell voltage could be expected up to 1 V (Fig. 4.1) for both applications. Early on, the proper choice and improvement of ODC, deriving mainly from the DeNora group, led to results with voltages as predicted in short tests as well as in endurance tests conducted over dozens of months at the Bayer endurance test facilities. 

Oxygen-depolarised cathode for aqueous hydrochloric acid electrolysis... 

In conventional hydrochloric acid electrolysis [1], aqueous hydrochloric acid (HClaq) is electrolysed in a cell, constructed basically from graphite, which is divided by a porous diaphragm or a membrane. The overall reaction is... 

The GDE for hydrochloric acid electrolysis is characterised by micro-scale hydraulic problems connected with the competition between the gas phase (oxygen), which has to diffuse towards the catalyst, and the liquid phase (water), which must be released. This competition is managed basically by a flow-through structure provided with hydrophobic channels of relatively large diameter. These are formed from PTFE (the binder of the structure) and catalyst particles and account for regulating the gas phase. Hydrophilic channels with smaller diameters (one order of magnitude smaller), which are located in the micro-porous carbon particles of the catalyst support (e.g. Vulcan XC-72), act as water absorbers. A consequence of the electrolysis process is that the catalyst itself is partially covered by liquid. This reduces its effectiveness and accounts for extra voltage. 

Balko, E.N. (1979) SPE hydrochloric acid electrolysis cells performances. Cell Configuration, Oronzio DeNora Symposium - Chlorine Technology. See United States Patent No. 4,311,568 and United States Patent No. 4,294,671. 

Figure 7.2 Cell designs, (a) Conventional alkaline electrolysis (b) advanced alkaline electrolysis (zero gap) (c) SPE configuration (acid electrolysis).

For a long time, conventional alkaline electrolyzers used Ni as an anode. This metal is relatively inexpensive and a satisfactory electrocatalyst for O2 evolution. With the advent of DSA (a Trade Name for dimensionally stable anodes) in the chlor-alkali industry [41, 42[, it became clear that thermal oxides deposited on Ni were much better electrocatalysts than Ni itself with reduction in overpotential and increased stability. This led to the development of activated anodes. In general, Ni is a support for alkaline solutions and Ti for acidic solutions. The latter, however, poses problems of passivation at the Ti/overlayer interface that can reduce the stability of these anodes [43[. On the other hand, in acid electrolysis, the catalyst is directly pressed against the membrane, which eliminates the problem of support passivation. In addition to improving stability and activity, the way in which dry oxides are prepared (particularly thermal decomposition) develops especially large surface areas that contribute to the optimization of their performance. 

Perselenic Acid.—Electrolysis of a cold concentrated solution of potassium selenate containing some free selenie acid has been found to produce a similar effect to that observed with the corresponding sulphate.7 A potassium perselenate was formed at the anode, the chemical process probably being of the same nature as in the formation of perdisulphate, but on account of its instability the product was not obtainable in a higher degree of purity than approximately 75 per cent. 

Also in the cathodic reduction of carboxylic acids, electrolysis is in competition with catalytic methods. However, catalytic hydrogenations in this area do not always proceed so smoothly that electrochemical processes are without any prospects from the outset. 

Typical values are between 0.2 (optimistic goals for industrial high temperature electrolysis) to 0.6-0.8 V (effective overvoltage of alkaline electrolysis) or even more for some acid electrolysis. 

Farbman, G. H. Krasicki, B. R. Hardman, C. C. Lin, S. S. Parker, G. H. EPRI-EM-789 "Economic Comparison of Hydrogen Production Using Sulfuric Acid Electrolysis and Sulfur Cycle Water Decomposition" March 1978 prepared by Westinghouse AESD for Electric Power Research Institute Palo Alto, California 94304. 

You should understand what is meant by chemical reactions and ions , and have a basic knowledge of what is meant by the words acid , electrolysis and oxidation. ... 

To the extent that it is discernible in the products and processes, appropriate aspects have been incorporated in the revision, for example see membrane technology in the chloralkali and hydrochloric acid electrolysis. 

The preparation of organic chloro-compounds usually involves chlorination according to RH + CI2 — RC1 + HC1. Then half of the chlorine necessary for the reaction ends up as either HC1 gas or aqueous hydrochloric acid. One possibility for the recycling of HC1 is the hydrochloric acid electrolysis to give CI2 and H2 again. 

Oxygen depolarized cathodes can be used in hydrochloric acid electrolysis, too. 

D.J. Eames and J. Newman, Electrochemical conversion of anhydrous HC1 to Cl2 using a solid-polymer-electrolyte electrolysis cell, J. Electrochem. Soc., 1995, 142, 3619-3625 I. Uehara, Y. Kawami, N. Wakabayashi, M. Motone and H. Takenaka, Hydrochloric acid electrolysis using solid polymer electrolysis. I. Cell voltage characteristics, Denki Kagaku (J Electrochem. Soc. Jpn.), 1990, 58, 360-366 II. Gas purity and current efficiency, ibid., 1990, 58, 459-467. 

And, at last, shortcomings of techniques, mentioned above, have caused a number of attempts to use hydrochloric acid as an electrolyte for hydrogen and chlorine obtaining. As far back as 1964, the firm "Hoechst" obtained 100 tons of chlorine per day with the help of a cell, worked out by the firm "Friedrich Uhde". This technique is widely used nowadays about 1 min tons of chlorine has been obtained by hydrochloric acid electrolysis by 1990. 

In spite of the obvious successes in the field of hydrochloric acid electrolysis, utilization of acid solutions, containing admixtures of organic compounds, remains to be an unsolved problem.. As it will be shown in the Chapter 1, attempts to utilize such solutions have failed electTOlysis of waste hydrochloric acid at the diaphragm cell requires expensive thorough purification from the organic admixtures, otherw/ise diaphra appears to be out of action. 

Hydrochloric acid electrolysis producing chlorine and hydrogen. 

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