Aqueous solutions electrolysis

Colourless crystals m.p. I25°C, soluble in water and alcohol. In aqueous solution forms equilibrium with its lactones. Gluconic acid is made by the oxidation of glucose by halogens, by electrolysis, by various moulds or by bacteria of the Acetobacter groups. 

The general characteristics of all these elements generally preclude their extraction by any method involving aqueous solution. For the lighter, less volatile metals (Li, Na, Be, Mg, Ca) electrolysis of a fused salt (usually the chloride), or of a mixture of salts, is used. The heavier, more volatile metals in each group can all be similarly obtained by electrolysis, but it is usually more convenient to take advantage of their volatility and obtain them from their oxides or chlorides by displacement, i.e. by general reactions such as... 

The reactions with water are summarised in Table 6.3. Since the metals are powerful reducing agents (p. 98) they cannot be prepared in aqueous solution electrolysis of the fused anhydrous halides is usually employed using a graphite anode. 

Each of these elements can be extracted by reduction of the respective oxide at high temperature, using either carbon or hydrogen or by electrolysis of an aqueous solution of a salt of the required element. 

The aqueous solution of sodium chlorate(I) is an important liquid bleach and disinfectant. It is produced commercially by the electrolysis of cold aqueous sodium chloride, the anode and cathode products being mixed. The sodium chloride remaining in the solution does not usually matter. There is evidence to suggest that iodic(I) acid has some basic character... 

Pure iron is prepared by reduction of iron(II) oxide with hydrogen, or by electrolysis of an iron(II)-containing aqueous solution. It is a fairly soft metal, existing in different form according to temperature ... 

Chlorine and caustic soda are coproducts of electrolysis of aqueous solutions of sodium chloride [7647-14-5] NaCl, (commonly called brine) following the overall chemical reaction... 

These reactions can be carried out at room temperature. Hydrogen gas can also be produced on a laboratory scale by the electrolysis of an aqueous solution. Production of hydrogen through electrolysis is also used industrially. This involves the following reaction at the cathode of the electrochemical cell ... 

The deposition of RE metals from aqueous solutions does not work because of the highly electropositive nature of the REE. Therefore, industrial production of RE metals is carried out by fused salt electrolysis or metaHothermic reduction. 

Electrolysis of Aqueous Solutions. The electrolytic process for manganese metal, pioneered by the U.S. Bureau of Mines, is used in the Repubhc of South Africa, the United States, Japan, and beginning in 1989, Bra2il, in decreasing order of production capacity. Electrolytic manganese metal is also produced in China and Georgia. 

Mercury layers plated onto the surface of analytical electrodes serve as Hquid metal coatings. These function as analytical sensors (qv) because sodium and other metals can be electroplated into the amalgam, then deplated and measured (see Electro analytical techniques). This is one of the few ways that sodium, potassium, calcium, and other active metals can be electroplated from aqueous solution. In one modification of this technique, a Hquid sample can be purified of trace metals by extended electrolysis in the presence of a mercury coating (35). 

Stannous Sulfate. Stannous sulfate (tin(Il) sulfate), mol wt 214.75, SnSO, is a white crystalline powder which decomposes above 360°C. Because of internal redox reactions and a residue of acid moisture, the commercial product tends to discolor and degrade at ca 60°C. It is soluble in concentrated sulfuric acid and in water (330 g/L at 25°C). The solubihty in sulfuric acid solutions decreases as the concentration of free sulfuric acid increases. Stannous sulfate can be prepared from the reaction of excess sulfuric acid (specific gravity 1.53) and granulated tin for several days at 100°C until the reaction has ceased. Stannous sulfate is extracted with water and the aqueous solution evaporates in vacuo. Methanol is used to remove excess acid. It is also prepared by reaction of stannous oxide and sulfuric acid and by the direct electrolysis of high grade tin metal in sulfuric acid solutions of moderate strength in cells with anion-exchange membranes (36). 

Electrolysis. Electrowinning of zirconium has long been considered as an alternative to the KroU process, and at one time zirconium was produced electrolyticaHy in a prototype production cell (70). Electrolysis of an aH-chloride molten-salt system is inefficient because of the stabiUty of lower chlorides in these melts. The presence of fluoride salts in the melt increases the stabiUty of in solution, decreasing the concentration of lower valence zirconium ions, and results in much higher current efficiencies. The chloride—electrolyte systems and electrolysis approaches are reviewed in References 71 and 72. The recovery of zirconium metal by electrolysis of aqueous solutions in not thermodynamically feasible, although efforts in this direction persist. 

The metals that are produced by electrolysis (81) are included in Table 1. Fused salt processes are used when the reactivity of the metal does not allow electrowinning from aqueous solutions. Manganese is the most reactive metal that is produced by electrolysis of an aqueous solution. 

Table 6. Production of Metals by Electrolysis of Aqueous Solutions ...

Explosion Hazards. The electrolysis of aqueous solutions often lead to the formation of gaseous products at both the anode and cathode. Examples are hydrogen and chlorine from electrolysis of NaCl solutions and hydrogen and oxygen from electrolysis of water. The electrode reactions. 

A.sahi Chemical EHD Processes. In the late 1960s, Asahi Chemical Industries in Japan developed an alternative electrolyte system for the electroreductive coupling of acrylonitrile. The catholyte in the Asahi divided cell process consisted of an emulsion of acrylonitrile and electrolysis products in a 10% aqueous solution of tetraethyl ammonium sulfate. The concentration of acrylonitrile in the aqueous phase for the original Monsanto process was 15—20 wt %, but the Asahi process uses only about 2 wt %. Asahi claims simpler separation and purification of the adiponitrile from the catholyte. A cation-exchange membrane is employed with dilute sulfuric acid in the anode compartment. The cathode is lead containing 6% antimony, and the anode is the same alloy but also contains 0.7% silver (45). The current efficiency is of 88—89%, with an adiponitrile selectivity of 91%. This process, started by Asahi in 1971, at Nobeoka City, Japan, is also operated by the RhcJ)ne Poulenc subsidiary, Rhodia, in Bra2il under Hcense from Asahi. 

Electrolysis of acidified water using platinum electrodes is a convenient source of hydrogen (and oxygen) and, on a larger scale, very pure hydrogen (>99.95%) can be obtained from the electrolysis of warm aqueous solutions of barium hydroxide between nickel electrodes. The method is expensive but becomes economical... 

At present about 77% of the industrial hydrogen produced is from petrochemicals, 18% from coal, 4% by electrolysis of aqueous solutions and at most 1% from other sources. Thus, hydrogen is produced as a byproduct of the brine electrolysis process for the manufacture of chlorine and sodium hydroxide (p. 798). The ratio of H2 Cl2 NaOH is, of course, fixed by stoichiometry and this is an economic determinant since bulk transport of the byproduct hydrogen is expensive. To illustrate the scde of the problem the total world chlorine production capacity is about 38 million tonnes per year which corresponds to 105000 toimes of hydrogen (1.3 x I0 m ). Plants designed specifically for the electrolytic manufacture of hydrogen as the main product, use steel cells and aqueous potassium hydroxide as electrolyte. The cells may be operated at atmospheric pressure (Knowles cells) or at 30 atm (Lonza cells). 

The spectacular success (in 1807) of Humphry Davy, then aged 29 y, in isolating metallic potassium by electrolysis of molten caustic potash (KOH) is too well known to need repeating in detail." Globules of molten sodium were similarly prepared by him a few days later from molten caustic soda. Earlier experiments with aqueous solutions had been unsuccessful because of the great reactivity of these new elements. The names chosen by Davy reflect the sources of the elements. 

It is clear that, if synthetic routes could be devised which would mechanistically hinder disproportionation, such compounds might be preparable. Although univalent compounds of the Group 2 metals have not yet been isolated, there is some evidence for the formation of Mg species during electrolysis with Mg electrodes. Thus H2 is evolved at the anode when an aqueous solution of NaCl is electrolysed and the amount of Mg lost from the anode corresponds to an oxidation state of 1.3. Similarly, when aqueous Na2S04 is electrolysed, the amount of H2 evolved corresponds to the oxidation by water of Mg ions having an average oxidation state of 1.4 ... 

Magnesium is reduced from a mixture of magnesium, calcium, and sodium chlorides. Electrolysis from aqueous solution is also possible zinc, copper, and manganese dissolved as sulfates in water can be reduced electrolytically from aqueous solution. 

The electrolysis of an aqueous solution of NaCl has the overall equation... 

We have dealt with electrolysis before—every time we discussed or measured the electrical conductivity of an electrolyte solution. To see this, let s consider the processes that occur when we cause electric charge to pass through an aqueous solution of hydrogen iodide. 

E.28 A chemist wants to extract the gold from 35.25 g of gold(III) chloride dihydrate, AuCI3-2H20, by electrolysis of an aqueous solution (this technique is described in Chapter 12). What mass of gold could be obtained from the sample ... 

An electrochemical cell in which electrolysis takes place is called an electrolytic cell. The arrangement of components in electrolytic cells is different from that in galvanic cells. Typically, the two electrodes share the same compartment, there is only one electrolyte, and concentrations and pressures are far front standard. As in all electrochemical cells, the current is carried through the electrolyte by the ions present. For example, when copper metal is refined electrolytically, the anode is impure copper, the cathode is pure copper, and the electrolyte is an aqueous solution of CuS04. As the Cu2f ions in solution are reduced and deposited as Cu atoms at the cathode, more Cu2+ ions migrate toward the cathode to take their place, and in turn their concentration is restored by Cu2+ produced by oxidation of copper metal at the anode. 

Predict the likely products of electrolysis of an aqueous solution from standard potentials (Example 12.1 I). 

---