Purification of electrolytes

Purification of solvents and salts is essential for reliable electrochemical studies and measurements. A water content of 20ppm already corresponds to a 10 3molL solution. This is in the concentration range of dilute solutions used in conductivity studies for the determination of association constants (see Sec.7.3.2). Traces of water may affect chemical equilibria and therefore act on specific conductivities and limiting ion conductivities. For example, addition of 30 ppm water to a 2xl0-4 mol LT1 solution of LiBF4 in THF at 15 °C increases its conductivity by 4.4 percent (precision of measurements about 0.02 percent) 380 ppm water causes an increase by 51.7 percent see Fig. 3 [20J. 

For electrode reactions at corroding electrodes the purity requirements are even more stringent a water content of 2x10 2 ppm suffices to produce a monolayer of LiOH on a lithium surface of 1 cm in contact with 1 cm electrolyte [1], However, despite good purification procedures [84-86], equipment, and purity control, even recent publications are based on materials used as received without (at least) purity control. As a consequence, results disagree among various authors. 

Other essentials which must be considered to obtain rehable measurements are the storage of purified solvents and salts in high-vacuum glassware equipped 

For ILs the situation is even worse. Synthesized ILs often contain solvents, chloride, and water. Chloride is an impurity that results from a metathesis reaction. 

As the properties of ILs are changed by these impurities, it is necessary to use synthesis routes that avoid them. 

To give an example, during a discussion in a joint project on dye solar cells [174-177], we insisted that work with ILs has to be done under an inert atmosphere at low and controlled water levels. Our cooperation partners felt that filling of thin-layer cells within a few minutes could be carried out under the atmosphere. So we repeated an experiment to determine diffusion coefficients of triiodide in thin-layer cells filled under the atmosphere. The result was overwhelming diffusion coefficients increased by about 100% when compared with results from measurements done under inert atmosphere at low and controlled water levels. 

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An important use of a mercury cathode is in the purification of electrolyte solutions, for example the removal of traces of heavy metals from potassium chloride solutions.-All such impurities have much more positive deposition... 

It is possible to attain the theoretical potentials of the reaction (1) only at the special conditions of the electrodes, and provided the deep purification of electrolytes. Stationary electrode potentials are usually lower than the theoretical potentials by 100-500 mV. The reason for that effect is occurance of parallel and side reactions in the cell. 

Hori et al. pointed out that the deactivation takes place due to the presence of heavy metal impurities originally contained in chemical reagents used as the electrolytes. Heavy metal ions in the electrolyte solution are cathodically reduced and deposited on the electrode surface during the CO2 reduction, deteriorating the electrocatalytic properties of metal electrodes. They apphed a classically established technique of preelectrolysis to purification of electrolyte solutions since their early works. Frese also referred to the impurity heavy metals, and mentioned the presence of Fe and Zn on the Cu electrode after electrolysis on the basis of the surface analysis by XPS. The importance of the purity of the electrolyte solution was mentioned in Section I1.2(zz) as well. The mechanism of the deactivation was recently established, and sununarized below. ... 

Stability, Long Bath Lifetime, Minimum Waste Generation, and Self-Purification of Electrolytes... 

Oka and co-workers (1,2) did find it possible to determine sodium in the presence of potassium by controlled-potential techniques with moderate accuracy, but were unable to carry out the analysis of potassium in the presence of sodium. Controlled-potential techniques are well suited, however, for the purification of electrolytes by preelectrolysis. Meites (3) removed the alkali metals from a 0.1 M tetramethylamine solution in 50 per cent ethanol by electrolysis with a mercury cathode at a potential of —2.35 V vs. SCE. 

Meites (174) has used controlled-potential reduction of nickel for the purification of electrolytes and the precise determination of nickel in the 10 to 10 per cent range (177). Ultra-pure nickel compounds have been prepared by Lingane and Page (172) using this technique. Applications of controlled-potential coulometry to the determination of nickel in plutonium solutions and copper alloys have been reported by Bergstresser (178) and Tanaka (173), respectively. 

Potassium removal is required because the presence of potassium during electrolysis reportedly promotes the formation of the a-Mn02 phase which is nonbattery active. Neutralization is continued to a pH of approximately 4.5, which results in the precipitation of additional trace elements and, along with the ore gangue, can be removed by filtration. Pinal purification of the electrolyte Hquor by the addition of sulfide salts results in the precipitation of all nonmanganese transition metals. 

The cementation of gold and the purification of the ziac electrolyte ate usually carried out ia cylindrical vessels usiag mechanical agitation. The cementation of copper is carried out ia long narrow tanks called launders, ia rotating dmms, or ia an iaverted cone precipitator (see Copper). 

Zinc. The electrowinning of zinc on a commercial scale started in 1915. Most newer faciUties are electrolytic plants. The success of the process results from the abiUty to handle complex ores and to produce, after purification of the electrolyte, high purity zinc cathodes at an acceptable cost. Over the years, there have been only minor changes in the chemistry of the process to improve zinc recovery and solution purification. Improvements have been made in the areas of process instmmentation and control, automation, and prevention of water pollution. 

The standard electrode potential for zinc reduction (—0.763 V) is much more cathodic than the potential for hydrogen evolution, and the two reactions proceed simultaneously, thereby reducing the electrochemical yield of zinc. Current efficiencies slightly above 90% are achieved in modem plants by careful purification of the electrolyte to bring the concentration of the most harmful impurities, eg, germanium, arsenic, and antimony, down to ca 0.01 mg/L. Addition of organic surfactants (qv) like glue, improves the quaUty of the deposit and the current efficiency. 

Metals less noble than copper, such as iron, nickel, and lead, dissolve from the anode. The lead precipitates as lead sulfate in the slimes. Other impurities such as arsenic, antimony, and bismuth remain partiy as insoluble compounds in the slimes and partiy as soluble complexes in the electrolyte. Precious metals, such as gold and silver, remain as metals in the anode slimes. The bulk of the slimes consist of particles of copper falling from the anode, and insoluble sulfides, selenides, or teUurides. These slimes are processed further for the recovery of the various constituents. Metals less noble than copper do not deposit but accumulate in solution. This requires periodic purification of the electrolyte to remove nickel sulfate, arsenic, and other impurities. 

Electrolytic Reductions. Both nitro compounds and nitriles can be reduced electrochemically. One advantage of electrochemical reduction is the cleanness of the operation. Since there are a minimum of by-products, both waste disposal and purification of the product are greatiy simplified. However, unless very cheap electricity is available, these processes are generally too expensive to compete with the traditional chemical methods. 

Salt that is substantially free of sulfate and other impurities is the cell feed. This grade may be purchased from commercial salt suppHers or made on site by purification of cmde sea or rock salt. Dried calcium chloride or cell bath from dismanded cells is added to the bath periodically as needed to replenish calcium coproduced with the sodium. The heat required to maintain the bath ia the molten condition is suppHed by the electrolysis current. Other electrolyte compositions have been proposed ia which part or all of the calcium chloride is replaced by other salts (61—64). Such baths offer improved current efficiencies and production of cmde sodium containing relatively Htde calcium. 

By far, the largest appHcation of zinc powder is for solution purification in electrolytic zinc plants. This appHcation consumed an estimated 17,700 t of powder in 1980. Zinc powder is also used in primary batteries, frictional materials, spray metallizing, mechanical plating, and chemical formulations. 

The final ceU product contains 250—300 g/L H2SO in the last stages of electrolyte purification, and antimony and bismuth precipitate, resulting in heavily contaminated cathodes that are recycled through the smelter. Arsenic and hydrogen evolved at the cathodes at these later stages react to form arsine, and hoods must be provided to collect the toxic gas. 

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. 

The propionamide can be dried over CaO. H2O and unreacted propionic acid were removed as their xylene azeotropes. It was vacuum dried. Material used as an electrolyte solvent (specific conductance less than 10 ohm cm" ) was obtained by fractional distn under reduced pressure, and stored over BaO or molecular sieves because it readily absorbs moisture from the atmosphere on prolonged storage. [Hoover Pure Appl Chem 37 581 I974 Recommended Methods for Purification of Solvents and Tests for Impurities, Coetzee Ed., Pergamon Press, 1982.]... 

The products of this electrolysis have a variety of uses. Chlorine is used to purify drinking water large quantities of it are consumed in making plastics such as polyvinyl chloride (PVC). Hydrogen, prepared in this and many other industrial processes, is used chiefly in the synthesis of ammonia (Chapter 12). Sodium hydroxide (lye), obtained on evaporation of the electrolyte, is used in processing pulp and paper, in the purification of aluminum ore, in the manufacture of glass and textiles, and for many other purposes. 

Alcohol sulfates commonly have free alcohol and electrolytes as impurities. Other hydrophobic impurities can also be present. A method suitable for the purification of surfactants has been proposed by Rosen [120]. Consequently, commercial products have CMCs that deviate from the accepted reference values. This was demonstrated by Vijayendran [121] who studied several commercial sodium lauryl sulfates of high purity. The CMC was determined both by the conductimetric method and by the surface tension method. The values found were similar for both methods but while three samples gave CMC values of 7.9, 7.8, and 7.4 mM, close to the standard range of 8.0-8.2 mM, three other samples gave values of 4.1, 3.1, and 1.7 mM. The sample with a CMC of 7.9 mM was found to have a CMC of 8.0 mM with no detectable surface tension minima after purification and recrystallization. This procedure failed in all other cases. 

It is a typical feature of aqueous electrolyte solutions that one can, within wide limits, change the solute concentrations and hence the conductivities themselves. Pure water has a very low value of o it is about 5 pS/m at room temperature after careful purification of the water. In the most highly conducting solutions (i.e., concentrated solutions of acids and bases), values of 80 S/m can be attained at the same temperature values seven orders of magnitude higher than those found for pure water. 

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