Inert solvents, liquid electrolytes

An important characteristic of liquid ionic systems is that they lack an inert solvent they are pure electrolytes. Owing to this characteristic, some aspects of transport phenomena in pure molten salts are simpler than similar phenomena in aqueous solutions. 

Case I Pure Liquids and Inert Electrolytes. In the absence of significant impurity currents, no faradaic current will flow if the applied bias between the tip and substrate, AEt, is less than the total potential difference, AEp rev, required to drive faradaic reactions at the STM tip and at the substrate. This condition can be easily calculated from the electrochemical potential data for the solvent/electrolyte system under study. This situation is most likely to exist in pure liquids or in solutions of nonelectroactive electrolytes where the faradaic reactions at both electrodes are... 

All binding processes in real-life systems occur in some solvent. The solvent is, in general, a mixture of many components, including water electrolytes and nonelectrolytes. At present, it is impossible to account for all possible solvent effects, even when the solvent is pure water. Yet, the solvent, whether a single or multi-component, cannot be ignored. Any serious molecular theory of cooperativity must deal with solvent effects. We shall see in this chapter that this is not an easy task even when the solvent is inert, such as argon, or a simple hydrocarbon liquid. ... 

For most potentiometric measurements, either the saturated calomel reference electrode or the silver/silver chloride reference electrode are used. These electrodes can be made compact, are easily produced, and provide reference potentials that do not vary more than a few mV. The silver/silver chloride electrode also finds application in non-aqueous solutions, although some solvents cause the silver chloride film to become soluble. Some experiments have utilised reference electrodes in non-aqueous solvents that are based on zinc or silver couples. From our own experience, aqueous reference electrodes are as convenient for non-aqueous systems as are any of the prototypes that have been developed to date. When there is a need to exclude water rigorously, double-salt bridges (aqueous/non-aqueous) are a convenient solution. This is true even though they involve a liquid junction between the aqueous electrolyte system and the non-aqueous solvent system of the sample solution. The use of conventional reference electrodes does cause some difficulties if the electrolyte of the reference electrode is insoluble in the sample solution. Hence, the use of a calomel electrode saturated with potassium chloride in conjunction with a sample solution that contains perchlorate ion can cause dramatic measurements due to the precipitation of potassium perchlorate at the junction. Such difficulties normally can be eliminated by using a double junction that inserts another inert electrolyte solution between the reference electrode and the sample solution (e.g., a sodium chloride solution). 

The plasma ionic liquid interface is interesting from both the fundamental and the practical point of view. From the more fundamental point of view, this interface allows direct reactions between free electrons from the gas phase without side reactions - once inert gases are used for the plasma generation. From the practical point of view, ionic liquids are vacuum-stable electrolytes that can favorably be used as solvents for compounds to be reduced or oxidised by plasmas. Plasma cathodic reduction may be used as a novel method for the generation of metal or semiconductor particles, if degradation reactions of the ionic liquid can be suppressed sufficiently. Plasma anodic oxidation with ionic liquids has yet to be explored. In this case the ionic liquid is cathodically polarized causing an enhanced plasma ion bombardment, that leads to secondary electron emission and fast decomposition of the ionic liquid. 

Solvent Extraction. A modified, one-cycle PUREX process is used at Rocky Flats to recover plutonium from miscellaneous Pu-U residues (11). The process utilizes the extraction of uranium (VI) into tributyl phosphate (TBP), leaving plutonium (III) in the raffinate. The plutonium is then sent to ion exchange for purification. An extraction chromatography method is being studied as a possible substitute for the liquid-liquid extraction process (12) TBP is sorbed on an inert support so ion exchange column equipment can be used. Electrolytic valence adjustment could significantly improve this process. 

DMSO is an excellent solvent for many inorganic salts and organic compounds. It is difficult to reduce and fairly resistant to electrolytic oxidation. Its dielectric constant is high (s = 47). It thus has many of the qualities desirable for a solvent for electrolysis, and it shows promise of being one of the most important electrochemical media [387]. The liquid range is from 18 to 189°C, which makes it somewhat inconvenient to get rid of DMSO in the workup. When used as solvent for electrolysis it must be considered that DMSO is not always inert but has a fair reactivity in certain reactions. DMSO is unfit for UV spectroscopy. Its autoprotolysis constant is 31.8. 

A common feature of all of these methods is that measurement is carried out in ultra-high vacuum (UHV) (<10 torr) thus any electrode surface to be examined must be removed from the cell, possibly rinsed, dried of solvent, and then place in vacuo. Electrodes cannot be examined in situ, since liquids will absorb and block the beams of electrons and ions. The sample must be transferred into a system where there is no electrolyte. This always raises the possibility that the analyzed interface differs significantly from the one in the cell, which is the actual point of interest. For example, hydrated solids will lose water in vacuum and may change composition. Also, exposure of the electrode to the air during transfer can cause oxidation of surface species. Special apparatus has been designed to minimize the problems of exposure to the atmosphere by allowing the sample to be removed from the cell in an inert atmosphere and moved directly into the UHV (Figure... 

Ion-exchange resin methods, which are well known as useful preconcentration methods for trace ions, have some drawbacks slow adsorption and desorption rates, poor selectivity, and requirement for a concentrated solution of electrolyte such as acid, base, or neutral salts for recovery. Chromatographic methods in which solvent extraction procedures have been used in a continuous separation process using inert supports impregnated with the extractants combine many of the advantages of both liquid-liquid extraction and ion-exchange chromatography, which are the two of most im-... 

The co-adsorption of neutral and/or ionic species plays a dominant role in the retention characteristics of solutes in electrochemically modulated chromatographic columns. The electrochemically modulated liquid chromatography is a new and promising technique, which uses conductive stationary phases and the whole column is configured as an electrochemical cell. ° If the mobile phase consists of the polar solvent S, an inert electrolyte, the organic modifier B and the eluite A, the capacity factor, k, for eluite A is given by ° ° ... 

An ideal electrolyte solvent for Li-ion cells shall meet the following minimal criteria, namely, high dielectric constant, to be able to dissolve salts of sufficient concentration, lower viscosity for facile ion transport, inert to all cell components, especially the charged surfaces of the electrodes, lower melting point, and higher boiling point to remain liquid in a wide temperature range. 

In other cases, the method of removal depends upon the nature of the product, e.g. gases may be (1) vented from the reactor, possibly via a slight reduction In pressure (2) displaced from the electrolyte via inert gas sparging (3) segregated via a solid polymer electrolyte (section 5.2) or recirculated via a gas-liquid separator. Liquid products may be (1) separated by flotation or settlement if they are immiscible and have a markedly different density to the electrolyte or (2) emulsified by mixing, then swept out of the reactor. Solid products can be separated via (I) flotation or settlement (2) fluidization or tangential shear to remove them from the reactor (3) solvent extraction or incorporation into a mercury phase, e.g. amalgamation of metals. 

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