Solubility of Metal Salts

Solubility of metal salts in ILs is extremely important in electrodeposition. In this section, the solubility of metal salts in air stable ILs is summarized. The solubility of metal salts in halometalate type ILs has been summarized in previous reports [90, 91]. In addition, many IL systems have been reported as electrolytes for lithium-ion secondary batteries. Some metal salts were reported to be soluble above 50 mol%. However, these systems were obtained by mixing ILs with metal salts in organic solvent or water followed by removal of the solvent this may produce supersaturated solutions. In this section, these systems are omitted due to space limitations. 

As another approach to enhancing the interaction between ILs and metal salts, an extractant highly compatible with both ILs and the metal salt was added. There are many reports on the extraction of metal ions from the aqueous phase into an IL phase with such extractants. Typical examples include crown ethers [40b,95], 

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In addition, the temperature dependence of the diffusion potentials and the temperature dependence of the reference electrode potential itself must be considered. Also, the temperature dependence of the solubility of metal salts is important in Eq. (2-29). For these reasons reference electrodes with constant salt concentration are sometimes preferred to those with saturated solutions. For practical reasons, reference electrodes are often situated outside the system under investigation at room temperature and connected with the medium via a salt bridge in which pressure and temperature differences can be neglected. This is the case for all data on potentials given in this handbook unless otherwise stated. 

The popularity of MSA as an electrolyte in electrochemical applications has developed as a result of the following unique physical and chemical properties (/) exhibits low corrosivity and is easy to handle, (2) nonoxidizing, (f) manufacturing process yields a high purity acid, (4) exceptional electrical conductivity, (5) high solubility of metal salts permits broad applications, (6) MSA-based formulations are simpler, (7) biodegradable, and (8) highly stable to heat and electrical current. 

Although the deposition of metals from ionic liquids has been possible for over 50 years, to date no processes have been developed to a commercial scale. There are numerous technical and economic reasons for this, many of which will be apparent from the preceding chapters. Notwithstanding, the tantalizing prospect of wide potential windows, high solubility of metal salts, avoidance of water and metal/water chemistry and high conductivity compared to non-aqueous solvents means that, for some metal deposition processes, ionic liquids must be a viable proposition. 

Electrochemistry in RTILs has recently been reviewed, and a book has been published on the topic. a large number of metals have been deposited from ionic liquids (Table 6.5) and a book has also been published on electrodeposition from these media. Alloys, semiconductors and conducting polymers have also been deposited from ionic liquids. The key advantages of ionic liquids for electrodeposition and electrochemical applications are their wide potential window, the high solubility of metal salts, the avoidance of water and their high conductivity compared to non-aqueous solvents. There are numerous parameters that can be varied to alter the deposition characteristics including temperature, the cation and anion used, diluents and additional electrolytes. ... 

In our laboratory, we have modified the supercritical fluid processing method known as RESS (Rapid Expansion of Supercritical Solution) (7 J-7S) by expanding the supercritical solution into a liquid solvent, or RESOLV (Rapid Expansion of a Supercritical Solution into a Liquid SOLVent), to produce nanoscale semiconductor and metal particles (7, 9, 19-22). For the solubility of metal salts, supercritical ammonia, THF, and acetone were used in the rapid expansion at relatively higher temperatures. The nanoparticles thus obtained were small (less than 10 nm), with relatively narrow size distributions. In an effort to replace the organic solvents with C02-based systems for RESOLV at ambient temperatures, we used a water-in-C02... 

The general rule is that the solubility of metal salts increases in the presence of suitable Lewis bases, such as NH3, CN , or OH , provided the metal forms a complex with the base. The ability of metal ions to form complexes is an extremely important aspect of their chemistry. 

The solubility of metal salts is also affected by the presence of certain Lewis bases that react with metal ions to form stable complex ions. Complex-ion formation in aqueous solution involves the displacement by Lewis bases (such as NH3 and CN ) of water molecules attached to the metal ion. The extent to which such complex formation occurs is expressed quantitatively by the formation constant for the complex ion. Amphoteric oxides and hydroxides are those that are only slightly soluble in water but dissolve on addition of either acid or hase. 

The Emf Series is an orderly arrangement of the standard potentials for all metals. The more negative values correspond to the more reactive metals (Table 3.2). Position in the Emf Series is determined by the equilibrium potential of a metal in contact with its ions at a concentration equal to unit activity. Of two metals composing a cell, the anode is the more active metal in the Emf Series, provided that the ion activities in equilibrium are both unity. Since unit activity corresponds in some cases to impossible concentrations of metal ions because of restricted solubility of metal salts, the Emf Series has only limited use for predicting which metal is anodic to another. 

The ability to form stable complexes with metal salts is another unique property of the crown ethers. The metal salts described previously include Group I and II elemental as well as in group III. O In addition, ammonium cation complexed with cyclic polyether has also been reported.It is interesting to note that the presence of cyclic polyethers strongly increase the solubility of metal salts in nonpolar solvents. This property can drastically affect the structure and reactivity of ion pairs 25 and also affect the rate of the reactions.26, 27... 

It is known that the solubility of metal salts in nonpolar media is drastically increased if small amounts of crown ethers (cyclic polyethers) are used as complexation agents. Such a concept has been demonstrated in various areas of chemistry. For example, they are used as phase-transfer catalysts in organic synthesis. Moreover, Cheng27 and Schue28 have expanded this idea in the areas of anionic polymerization. 

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