Nonaqueous solutions conductance

So far, the classical work on polyelectrolyte nonaqueous solutions conducted during the 1950s has been described. Here we will describe more recent work on polyelectrolyte nonaqueous solutions, conducted during the 1980s and 1990s, in particular the work on the solution properties of ionomers in a polar solvent, which may be considered as weakly charged polyelectrolytes. 

Barthel, J. Temperature Dependence of Conductance of Electrolytes in Nonaqueous Solutions 13... 

For this reason, other types of electrolytes are used in addition to aqueous solutions (i.e., nonaqueous solutions of salts (Section 8.1), salt melts (Section 8.2), and a variety of solid electrolytes (Section 8.3). More recently, a new type of solid electrolyte is being employed more often (i.e., water-impregnated ionically conducting polymer films more about them in Chapter 26). 

In aqueous electrolyte solutions the molar conductivities of the electrolyte. A, and of individual ions, Xj, always increase with decreasing solute concentration [cf. Eq. (7.11) for solutions of weak electrolytes, and Eq. (7.14) for solutions of strong electrolytes]. In nonaqueous solutions even this rule fails, and in some cases maxima and minima appear in the plots of A vs. c (Eig. 8.1). This tendency becomes stronger in solvents with low permittivity. This anomalons behavior of the nonaqueous solutions can be explained in terms of the various equilibria for ionic association (ion pairs or triplets) and complex formation. It is for the same reason that concentration changes often cause a drastic change in transport numbers of individual ions, which in some cases even assume values less than zero or more than unity. 

The techniques and apparatus which have been developed to measure electrolytic conductivities in nonaqueous solutions have been adapted from aqueous conductivity measurements with some modifications. Direct current measurements suffer the limitation of requiring reversible electrodes - a serious limitation in nonaqueous solvents. Although this problem can be circumvented U in some instances, virtually all precision conductance data have been taken using the alternating current method. General descriptions of this method are given in several sources. 2>3)... 

The temperature coefficient of conductance is approximately 1-2 % per °C in aqueous 2> as well as nonaqueous solutions 27). This is due mainly to thetemper-ature coefficient of change in the solvent viscosity. Therefore temperature variations must be held well within 0.005 °C for precise data. In addition, the absolute temperature of the bath should be known to better than 0.01 °C by measurement with an accurate thermometer such as a calibrated platinum resistance thermometer. The thermostat bath medium should consist of a low dielectric constant material such as light paraffin oil. It has been shown 4) that errors of up to 0.5 % can be caused by use of water as a bath medium, probably because of capacitative leakage of current. 

Recently Hoover 29> compared various extrapolation methods for obtaining true solution resistances concentrated aqueous salt solutions were used for the comparisons. Two Jones-type cells were employed, one with untreated electrodes and the other with palladium-blacked electrodes. The data were fitted to three theoretical and four empirical extrapolation functions by means of computer programs. It was found that the empirical equations yielded extrapolated resistances for cells with untreated electrodes which were 0.02 to 0.15 % lower than those for palladium-blacked electrodes. Equations based on Grahame s model of a conductance cell 30-7> produced values which agreed to within 0.01 %. It was proposed that a simplified equation based on this model be used for extrapolations. Similar studies of this kind are needed for dilute nonaqueous solutions. 

From a review of the recent Russian electrochemical literature it can be concluded that intensive research on nonaqueous batteries is carried out in the U.S.S.R. Although no results of the performance of such batteries are published, papers on properties of Li solutions in solvents used in Li cells, and on the behaviour of metallic Li in nonaqueous solutions, which have appeared recently in Russian literature, can be inferred to be the tip of an iceberg of research in that field. For example, the electrodeposition of Li from dimethylformaunide solutions of its salts (40), or the behaviour of Li in aprotic solutions (4la) and the solubility and conductivity of its salts in these solutions (41b) emanate from an unnamed research institute in Moscow. 

Historically, the following four main steps must be mentioned the preparation of ethylammonium nitrate [C2H5NH3][N03] by Paul Walden in 1914 is recognized by many as the first IL. This compound has a melting point of 12°C but owing to its high reactivity has not really found a use [1]. This was the outcome of his studies of conductivity and electrical properties of salt solutions, especially nonaqueous solutions of organic salts. He conducted very systematic studies with different solvents and salts, and his special interest was in ammonium salts. But Walden himself pointed on the work... 

Lower Conductivity. The equivalent conductance of nonaqueous solutions a( infinite dilution is often comparable to that of aqueous systems, but it decreases with an increase in concentration more rapidly than the corresponding aqueous systems (the effect of the lower dielectric constant). Since the specific conductivity, K (that which determines the resistance between cathode and anode) is proportional to Ac, the equivalent conductance, the IR drop between the electrodes of a cell in which deposition from nonaqueous solutions is to lake place will be greater than that in aqueous solution (see Section 4.8.7). The electricity needed to deposit a given mass of metal is proportional to the total E between the electrodes, and this includes the IR between the electrodes, which is much greater in the nonaqueous than in the aqueous cases. Hence, nonaqueous deposition will be more costly in electricity (more kilowatt hours per unit of weight deposited) than a corresponding deposition in aqueous solution. The difference may be prohibitive. 

There are electronic compensation circuits available to reduce this error, and if the current density is low enough (or the solution highly conducting), it may be negligible. The high resistance of nonaqueous solutions could provide a difficulty (which, however, is not present in the decay transient approach). 

It is clear at once that in aqueous solutions, the reduction of C02 will be difficult because of competition with hydrogen evolution, so that catalysts with low i0 for this latter reaction will be best. Nonaqueous solutions would offer the advantage of a greater C02 solubility and no competition with H2, which must be balanced against reduced conductivity and the difficulty of keeping the system anhydrous over longer times. 

Nonaqueous liquid electrolyte solutions may be divided into subgroups according to several criteria based on the differences among the various polar aprotic solvents. The first division can be between protic or polar aprotic nonaqueous solvents and nonpolar solvents. In polar aprotic and protic nonaqueous systems, conductivity is achieved by the dissolution of the electrolytes and the appropriate charge separation of the dissolved species, allowing for their free migration under the electrical field. In nonpolar systems the conductance mechanism may be more... 

Properties of solutions, solubility of salts, their degree of dissociation, and the resulting conductivity of nonaqueous solutions. In this respect, the effect of the solvent mixture and various possible conducting mechanisms should be discussed. 

Electrical conductivity is a critical issue in nonaqueous electrochemistry, since the use of nonaqueous solvents, which are usually less polar than water, means worse electrolyte dissolution, worse charge separation, and, hence, worse electrical conductivity compared with aqueous solutions. In this section, a short course on electrical conductivity in liquid solutions is given, followed by several useful tables summarizing representative data on solution conductivity and conductivity parameters. 

An important approach to forming conductive nonaqueous solutions is the use of solvent mixtures. There are many important highly polar aprotic solvents... 

A nonactive electrode may include noble metals such as gold, silver, and platinum, the so-called sp-metals such as In, Ga, Cd, Bi, as well as transition (or d) metals such as nickel or cobalt. Carbon electrodes and semiconductors such as indium tin oxide [1], diamond [2], and conducting polymers may fall into the category of nonactive electrodes in appropriate solutions, as do composite materials that contain metal oxides or chalcogenides. The behavior of active electrodes in nonaqueous solution is discussed separately in the next chapter. 

Tahon [4] prepared poly(3,4-aIkoxythiophene), (III), derivatives to enhance the conductivity in nonaqueous printing. The process for preparing aqueous and nonaqueous solution dispersions of this agent are described by Louwet [5]. 

Electrochemical measurements (mainly conductances) have been made in both the organic and the pure electrolyte kind of nonaqueous solution for at least two generations. Why, then, is there talk of nonaqueous electrochemistry as one of the frontiers of the field ... 

A nonaqueous solution must be able to conduct electricity if it is going to be useful. What determines the conductivity of a nonaqueous solution Here, the theoretieal principles involved in the conductance behavior of true electrolytes in nonaqueous solvents will be sketched. However, before that, let the pluses and minuses of working with nonaqueous solutions (particularly those involving organic solvents) be laid out. 

On the other hand, the drawbacks of nonaqueous solutions include their lower conductivities and their toxicity and flammability. They need extreme purification and handling under a highly purified inert-gas atmosphere. They may not be exposed to the atmosphere because they will pick up water, which may give rise to the undesired co-deposition of hydrogen. 

The most important concentration range of conductivity studies for these electrolytes is below 10" mol dm". Their most determined enemy is water, which acts as a contaminant. If one considers that 20 ppm of water is equivalent to a 10" mol dm" solution of water in a nonaqueous solvent, it is no surprise that electrochemical quantities recorded in the literature are much less precise than those for aqueous solutions. Conductivities that are said to be as precise as 1% are often 10% in the nonaqueous literature. With materials that react with water (e.g., Li and Na) the water level has to be cut to less than 0.05 ppm and kept there otherwise a monolayer of oxide forms on the metals surfaces. 

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