Some Common Nonaqueous Solvents

Although water is used as a solvent more extensively than any other liquid, other solvents may offer some important advantages. For example, if any base stronger than OH- is placed in water, it reacts with water to produce OH-. If it is necessary to use a stronger base than OH- in some reaction, the best way is to use a solvent that is more basic than water because the anion from that solvent will be 331 

One of the disadvantages of using a nonaqueous solvent is that in most cases ionic solids are less soluble than in water. There are exceptions to this. For example, silver chloride is insoluble in water, but it is soluble in liquid ammonia. As will be illustrated later, some reactions take place in opposite directions in a nonaqueous solvent and water. 

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Table 10.1 Properties of Some Common Nonaqueous Solvents. ...

When an acid dissociates, as expressed by the relation HA A -i- H+, a major concern is the extent to which the solvent stabilizes the species involved, and in particular the ionic species A and H+, since these species are normally more strongly affected. This stabilization will depend on both the nature of the solute and the characteristics of the solvent, such as its dielectric constant and whether it is protic or nonprotic. Solvents that stabilize ions poorly will drive the equilibrium to the left, decreasing the acidity and increasing the pK. Because water is an exceptionally good solvent for ionic species, the pK s observed in almost all other solvents are ligher than those in water. A list of several properties of water and some common nonaqueous solvents is given in Table 8.1. 

Use an ideal value for the van t Hoff factor unless the question clearly indicates to do otherwise, as in the following example and in some of the end-of-chapter exercises. For a strong electrolyte dissolved in water, the ideal value for its van t Holf factor is listed in Table 14-3. For nonelectrolytes dissolved in water or any solute dissolved in common nonaqueous solvents, the van t Hoff factor is considered to be 1. For weak electrolytes dissolved in water, the van t Hoff factor is a little greater than 1. 

A major class of nonaqueous solvents is the fixed oils. The USP [1] recognizes the use of fixed oils as parenteral vehicles and lists their requirements. The most commonly used oils are corn oil, cottonseed oil, peanut oil, and sesame oil. Because fixed oils can be quite irritating when injected and may cause sensitivity reactions in some patients, the oil used in the product must be stated on the label. 

Stability limits for some nonaqueous solvents commonly used in lithium-based battery research. Despite the inconsistency created by the varying measurement conditions, these data express a general trend that we have discussed in section 2 that is, carbonates and esters are more anodically stable, while ethers are more resistant to cathodic decompositions. 

Some Practical Considerations in the Use of Salt Bridges. Salt bridges are most commonly used to diminish or stabilize the junction potential between solutions of different composition and to minimize cross-contamination between solutions. For example, in working with nonaqueous solvents an aqueous reference electrode often is used that is isolated from the test solution by a salt bridge that contains the organic solvent. However, this practice cannot be recommended, except on the grounds of convenience, because there is no way at present to relate thermodynamically potentials in different solvents to the same aqueous reference-electrode potential furthermore, there is a risk of contamination of the nonaqueous solvent by water. 

As a result, special techniques are often required in nonaqueous solvent chemistry. The physical properties of some of the commonly used nonaqueous solvents are shown in Table 5.4, and solubilities of inorganic compounds in some solvents have been described in Chapter 4. 

Proteins are generally not directly soluble in the common nonpolar solvents, or in the usual polar solvents such as alcohol and acetone. If they were, much more extensive studies of nonaqueous solutions of proteins would probably have been carried out long ago. However, they are directly soluble in strongly protic solvents, as had been discovered early in the history of protein chemistry, and in some polar solvents which have only relatively recently become commercially available. Furthermore, by... 

Between 1879 and 1885 Kohlrausch made a number of determinations of the Ao values, and observed that they exhibited certain regularities. Some values are given in Table 6.2 for corresponding sodium and potassium salts. We see that the difference between the molar conductivities of a potassium and a sodium salt of the same anion is independent of the nature of the anion. Similar results were obtained for a variety of pairs of salts with common cations or anions, both in aqueous and nonaqueous solvents. 

Ultrasonic techniques are so numerous that a comprehensive discussion is not possible. Since an ultrasonic wave is an adiabatic pressure wave, in general both a temperature and a pressure perturbation of the system occurs. In most nonaqueous solvents, the temperature perturbation is of primary importance, because chemical equilibria are generally much more sensitive to temperature changes than to pressure changes. In aqueous solutions, however, the pressure perturbation is usually of primary importance, because the thermal-expansion coefficient of water is very small, so that the pressure wave is almost isothermal. A serious disadvantage of ultrasonic methods is that rather large volumes of solution are required for low-frequency measurements and relatively high concentrations (> 10" M) of reactants are required at all frequencies. Recent experimental innovations have alleviated these problems to some extent. The most common ultrasonic... 

Table 7.2 presents some thermodynamic data for the solution of methane in water and in nonaqueous solvents. The most outstanding feature is the relatively large negative entropy and enthalpy of solution in water. In all of the following theoretical developments, we refer to the values of zl5 °(II) and Zl// °(II) as the standard entropy and enthalpy of solution, respectively. The connecting relations to the more common standard states are given in Section 4.11. 

Lithium batteries use nonaqueous solvents for the electrolyte because of the reactivity of lithium in aqueous solutions. Organic solvents such as acetonitrile, propylene carbonate, and dimethoxyethane and inorganic solvents such as thionyl chloride are typically employed. A compatible solute is added to provide the necessary electrolyte conductivity. (Solid-state and molten-salt electrolytes are also used in some other primary and reserve lithium cells see Chaps. 15, 20, and 21.) Many different materials were considered for the active cathode material sulfur dioxide, manganese dioxide, iron disulfide, and carbon monofluoride are now in common use. The term lithium battery, therefore, applies to many different types of chemistries, each using lithium as the anode but differing in cathode material, electrolyte, and chemistry as well as in design and other physical and mechanical features. 

In some cases, the electrode material is the limiting factor of the electrochemical stability window. In a metal salt solution, underpotential deposition (UPD) may occur. In some examples, such as gold or platinum electrodes in the presence of lithium ions, the UPD appears at potentials that are substantially higher than the bulk metal deposition [4-6], In addition, some metals may possess catalytic activity for specific reduction or oxidation processes [7-12], Many nonactive metals (distinguished from the noble metals), including Ni, Cu, and Ag, which are commonly used as electrode materials, may dissolve at certain potentials that are much lower than the oxidation potentials of the solvent or the salt. In addition, some electrode materials may be catalytic to certain oxidation or reduction processes of the solution components, and thus we can see differences in the stability limits of nonaqueous systems depending on the type of electrode used. 

In reviewing the intrinsic electrochemical behavior of nonaqueous systems, it is important to describe reactions of the most common and unavoidable contaminants. Some contaminants may be introduced by the salts (e.g., HF in solutions of the MFX salts M = P, B, As, etc.). Other possible examples are alcohols, which can contaminate esters, ethers, or alkyl carbonates. We examined the possible effect of alcoholic contaminants such as CH3OH in MF and 1,2-propylenegly-col at concentrations of hundreds of ppm in PC solutions. It appears that the commonly used ester or alkyl carbonate solvents are sufficiently reactive (as described above), and so their intrinsic reactivity dominates the surface chemistry if the concentration of the alcoholic contaminant is at the ppm level. We have no similar comprehensive data for ethereal solutions. However, the most important contaminants that should be dealt with in this section, and which are common to all of these solutions, are the atmospheric ones that include 02, H20, and C02. The reduction of these species depends on the electrode material, the solvent used, and their concentration, although the cation plays the most important role. When the electrolyte is a tetraalkyl ammonium salt, the reduction products of H20, 02 or C02 are soluble. As expected, reduction of water produces OH and... 

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