Nonaqueous solvents, conductance

The majority of titrations involving basic analytes, whether conducted in aqueous or nonaqueous solvents, use HCl, HCIO4, or H2SO4 as the titrant. Solutions of these titrants are usually prepared by diluting a commercially available concentrated stock solution and are stable for extended periods of time. Since the concentrations of concentrated acids are known only approximately,the titrant s concentration is determined by standardizing against one of the primary standard weak bases listed in Table 9.7. 

Table 3 shows conductivity of 2mol/dm3 solutions of EMImBF4 and EMImPF6 in a number of molecular solvents. A high increase of conductivity, in comparison to neat ionic liquids, can be observed after dilution with electrically neutral molecular liquids. However, solutions of ionic liquids in molecular liquids are simply conventional solutions of organic salts in nonaqueous solvents, and no distinction can be seen between them and commonly employed solutions of (C2H5)4NBF4. 

CrystalSulf A process which uses a nonaqueous solvent/catalyst system to remove sulfuric acid from high-pressure natural gas. This project, part of the GRI Basic Research programme, has been conducted by Radian Corporation. 

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)... 

Variations of resistance with frequency can also be caused by electrode polarization. A conductance cell can be represented in a simplified way as resistance and capacitance in series, the latter being the double layer capacitance at the electrodes. Only if this capacitance is sufficiently large will the measured resistance be independent of frequency. To accomplish this, electrodes are often covered with platinum black 2>. This is generally unsuitable in nonaqueous solvent studies because of possible catalysis of chemical reactions and because of adsorption problems encountered with dilute solutions required for useful data. The equivalent circuit for a conductance cell is also complicated by impedances due to faradaic processes and the geometric capacity of the cell 2>3( . 

Solubility is highly influenced by the solid-state form (e.g., crystalline or amorphous) of the drug. Rigorous solubility studies using the final solid form (i.e., salt form or crystal form) as a function of temperature (i.e., 25 and 37°C) and pH (range 1 to 7.5) are conducted during preformulation. Solubility in nonaqueous solvents is also screened. Solubility in simulated gastrointestinal fluids is also important. 

Dolye, M., Lewittes, M. E., Roelofs, M. G. and Perusich, S. A. 2001. Ionic conductivity of nonaqueous solvent-swoUen ionomer membranes based on fluorosulfonate, fluorocarboxylate, and sulfonate fixed ion groups. Journal of Physical Chemistry B 105 9387-9394. 

Like LiAsFe, LiBF4 is a salt based on an inorganic superacid anion and has moderate ion conductivity in nonaqueous solvents (Table 3). It was out of favor in the early days of lithium battery research because the ether-based electrolytes containing it were found to result in poor lithium cycling efficiencies, which decayed rapidly with cycle number. ° The reactivity of LiBF4 with lithium was suspected as discoloration occurred with time or heating. 

One major drawback of these sulfonate salts is their poor ion conductivity in nonaqueous solvents as compared with other salts. In fact, among all the salts listed in Table 3, LiTf affords the lowest conducting solution. This is believed to be caused by the combination of its low dissociation constant in low dielectric media and its moderate ion mobil-ityi29 3 compared with those of other salts. Serious ion pairing in LiTf-based electrolytes is expected, especially when solvents of low dielectric constant such as ethers are used. ... 

In nonaqueous solvents based on mixed alkyl carbonates, LiPFe remains one of the most conducting salts. For example, in EC/DMC (1 1) the conductivity is 10.7 mS cm only fractionally lower than that of... 

Electrochemical studies on copper systems are frequently conducted in nonaqueous solvents, principally for the purpose of improving the solubility of complexing ligands of interest. The largest number of such studies have been reported in... 

The effect of the addition of water and molecular solvents such as propylene carbonate, N-methylformamide, and 1-methylimidazole on the conductivity of [C4Cilm][Br] and [C2Cilm][BF4] was measured at 298 K [211]. The mixture of the IL and the molecular solvent or water showed a maximum on the conductivity/mole fraction IL curves. The maximum for nonaqueous solvents was at the level of approximately 18-30 mScm at low mole fraction of the IL and the maximum for water was at level approximately 92-98 mScm [211]. The conductivity of a mixture of these two ILs depends monotonically on the composition. The temperature dependence of the conductivity obeys the Arrhenius law. Activation energies, determined from the Arrhenius plot, are usually in the range of 10-40 kj mol / The mixtures of two ILs or of an IL with molecular solvents may find practical applications in electrochemical capacitors [212]. 

Direct conductivity measurements do not provide a satisfactory index of added water in milk. However, it has been reported (Rao et al. 1970) that measurement of conductivity in nonaqueous solvents can be useful in detecting adulteration. The conductivities of extracts using two different solvent systems were correlated with the percentage of added water in the original milk. One solvent system consisted of 10 ml acetone and 90 ml methanol plus 3 g sodium chloride, and the other contained 2.65 g formic acid in 100 ml acetone. 

Fusion with Alkali and Cupric Oxide in Nonaqueous Solvents. Alkali lignin was fused with potassium hydroxide and cupric oxide in methanol under conditions suggested by Tiemann (20) and in n-amyl alcohol as suggested by Klages (4). These procedures were very effective in earlier model compound studies in our laboratories (12). Ether extracts obtained were less than those from corresponding experiments in aqueous solution, and qualitative compositions were essentially the same. In the case of the amyl alcohol experiments, artifacts with the cupric oxide were obtained. Again, experiments were conducted under more dilute conditions in a bomb under superatmospheric conditions, but results were no better. 

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... 

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. 

Empirical formulas exist to correct for the temperature dependence of the reference potentials in aqueous solution. When one must work in nonaqueous solvents, because of their conveniently large "window," one must add a 0.1 M to 1.0 M salt (see Fig. 11.67) to help conduct current, but there can be a problem with referencing the working electrode potential to a standard electrode. SCE can be used in many nonaqueous solvents, but in some cases such a direct experiment does not work one must use the Ag Ag+ ion... 

Solubility and high conductivity in nonaqueous solvents and polyether solids. 

To the contrary, however, many of the nonaqueous solvents possess lower polarity and thus form electrolyte solutions with a lower conductivity, but have a wider electrochemical window than that of water. - nonaqueous solvents have a wide application in - lithium batteries (both primary and secondary). 

Liquid SO2 has a number of important uses. It has been used as a refrigerant, although ammonia and chlorofluorocarbons have also been used for this purpose. Liquid S02 has been extensively utilized as a nonaqueous solvent (see Chapter 5). Its low electrical conductivity is more likely to be due to the presence of trace amounts of impurities than a very slight degree of ionization. If ionization of liquid SO2 did occur, it could be represented as... 

Selenyl chloride is the most important of the three selenium compounds having the formula SeOX2, and it has been used as a nonaqueous solvent. The liquids have substantial conductivity so it is presumed that some autoionization occurs in both SeOCl2 and SOCl2 ... 

Sulfuric acid has received considerable study as a nonaqueous solvent (see Chapter 5). It is, of course, a strongly acidic solvent, and it has a Kf value of -6.15 °/molal. In 100% H2S04, conductivity measurements and cryoscopic studies show that protonation of many substances occurs even though they are not normally bases in the usual sense. For example, organic compounds such as acetic acid and ether function as proton acceptors ... 

A small degree of autoionization of the XX 3 interhalogens is indicated by their electrical conductivity. Some of these compounds have been extensively used as nonaqueous solvents in which their behavior indicates dissociation as shown in the following case for BrF3 ... 

Regarding hydrochloric acid, in a concentration range of 30.10 4 to 300.10 4 mol/L, equivalent conductance assumes an extremely low and constant value of 0.03 S cm2/mol, as seen in Figure 3. This behavior certainly cannot be explained on the basis of simple dissociation phenomena. Thus we have interpreted these results on the basis of theoretical work by Caruso and co-workers (31) who consider the conductometric, potentiometric, and spectrophotometric behavior of weak acids and bases in nonaqueous solvents. In these solvents a weak acid, HA, besides undergoing simple ionic dissociation, also may undergo conjugation phenomena by the H+ and A" ions which lead to the formation of ionic complex species A(HA)/ or H(HA)/. Caruso shows that the... 

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. 

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. 

So the question of the specific conductivity of nonaqueous solutions vis-a-vis aqueous solutions hinges on whether the dielectric constant of nonaqueous solvents is lower or higher than that of water. Table 4.23 shows that many nonaqueous solvents have fi s considerably lower than that of water. There are some notable exceptions, namely, the hydrogen-bonded liquids. 

In summary, it is the lower dielectric constants of the typical nonaqueous solvent that cause a far greater decrease in equivalent conductivity with an increase of concentration than that which takes place in typical aqueous solutions over a similar concentration range. Even if the infinite-dilution value A makes a nonaqueous electrochemical system look hopeful, the practically important values of the specific conductivity (i.e., the ones at real concentrations) are nearly always much less than those in the corresponding aqueous solution. That is another unfortunate aspect of nonaqueous solutions, to be added to the difficulty of keeping them free of water in ambient air. 

Specific Conductivities of Eiectrolytes in Aqueous and Nonaqueous Solvents at the Same Concentration... 

In the study of nonaqueous electrolytes, the ion-pair effect is a sevae factor affecting ion conduction. The degree of association of salts in nonaqueous solvents (or the solubilizing ability of the different solvents toward the salt) is often estimated by comparing the Walden product, that is, Arf. Justify this method and explain what hypothesis is included and how it holds. (Xu)... 

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