Conductance in nonaqueous solvents

The principles governing conductivity in nonaqueous solvents are the same as those for aqueous solutions, of course. The dependence of the conductivity on the viscosity of the solvent was discussed in Section 31.11. However, in solvents having low dielectric constants, there is a lessening of the degree of ionization of many substances. Electrolytes that are completely dissociated in water may be only partially dissociated in a low dielectric constant solvent. Hydrochloric acid is completely dissociated in water HCl is a strong acid. In ethyl alcohol, however, HCl is a half-strong acid, with a dissociation constant of about 1.5 X 10.  

Suppose we compare the energy of interaction of two ions having charges +ze and — ze at a distance r in a medium of dielectric constant e. This energy is 

is large (in HjO, e, 80), the ions must come rather close together before the energy of interaction becomes appreciable. If we choose ethyl alcohol (e, 24), then at the same 

The large ionic interaction often renders the Onsager equation useless (it is still presumably correct) for the extrapolation to obtain A . The solutions for which the Onsager relation is valid are so dilute that it is not possible to obtain reliable measurements of their conductivity. In these cases, special methods of obtaining A are used. If the electrolyte is weakly dissociated, then the A can be obtained by application of the Ostwald dilution law, modifying it in precise work to correct for the interionic forces. 

In solvents of low dielectric constant, ion association occurs. The appearance of ion pairs A B and ion triplets A B A and B A B results in a very rapid variation of conductivity with concentration. 

---

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. ... 

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... 

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. 

Solubility and high conductivity in nonaqueous solvents and polyether solids. 

Reactions can be conveniently monitored by H NMR spectroscopy or UV/vis spectrophotometry, and when conducted in nonaqueous solvents proceed without detectable core degradation. In aqueous solution at neutral and basic pH values, ligand substitution seems to be the first step in hydrolytic core degradation (68, 69). Therefore, when these reactions are carried out in an aprotic solvent, or in an aprotic-aqueous solvent mixture. 

It is important that the surface of a glass membrane be swollen before measurements are taken. Prior to first use the electrode should therefore be allowed to swell overnight in a 3 mol/L KCl solution. For the same reason the electrodes should also be stored in a solution of this type between measurements and after measurements have been conducted in nonaqueous solvents. Here, the choice of a double junction reference electrode with the same solvent as in the mea.suring solution and added organic salt for sufficient conductivity is important. In the case of endpoint detection a platinum/ferrocene redox element without a liquid junction could also be used. 

Polymerizations conducted in nonaqueous media in which the polymer is insoluble also display the characteristics of emulsion polymerization. When either vinyl acetate or methyl methacrylate is polymerized in a poor solvent for the polymer, for example, the rate accelerates as the polymerization progresses. This acceleration, which has been called the gel effect,probably is associated with the precipitation of minute droplets of polymer highly swollen with monomer. These droplets may provide polymerization loci in which a single chain radical may be isolated from all others. A similar heterophase polymerization is observed even in the polymerization of the pure monomer in those cases in which the polymer is insoluble in its own monomer. Vinyl chloride, vinylidene chloride, acrylonitrile, and methacryloni-trile polymerize with precipitation of the polymer in a finely divided dispersion as rapidly as it is formed. The reaction rate increases as these polymer particles are generated. In the case of vinyl chloride ... 

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. 

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. 

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... 

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. 

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... 

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. 

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)... 

Bisbipyridylsilver(H) perchlorate has been examined spectroscopically and a square planar structure was postulated (36) it is possible however that a structure of lower symmetry may be the correct one. It is apparently very difficult to prepare bisbipyridyl or phenanthroline complexes of gold(III). The compounds [AuLX2]X and [AuLX2]C104 have been prepared (X = Cl or Br). In nonaqueous solvents the compounds AuLXs have low conductivities this is attributed to the rapid establishment of the following equilibrium ... 

A similar linear relationship between Cu(II/I) potential values and logarithmic Cu L stability constants may exist in nonaqueous solvents, but such a relationship has not been adequately established. Measurements conducted in our laboratories on a variety of polythioether complexes have shown that the Cu L stability constants tend to increase by approximately IO on going from water to acetonitrile, whereas the Cu L stability constants tend to decrease by a similar order of magnitude [54]. These values obviously reflect the preference ofCu(II) to be solvated by water and the corresponding preference of Cu(I) to be coordinated to acetonitrile [111]. 

The polarographic behavior of 1,10-phenanthroline, " 1,7-phenanthroline, and 4,7-phenanthroline has been studied in aqueous solution, but the interpretation of the reduction waves is not always certain because of complications due to adsorption and catalytic hydrogen waves. Some substituted 1,10-phenanthrolines have also been investigated in this way. - Two clear reduction waves were obtained with 1,10-phenanthroline in dimethylformamide, however, and an attempt was made to correlate the reduction potentials with the energy levels of the molecule. Other studies in nonaqueous solvents with 1,10-, 1,7-, and 4,7-phenanthrolines also gave distinct waves. 1,10-Phenanthroline is a semiconductor. Its electrical conductivity is superior to that of phenanthrene and increases on melting. " ... 

In the nonaqueous polymerization processes that are conducted in organic solvent or diluent, many of the carboxyl-containing monomers cited have successfully been used in the preparation of ASNE and HASNE ASTs. Reactivity ratios, rates of reaction, monomer-polymer solubilities, economics, and degree of polymerization were among the criteria considered in monomer selection. However, in the important aqueous processes of suspension and particularly emulsion polymerization, the water solubility and hydrophilicity of the carboxylic monomer was found to be of considerable importance. Fordyce and Ham (13) observed that a significant portion of itaconic acid polymerized in the aqueous phase when emulsion polymerization was carried out with styrene. Fordyce et al. (9, 10) reported that... 

The site of protonation of phospholes in nonaqueous solvents by dry HCl or CFjSOjH has been found to be the phosphorus atom when the reaction is conducted at — 90°C <86JCS(P2)629> in pyrroles it is a ring carbon that undergoes protonation. The 1 //-phospholium ions are not stable with Cl as counterion, and consequences of the P-protonation by HCl are discussed in Section 2.15.5.3. When the counterion is nonnucleophihc, the l//-phospholium ions are sufficiently stable to be directly observable <71JHC877, 72CJC1008>. 

The thermodynamic affects of complexation of 25, its monomeric component and 23 with metal ions monitored through NMR studies in nonaqueous solvents also show that in these conjugates, the positions of the pyridyl nitrogen and ethereal oxygen play a primary role in their hosting abilities for metal cations and as the distance of N and O increases, the ability of the ligand to coordinate decreases. Thus, 23 was able to interact with alkali metal cations, but this ability is lost for 25. The conductance measurements show that for all cations except Hg°, the composition of complexes of 25 is 1 1. However, 25 hosts two Hg° cations (2005JPC(B)14735). 

To a good approximation, there is an inverse relationship between the viscosity and the resulting current doubling the buffer viscosity will halve the current and ATRadiai- In nonaqueous solvents, electrical currents are generally lower than that for the same electrolyte dissolved in water, but predicting ATRadiai becomes complex as one needs to take into account variations in the mobilities of species, dissociation equilibria, dielectric constant, zeta potential, and thermal conductivity [18]. 

---