Molar triple ions

Figure 5. (a) The ( A, SO,) anion symmetric streching mode of polypropylene glycol)- LiCF,SO, for 0 M ratios of 2000 1 and 6 1. Solid symbols represent experimental data after subtraction of the spectrum corre-ponding to the pure polymer. Solid curves represent a three-component fit. Broken curves represent the individual fitted components, (b) Relative Raman intensities of the fitted profiles for the ( Aj, SO,) anion mode for this system, plotted against square root of the salt concentration, solvated ions ion pairs , triple ions, (c) The molar conductivity of the same system at 293 K. Adapted from A. Ferry, P. Jacobsson, L. M. Torell, Electrnchim. Acta 1995, 40, 2369 and F. M. Gray, Solid State Ionics 1990, 40/41, 637. 

In conductimetric studies of triple ion formation, it is often assumed that M2A+ and MA2 have the same formation constants (Kt= Kt+= Kt ) and the same molar conductivities at infinite dilutions (2 = = 2 ). If we denote by a and a, the... 

As mentioned in Section 7.1, if we determine the molar conductivity of an electrolyte as a function of its concentration and analyze the data, we can get the value of limiting molar conductivity A°° and quantitative information about ion association and triple-ion formation. If we determine the limiting molar conductivity of an ion (7 °) by one of the methods described in Section 7.2, we can determine the radius of the solvated ion and calculate the solvation number. It is also possible to judge the applicability of Walden s rule to the ion under study. These are the most basic applications of conductimetry in non-aqueous systems and many studies have been carried out on these problems [1-7]. 

If at much higher molarities, above the conductance minimum, the content of single ions is insignificant in comparison with the concentration of triple ions, then CLiBrLi+ = BrLiBr- and eqn. 5.8.7 approximates to... 

Figure 6 shows the dependence on concentration and temperature of the molar conductivity of 1,2-dimethoxyethane solutions of LiBF4 from infinite dilution to saturation. The plots of A versus show a minimum at moderate concentrations and a maximum at high concentrations. Although the minimum is only weakly dependent on temperature, the maximum exhibits a strong displacement. The minimum is a general feature of bilateral triple-ion formation ... 

The models discussed above hold for aqueous solutions. More complex behaviour is observed in non-aqueous solutions, particularly in media of low polarity. In such cases, extensive ion-pairing is observed. That is, electrically neutral ion-pairs will not contribute to solution conductivity. Conversely, under some conditions charged triple ions can form. The formation of such species is clearly favoured by higher concentrations. Thus, in some cases, molar conductivity can increase with ionic activity due to triple ion formation. Theories have been developed to predict the formation constants of such species (14). 

Kt is the formation constant of triple ions and A°r the infinite dilution molar conductivity of the triple ions. Usually Ka and Kj are obtained from Equation (4.20) by employing an estimated value of A° calculated from Walden s rule (k°p = constant) and assuming that k°r = 2k°/3. 

At salt concentrations below those shown in Figure 18.5, molar conductivity behavior has been identified with the formation of electrically neutral ion pairs [8]. Between concentrations of 0.01 and 0.1 mol (up to an O M ratio of 50 1) the molar conductivity rises and this can be explained by the formation of mobile charged clusters such as triple ions, a progressive dissociation of ion pairs, or a combination of both. Up to O M = 50 1, however, spectral data indicate very little change in the species concentrations, and this may instead indicate an enhancement in ionic mobility. With a charge separation <5 A and polymer motion restricted by ion coordination, an anion-assisted (Grotthus-like) transport mechanism could be envisaged as Equations 18.5 and 18.4. 

Ion conduction behavior in an organic solvent is significantly different from that in aqueous solution. For example, in the case of the LiBp4 electrolyte in an ether system, its infinite dilution molar conductivity (Aq) increases with salt concentration, starting with a minimum at 0.05 M concentration. It then increases to a maximum near 1.0 M and subsequently levels off and even decreases. This behavior is very complex and difficult to explain using the general theory. When the salt concentration is low, solvated lithium ions are formed (Equation 9.4). Triple ions are formed when the concentration is very high (Equations 9.5 and 9.6) in which the solvent is omitted. 

As shown in Figure 10.14, the ionic conductivity of a polymer electrolyte correlates strongly with the salt and its concentration [12]. The molar conductivity increases as the salt concentration is raised from 0.01 to 0.1 mol/L, i.e., the molar ratio of ether O to metal cation (O/M) increases to about 50 1. The charged ions such as triple ions and ion clusters fiuther ionize. When the salt concentration is lower, neutral ion pairs are mainly formed. When the O/M ratio is above 50 1, the concentrations of the ions and other species do not change, and the main process is the increase in ionic mobility. The motion of the polymer chain is apparently limited by ion complexing since the distance between the electric charges is less than 0.5 nm, and may be transferred under the effect of anions as shown in Equations 10.17 and 10.18 ... 

Relative Raman intensity of the symmetrical stretch in the anion (Ai, SO3) mode, and molar conductivity at 30°C with changing concentration of the polypropylene glycol (PPG)-LiCF3 03 system with ions (O), ion pairs ( ), and triple ions ( ). (Adapted from Ferry, A. et al., Electrochim. 

Montmorillonite is the name given to day found near MontmoriUonin in France, whereit was identified by Knight in 1896 (Utracki, 2004). Montmorillonite is a 2 1 layered hydrated aluminosilicate, with a triple-sheet sandwich structure consisting of a central, hydrous alumina octahedral sheet, bonded to two silica tetrahedral sheets by shared oxygen ions (Fig. 3). The unit cell of this ideal structure has a composition [Al2(0H)2(Si205)2]2 with a molar... 

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