Non-aqueous and Mixed Solvents

The formation rate constant for the nickel monoammine complex in methanol-water mixtures (at 25 °C and ionic strength O.IOM) varies as follows  

In order to interpret quantitatively the variation in kei with solvent composition, the relative concentrations of the individual solvated Ni + species at the various compositions are evidently required. Water is a strongly preferred ligand to methanol and it was found that, for reasonably low nickel-ion concentrations, only the nickel species containing zero, one, and two methanol molecules need be considered in methanol-water 

The rates of complex formation of Ni + with SCN and the murexide anion (2) in DMSO have been reported and compared with the rate of the corresponding reaction with bipy. All three complex-formation rates 

If this is the correct interpretation of the low rate constants it should be possible to increase the overall rate of complex formation by diluting the DMSO with an unreactive solvent of similar dielectric constant [thereby displacing the first step in equation (1) to the right]. When nitromethane was used as a diluent the rate was enhanced by a factor of more than ten at the lowest DMSO concentrations used (2M). Such an observation would be difficult to explain on the basis of the normal mechanism. The authors suggest that the reason for the presence of a kinetically detectable intermediate in this case might be steric. 

A different approach to the mechanism of complex formation in non-aqueous solvents has been developed by Bennetto and Caldin. They point out that, even for reactions in aqueous solution, equation (2) 

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The exchange reaction between Fe(III) and Fe(II) in non-aqueous and mixed solvents... 

The separation of a reactant system (solute) from its environment with the consequent concept of solvent or surrounding medium effect on the electronic properties of a given subsystem of interest as general as the quantum separability theorem can be. With its intrinsic limitations, the approach applies to the description of specific reacting subsystems in their particular active sites as they can be found in condensed phase and in media including the rather specific environments provided by enzymes, catalytic antibodies, zeolites, clusters or the less structured ones found in non-aqueous and mixed solvents [1,3,6,8,11,12,14-30],... 

The above conceptual and operational pH definitions for solutions in non-aqueous and mixed solvents are very similar to those for aqueous solutions [16]. At present, pH values are available for the RVS and some primary standards in the mixtures between water and eight organic solvents (see 5 in Section 6.2) [17]. If a reliable pH standard is available for the solvent under study, the pH can be determined with a pH meter and a glass electrode, just as in aqueous solutions. However, in order to apply the IUPAC method to the solutions in neat organic solvents or water-poor mixed solvents, there are still some problems to be solved. One of them is that it is difficult to get the RVS in such solvents, because (i) the solubility of KHPh is not enough and (ii) the buffer action of KHPh is too low in solutions of an aprotic nature [18].8) Another problem is that the response of the glass electrode is often very slow in non-aqueous solvents,9 although this has been considerably improved by the use of pH-ISFETs [19]. Practical pH measurements in non-aqueous solutions and their applications are discussed in Chapter 6. 

This pH definition for non-aqueous and mixed solvent systems is practically the same as that for aqueous solutions (Section 6.2.1). Thus, if a pH standard is available for the solvent or mixed solvent under study, the glass electrode is calibrated with it and then the pH of the sample solution is measured. The pHRVs values for 0.05 mol kg-1 KHPh have been assigned to aqueous mixtures of eight organic solvents (see 5 for pHRVs at 25 °C). Although they are for discrete solvent compositions, the pHRVs in between those compositions can be obtained by use of a multilinear regression equation [14b],... 

Galus, Z., in Advances in Electrochemical Science and Engineering, Vol. 2 (Eds H. Gerischer, C.W. Tobias), VCH, Weinheim, 1994, pp. 217-295. Thermodynamics and kinetics of electrode reactions in non-aqueous and mixed solvents. 

Case, B. and Parsons, R. 1967, The real free energies of solvatation of ions in some non-aqueous and mixed solvents. Trans. Faraday Soc., 63,1224-1239. 

Marcus Y (1986) Thermodynamics of transfer of single ions from water to non-aqueous and mixed solvents, 4. The selection of the extrathermodynamic assumptions. Pure Appl Chem 58 1721— 1736... 

It is, of course, natural from many points of view that aqueous solutions have been in the foreground for studies of electrolyte solutions, while studies of halide ion quadrupole relaxation in non-aque-ous solvents are quite few. However, studies of non-aqueous and mixed solvent systems are in certain respects highly relevant. For example, in order to test relaxation theories the possibility of making marked changes in solvent dipole moment, molecular size, dielectric constant, solvation number etc. should be very helpful. Also, the elucidation of certain general aspects of interactions and particle distributions in electrolyte solutions may be more easily achieved for non-aqueous systems. One such point is ion-pair formation, which for simple salts is not of great importance in water. Finally, of course, the quadrupole relaxation method may, as for aqueous solutions, be applied to more special problems such as ion solvation, complex formation etc. In studies of preferential solvation phenomena disorder effects in the first sphere may in certain cases be expected to lead to dramatic changes in the quadrupole relaxation rate. 

Fig. 5.12. Br transverse relaxation rates (from line widths) in non-aqueous and mixed solvent systems as a function of electrolyte concentration. Systems studied are denoted ...

Although the aims of the book are essentially practical, it also deals in some detail with those theoretical aspects considered most helpful to an understanding of buffer applications. We have cast our net widely to include pH buffers for particular purposes and for measurements in non-aqueous and mixed solvent systems. In recent years there has been a significant expansion in the range of available buffers, particularly for biological studies, largely in consequence of the development of many zwitterionic buffers by Good et al. (1966). These are described in Chapter 3. 

Application of glass electrodes for pH-measurements in non-aqueous media caused some controversies in the past " and in more recent literature. All the above remarks, related to accuracy and repeatability of results in non-aqueous and mixed-solvent media testify well on account on its applicability in such media. Some troubles and the non-compatibility of the results obtained with the expected ones may arise when the organic solvent (e.g. acetonitrile) is aggressive towards the electrode" or a solute tested. 

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