Ions, absolute properties

Absolute properties are distinguished from conventional properties i.e., a generic standard partial molal property of an aqueous ion j (H°) is related to the absolute property by... 

It is possible to effect some simplification in the equations defining the thermodynamic properties of the ions by introducing additional conventions (a convention can be defined somewhat facetiously as a convenient assumption that we know is not true). If, for example, we decide that the absolute free energies and enthalpies of all pure elements are to be set at zero, then the defining equation for free energies and enthalpies (equation 17.21) becomes the same as that for S, V, and Cp (equation 17.22). If in addition we define all properties of the hydrogen ion as zero, then the conventional ionic properties become the same as the corresponding absolute properties, and we could have stopped at equation (17.19). 

Conventional IMS includes methods based on absolute ion transport properties that could be measured using a time-independent electric field. 

The problem of the validity of methods for obtaining the so-called absolute property values of individual aqueous ions was discussed by Conway (1978) and more recently by Marcus (2008a) and by Hiinenberger and Reif (2010). These issues are treated in the following sections dealing with the properties of aqueous ions. 

If the properties F of ions of opposite charge are to be compared, it is necessary to use the so-called absolute property values of individual ionic species. These are also needed for comparison with and validation of theoretical values of these... 

Given the diversity of different SCRF models, and the fact that solvation energies in water may range from a few kcal/mol for say ethane to perhaps 100 kcal/mol for an ion, it is difficult to evaluate just how accurately continuum methods may in principle be able to represent solvation. It seems clear, however, that molecular shaped cavities must be employed, the electiostatic polarization needs a description either in terms of atomic charges or quite high-order multipoles, and cavity and dispersion terms must be included. Properly parameterized, such models appear to be able to give absolute values with an accuracy of a few kcal/mol." Molecular properties are in many cases also sensitive to the environment, but a detailed discussion of this is outside the scope of this book. ... 

The direct access to the electrical-energetic properties of an ion-in-solution which polarography and related electro-analytical techniques seem to offer, has invited many attempts to interpret the results in terms of fundamental energetic quantities, such as ionization potentials and solvation enthalpies. An early and seminal analysis by Case etal., [16] was followed up by an extension of the theory to various aromatic cations by Kothe et al. [17]. They attempted the absolute calculation of the solvation enthalpies of cations, molecules, and anions of the triphenylmethyl series, and our Equations (4) and (6) are derived by implicit arguments closely related to theirs, but we have preferred not to follow their attempts at absolute calculations. Such calculations are inevitably beset by a lack of data (in this instance especially the ionization energies of the radicals) and by the need for approximations of various kinds. For example, Kothe et al., attempted to calculate the electrical contribution to the solvation enthalpy by Born s equation, applicable to an isolated spherical ion, uninhibited by the fact that they then combined it with half-wave potentials obtained for planar ions at high ionic strength. 

Early attempts to purify the enzyme brought the quick realization that aconitase is easily inactivated (6,7). In the early 1950 s Dickman and Qoutier (8,9) found that inactivated aconitase could be reactivated by incubation with iron and a reduc-tant. From kinetic analyses of the iron and reductant effects on enzyme activity, Morrison argued that both formed Michaelis-Menten complexes wiA the enzyme (10). This refuted the earlier idea that the sole role of the reductant was to maintain iron in a reduced state (9). Of several metal cations tried, only ferrous ion was found to be capable of this reactivation process (8,11). Because of the absolute requirement for iron in activation, the known chelation properties of citrate, and Ogston s 3-point attachment proposal, Speyer and Dickman proposed that the active site iron provides three coordination sites for substrate binding - one for hydroxyl and two for carboxyl groups (12). 

It is known that relatively subtle solvent properties (. . the presence of trace metal ions or dissolved oxygen) can have a pronounced effect on Tj values (1 ). For this reason, we have emphasized studies based on comparing relative Tj values of resonances taken from the same spectrum of a given compound, rather than comparing absolute Tj values taken from different spectra. To insure reproducibility, duplicate Tj determinations were made in all cases. Monomer Tj values (e.g. methyl a-D-gluco-pyranoside) can be obtained in less than an hour. However, we have experienced difficulty in obtaining consistent absolute Tj values for successive samples of the same monosaccharide. Such reproducibility problems have not been observed for the polysaccharides, and we have observed no successive Tj value differences which can be attributed to solvent or sample preparation. 

In a perfect crystal, all atoms would be on their correct lattice positions in the structure. This situation can only exist at the absolute zero of temperature, 0 K. Above 0 K, defects occur in the structure. These defects may be extended defects such as dislocations. The strength of a material depends very much on the presence (or absence) of extended defects, such as dislocations and grain boundaries, but the discussion of this type of phenomenon lies very much in the realm of materials science and will not be discussed in this book. Defects can also occur at isolated atomic positions these are known as point defects, and can be due to the presence of a foreign atom at a particular site or to a vacancy where normally one would expect an atom. Point defects can have significant effects on the chemical and physical properties of the solid. The beautiful colours of many gemstones are due to impurity atoms in the crystal structure. Ionic solids are able to conduct electricity by a mechanism which is due to the movement of fo/ 5 through vacant ion sites within the lattice. (This is in contrast to the electronic conductivity that we explored in the previous chapter, which depends on the movement of electrons.)... 

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