Self-ionizing solvents

The solvents that are leveling to both acids and bases are self-ionized solvents, e.g., water, ammonia, alcohols, carboxylic acids, nitric... 

Self-ionizing solvents possessing both acid and base characteristics e.g. water) are designated amphiprotic solvents, in contrast to aprotic solvents, which do not self-... 

A common feature of self-ionizing solvents are the amphoteric properties of the solvent molecules, since they may act both as EPD and as EPA (27). It appears therefore that outer-sphere effects, similar to those existing between polarized solvent molecules of solvated ions, occur in the pure liquids resulting in mutual polarization of solvent molecules within associated units. From the functional point of view this is described in the following way ... 

To conclude this section, it should be emphasized that the solvosystem concept is restricted to self-ionizing solvents it is, therefore, not suitable for completely describing ionic solvents and those which have no ionizing ability. 

In a self-ionizing solvent, an acid is a substance that produces the cation characteristic of the solvent, and a base is a substance that produces the anion characteristic of the solvent. 

Halide donor-acceptor reactions (of XX ) are generally those in which X is donated to or accepted from an interhalogen. They include self-ionization reactions such as that of BrF3 shown in Equation (18.53). This property makes bromine trifluoride a common aprotic (without protons) self-ionizing solvent. In addition to its self-ionization, BrF3 readily accepts fluoride ions from other sources, such as alkali-metal fluorides, to produce salts containing the bromine tetrafluoride ion, as shown in Equation (18.54). Conversely, it can donate fluoride ions to produce salts containing the bromine difluoride cation, as shown in Equation (18.55). 

Experimental determinations of the conducting properties of electrolyte solutions are important essentially in two respects. Firstly, it is possible to study quantitatively the effects of interionic forces, degrees of dissociation and the extent of ion-pairing. Secondly, conductance values may be used to determine quantities such as solubilities of sparingly soluble salts, ionic products of self-ionizing solvents, dissociation constants of weak acids and to form the basis for conductimetric titration methods. 

Amphiprotic Self-ionizing solvent possessing both characteristics of Bronsted acids and bases, for example H2O and CH3OH, in contrast to aprotic solvent. ... 

Molten I2CI6 has been much less studied as an ionizing solvent because of the high dissociation pressure of CI2 above the melt. The appreciable electrical conductivity may well indicate an ionic self-dissociation equilibrium such as... 

In contrast to this, consider next a solution of sodium acetate. From vSec. 09 we know that in such a solution the thermal agitation raises a certain number of protons from the solvent molecules to the vacant proton levels of the (CH GOO) ions. In the aqueous solution of such a salt, this process is known as the hydrolysis of the salt and is traditionally regarded as a result of the self-ionization of the water. In Fig. 36, however, it is clear that in the proton transfer... 

The molecules of amphiprotic solvents which are the most important will be designated as SH. Self-ionization occurs to a small degree in these solvents according to the equation... 

The activity of the solvent molecule HS in a single-component solvent is constant and is included in Kus. The concentration of ions is mostly quite low. For example, self-ionization occurs in water according to the equation 2H20— H30+ + OH". The conductivity of pure water at 18°C is only 3.8 X 10"8 Q"1 cm-1, yielding a degree of self-ionization of 1.4xl0"19. Thus, one H30+ or OH" ion is present for every 7.2 x 108 molecules of water. Some values of Kus are listed in Table 1.5 and the temperature dependence of the ion product of water Kw is given in Table 1.6. 

Table 1.5 Self-ionization constants of solvents. (According to B. Tremillon)... 

If the solvent is not protogenic but protophilic (acetone, dioxan, tetrahydrofuran, dimethylformamide, etc.), self-ionization obviously does not occur. Consequently, the dissolved acids are dissociated to a greater or lesser degree but dissolved bases do not undergo protolysis. Thus, there can exist only strong acids but no strong bases in these solvents. The pH is not defined for a solution that does not contain a dissolved acid (i.e. in the pure solvent or in the solution of a base). The pKA value can be defined but not... 

It will be seen from these examples that the process of self-ionization in a protonic solvent involves the transfer of a proton from one solvent molecule to another. Thus, the solvent is acting simultaneously as a Lowry-Bronsted acid and as a base. 

It is very instructive to compare the kinetics and plausible mechanisms of reactions catalyzed by the same or related catalyst(s) in aqueous and non-aqueous systems. A catalyst which is sufficiently soluble both in aqueous and in organic solvents (a rather rare situation) can be used in both environments without chemical modifications which could alter its catalytic properties. Even then there may be important differences in the rate and selectivity of a catalytic reaction on going from an organic to an aqueous phase. TTie most important characteristics of water in this context are the following polarity, capability of hydrogen bonding, and self-ionization (amphoteric acid-base nature). 

Aurothiopropanolsulfonate, 36 18-19 [Au6(nPT) ], 40 444-445 Autoprotolysis, see Solvents, self-ionization Autotrophic bacteria, 36 105 growth, bacterial, 45 359-362 Axial ligands, substitution properties in quad-ruply bridged dinuclear complexes, 40 232-234... 

Differentiating solvents are solvents in which neither the acidity of acids nor the basicity of bases is limited by the nature of the solvent. These solvents are not self-ionized. The aliphatic hydrocarbons and the halogenated hydrocarbons are such solvents. 

Fortunately, many of the common solvents by themselves are capable of acting as acids and bases. These amphoteric or amphiprotic solvents undergo self-ionization [e.g., Eqs. (1.2) and (1.3)], which can be formulated in a general way as in Eq. (1.4). 

Autoprotolysis constant — The ion-product calculated from the ion activities of the conjugate acidic and basic species of an -> amphiprotic solvent (SH). The chemical equation of such self-ionization reactions can be schematized as 2HS H2S+ + S , where H2S+ is the conjugate cation, S the conjugate anion. The autoprotolysis constant can be formulated as JCauto = [H2S+] ... 

Ion product — A temperature-dependent constant related to pure substances that can dissociate forming ions and remain in equilibrium with them. It is the product of the ion activities raised to the stoichiometric coefficients of such ionic species in former pure substance. Since the concentration of the pure substance is practically a constant, it is not included in this equilibrium expression. Common pure substances characterized by an ion-product constant are -> amphiprotic solvents, and those salts that are partially dissolved in a given solvent. In the latter case, the ion product is synonymous with solubility product. The following table (Table 1) summarizes self-ionization ionic products and - autoprotolysis constants of some - amphiprotic solvents [i]. 

The pure liquid (bp -10°C) is a useful nonaqueous solvent despite its low dielectric constant (—15), and lack of any self-ionization. It is particularly useful as a solvent for superacid systems. 

Some ionizing solvents are of major importance in analytical chemistry whilst others are of peripheral interest. A useful subdivision is into protonic solvents such as water and the common acids, or non-protonic solvents which do not have protons available. Typical of the latter subgroup would be sulphur dioxide and bromine trifluoride. Non-protonic ionizing solvents have little application in chemical analysis and subsequent discussions will be restricted to protonic solvents. Ionizing solvents have one property in common, self-ionization, which reflects their ability to produce ionization of a solute some typical examples are given in table 3.2. Equilibrium constants for these reactions are known as self-ionization constants. 

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