Solvents autoprotolysis constants

The dissociation, or autoprotolysis constant for a solvent, SH, relates the concentration of the protonated solvent, SH2, to that of the deprotonated solvent, S . For amphoteric solvents, which can act as both proton donors and proton acceptors, the autoprotolysis reaction is... 

In dilute aqueous solutions (the only ones we consider in this chapter), the solvent, water, is very nearly pure, and so its activity may be taken to be 1. The resulting expression is called the autoprotolysis constant of water and is written Kw ... 

As a result, inert and aprotic solvent toluene is suitable for the titration of weak bases in non-aqueous media as solvent, although benzene which is more carcino-genic aromatic hydrocarbon used widely in literature for non-aqueous titrations. The major advantage of toluene is tliat it does not compete for protons with the reactant in the titrations because of its autoprotolysis constant approaching zero. The major disadvantages of solubility can be removed by using small amount of amphiprotic solvents. 

The recent introduction of non-aqueous media extends the applicability of CE. Different selectivity, enhanced efficiency, reduced analysis time, lower Joule heating, and better solubility or stability of some compounds in organic solvent than in water are the main reasons for the success of non-aqueous capillary electrophoresis (NACE). Several solvent properties must be considered in selecting the appropriate separation medium (see Chapter 2) dielectric constant, viscosity, dissociation constant, polarity, autoprotolysis constant, electrical conductivity, volatility, and solvation ability. Commonly used solvents in NACE separations include acetonitrile (ACN) short-chain alcohols such as methanol (MeOH), ethanol (EtOH), isopropanol (i-PrOH) amides [formamide (FA), N-methylformamide (NMF), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA)] and dimethylsulfoxide (DMSO). Since NACE—UV may present a lack of sensitivity due to the strong UV absorbance of some solvents at low wavelengths (e.g., formamides), the on-line coupling of NACE... 

Autoprotolysis constants exist for any amphiprotic solvent and can be determined from electric conductivity studies of the solvent and solutions. A few examples include ... 

Tab. 1.5 Chemical properties of organic solvents of electrochemical interest. Donor numbers (DN), accepptor numbers (AN), and autoprotolysis constants (p/[Pg.18]

Amphiprotic solvents have both acidic and basic properties in terms of the Bransted acid-base concept. If we denote an amphiprotic solvent by SH, it donates a proton by SH S +H+ and accepts a proton by SH + H+ = SH2. Overall, the autoprotolysis (autoionization) occurs by 2SH = SH2 + ST The extent of autoprotolysis is expressed by the autoprotolysis constant, Kjh = aSH2 aS, the values of which are also included in Table 1.5 as pKSH values (for more details, see Table 6.6). 

Most of the so-called dipolar aprotic solvents have appreciable Lewis base character with donor numbers greater than 10 and autoprotolysis constants smaller than 10 20 (pK > 20).20 They solvate cations better than anions, and radical anions often have appreciable stability in the rigorously purified solvents. 

For each corresponding acid-base pair, e.g., HA and A", the acidity constant Ka and the basicity constant AT are related to each other by the equation Kaut0 = KaKb, where Kaut0 is the - autoprotolysis constant of the solvent. 

The equilibrium can be described with the autoprotolysis constant, i.e., the product of the activities of the species produced as a result of autoprotolysis. For solvents in which no other ionization processes are significant the term is synonymous with ionic product . 

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]. 

Ion product — Table. Autoprotolysis constants (Aauto = [SH J ] - [S ]) of some amphiprotic solvents SH . Values at 25 ° C unless stated otherwise... 

This representation is over-simplified, each of the ions being further solvated in each acid. Autoprotolysis constants have been reported as 3 x 10 13 mol2 kg-2 (0°C) for HF[6], 3.8 x 10 8 (25°C) for HS03F[7] and 7.9 x 1(T7 (25°C) for CF3S03H[8]. Protonic media are made more acidic by addition of an entity which increases the proton concentration. Superacids are themselves so very weakly basic that very few, if any, compounds can act as Bronsted acids to donate protons to the solvent directly. Lewis acids combine with X- to shift the autoprotolysis equilibria to increase the proton concentration. Superacids are rendered basic by direct addition of the X species, the base of the system, (e.g. from an alkali metal compound MX) or by addition of compounds which accept protons from the medium, increasing the concentration of the base X. ... 

Eq. (3-11) reflects both the acidic and basic properties of a solvent, which are described quantitatively by the autoprotolysis constant A auto-... 

Autoprotolysis constants for some representative solvents are collected in Table A-12 (Appendix). The useful pH range of a solvent increases as the autoprotolysis constant decreases. The smaller the autoprotolysis constant, the greater the range of acid or base strengths which can exist in a solvent . 

The autoprotolysis constant The extent of ionization (4-19) of a pure amphiprotic solvent is measured by the autoprotolysis constant SH> defined as the product Since the autoprotolysis reaction results in the formation of both solvent cations and solvent anions, the autoprotolysis constant is a measure of the differentiating ability of a solvent. If a solvent has a large Ash value, the existence in it of a wide range of strengths of either adds or bases is not possible. In contrast, if the autoprotolysis constant is small, adds and bases of varying strengths show titration curves distinctly different from each other. 

Glacial acetic add represents the other extreme from ammonia, that of a solvent strongly addic but weakly basic compared with water. These two characteristics by themselves would cause gladal acetic add to have a relatively high autoprotolysis constant. Nevertheless, owing to the low dielectric constant (6.13), the autoprotolysis constant (p sh = 14.45) turns out to be about the same as that of water. ... 

Glacial acetic acid, though like water in being classed as amphiprotic, represents a solvent type distinctly different from water in that it is a much weaker base. This weak basicity makes it a useful solvent for the titration of weakly basic substances. As mentioned in Section 4-3, the autoprotolysis constant has a p T h value of 14.45 for the reaction... 

Ethylenediamine (en), NH2C2H4NH2, a strongly basic substance, may be considered to represent solvents that are weakly acidic compared with water. Ethylenediamine is therefore useful as a solvent for the titration of weakly acidic substances. It is a leveling solvent for adds whose ionization constants are larger than about 10 in water thus acetic add and hydrochloric acid are leveled to about equal strength. The titrant base normally used in en is sodium ethanolamine. The autoprotolysis constant of en is 5 x 10" for the equilibrium... 

Since the autoprotolysis constant of acetonitrile is small, the SH2 contribution from dissociation of the solvent may be neglected. 

From the practical point of view, nonaqueous solvents can be described broadly as acidic, basic, or neutral. For carrying out titrations, properties such as dielectric constant, melting point, boiling point, and (for amphiprotic solvents. Section 4-3) autoprotolysis constant are important (Table 6-1). 

The extent of the autoprotolysis is a measure of both the acidic and basic strengths of the solvent and is given by the autoprotolysis constant or ionic product for example, for water K p = [H3O+] [OH ] = lO" (25°) and for sulfuric acid Kpp = [H3SOi+] [HSO4-] = 1.7 X 10- (10°). The autoprotolysis constant of sulfuric acid is greater than that for any other solvent that has been studied. Such a large value implies that, in spite of its very high acidity, sulfuric acid must also be appreciably basic. 

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