Acetonitrile autoprotolysis constant

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... 

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

For reductions DMF has a usable potential range comparable to that of acetonitrile, but it is inferior for oxidations. In the presence of inorganic ions their discharge is the cathodic limiting reaction, whereas it is more uncertain whether it is the solvent or the cation that is reduced in solutions of tetraalkylammonium ions. The anodic limiting reaction at a Pt electrode is an oxidation of DMF, which involves the removal of an electron from the amide nitrogen. Its autoprotolysis constant is 29.4. DMF is a better hydrogen atom donor than MeCN and DMSO. 

The autoprotolysis constant is also important in determining the sharpness index of a titration. For a successful titration we want the sharpness index to be large at the equivalence point this corresponds to a large change in pH or potential per increment of titrant, indispensable for the precise location of the point. Since the sharpness index varies inversely with the autoprotolysis constant, it is advantageous to use solvents with small values of Ks. Useful titrations can therefore be made in water or acetonitrile, but they cannot be very successful in pure H2SO4. 

An exceptionally wide range of acid and base strengths can be studied in acetonitrile because its autoprotolysis constant is so low, less than... 

The pJCj values are now available for many hydride complexes. Extensive tables have been compiled recently by Bullock and by Tilset. The rate of proton transfer to and from transition metals is rather slow (see below), so it is often possible to detect separate NMR signals for M-H and M , and tiius to determine the position of proton transfer equilibria between hydride complexes (M-H) and bases (B), or metal bases (M") and organic acids (HA). The pX values in Table 3.1 have been obtained in acetonitrile, an excellent solvent for acid-base chemistry because it solvates cations well enough to minimize ion pair formation it is both a weak acid and a weak base, with a very low autoprotolysis constant (ion product). ... 

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