The influence of solvent in binary acetonitrile-water solvent systems on dissociation constants of protonated pyridine and its N-oxide

Using a 2,6-dinitrophenol-tetra-n-butylammonium 2,6-dinitrophenolate standa rdizing system, the application of the previously proposed method for pK(a) determination of potentiometric standards in binary solvent systems consis ting of an organic polar solvent (acetonitrile in this case) and water was checked. The reliability of the obtained pK(a) values of 2,6-dinitrophenol was checked in a system of protonated pyridine in acetonitrile water mixtur es over the range of 0 through 1 mole fraction of acetonitrile, for which l iterature data were available. The method checked in this way was subsequen tly used to determine pK(a) of protonated pyridine N-oxide in this binary s olvent system. pK(a) values served for discussion of the variations of pK(a )'s of cationic acids conjugated with pyridine and its Ar oxide and of the influence of composition of the binary solvent.