A new potentiometric method of determining the ionization constant of water based on the ionic liquid salt bridge consisting of N-ethyl-N-heptylpyrrolidinium bis(pentafluoroethanesulfonyl)amide

The ionization constant of water, KW, has been determined potentiometrically by use of an ionic liquid salt bridge (ILSB) consisting of a moderately hydrophobic N-ethyl-N-heptylpyrrolidinium bis(pentafluoroethanesulfonyl)amide at 25°C. The value obtained for pKW = -log10(KW) on the assumption that the liquid junction potential across the ILSB inserted between aqueous ca. 0.001 mol dm−3 solutions of HCl and NaOH is negligible was 13.81 ± 0.01(STDEV), which is smaller than 13.9965, reported about 90 years ago [1,2], based on potentiometry of the cells claimed to be without liquid junction. The present results seem to challenge the unproven assumption of no liquid junction in the so-called Harned cells in that if our Ecell value were greater by 10 mV, pKW would have been in perfect agreement with their 13.99, although it is very difficult to expect such a substantial magnitude of potential difference across the ILSB; logically, one may expect the opposite case, that is, that their value bears ca. 10 mV surplus possibly due to the hidden liquid junctions in the Harned cell. Relevant physicochemical properties of N-ethyl-N-alkylpyrrolidinium+ (alkyl = pentyl, hexyl and heptyl) and its ionic liquids formed in combination with bis(pentafluoroethanesulfonyl)amide− at 25°C are also reported. •The ionization constant of water has been determined potentiometrically at 25°C.•The cell with an ionic liquid salt bridge gave pKW = 13.81 ± 0.01(STDEV).•The value is considerably smaller than pKW = 13.99 by R. S. Harned et al. in 1930s.•The so-called Harned cells may contain hidden liquid junction potentials.•The above discrepancy corresponds to ca. 10 mV at the HCl|NaOH interface.