Stability of a Ag/AgCl Reference Electrode Equipped with an Ionic Liquid Salt Bridge Composed of 1-Methyl-3-octylimidazolium Bis(trifluoromethanesulfonyl)-amide in Potentiometry of pH Standard Buffers

The stability of a Ag/AgCl reference electrode equipped with a gelled ionic liquid, 1-methyl-3-octylimidazolium bis(trifluoromethanesulfonyl)amide (C8mimC1C1N), as a salt bridge, was examined in the potentiometry of pH standard solutions. The variation in the liquid junction potential (LJP) of the ionic liquid (IL)-type reference electrode, measured with respect to a double junction-type KCl reference electrode, was within 1 mV when one standard solution was replaced by another, except for the phthalate standard. The time course of the potential of the IL-type reference electrode showed a standard deviation of ±0.3 mV in all buffer solutions. The reproducible deviation of the potential of the IL-type reference electrode in the phthalate pH standard amounted to 5 mV. The deviation is due to the partition of the hydrogen phthalate in the C8mimC1C1N, influencing the phase boundary potential (PBP) across the interface between C8mimC1C1N and the phthalate standard. If a citrate standard is used instead of the phthalate buffer, the IL salt bridge works satisfactorily as a salt bridge for a reference electrode suitable for potentiometoric pH measurements.