PIEZOELECTRIC ADMITTANCE-BASED SENSING OF ELECTROLYTE-SOLUTIONS BY MONTMORILLONITE CLAY FILM-COATED QUARTZ-CRYSTAL OSCILLATORS

The piezoelectric admittance of quartz-crystal oscillators coated with a montmorillonite clay film or a clay/poly(vinyl alcohol) (PVA) compo site film in contact with an aqueous electrolyte solution has been mea sured and the possibility of applying the technique to chemical sensin g examined. The resistance of the equivalent circuit, which has been c onventionally accepted for a piezoelectric quartz crystal, of the day film-coated oscillator evaluated from the admittance measurement incre ases with decreasing Na2SO4 concentration, though the dependency is no t linear. This change in resistance may be ascribed to changes in the density and/or the viscosity of the clay film, which result from the s tructural change of the film induced by the cation. The sensitivity is suppressed by composition of the clay film with PVA. The clay/PVA com posite film-coated oscillator exhibits lower resistance for a cation w ith higher valence; the oscillator can measure the equivalent concentr ation or ionic strength of a cation.