N,N′-dibenzyl-4,13-diaza-18-crown-6
(A) and bis[(benzo-15-crown-5)-4′-ylmethyl] pimelate (B) were tested
as coatings for two piezoelectric crystals for potassium quantification.
Both sensors showed stability, reversibility and sensitivity
characteristics that allowed their use in quantitative analysis. However,
compound A is much more sensitive to potassium than B. Compound A also
shows a larger relative sensitivity for potassium with regard to sodium
than B. A pharmaceutical sample of known composition was analysed both by
an acoustic wave sensor with a crystal coated with compound A and by
conductivity. No statistically significant difference in the median of the
results was found (α = 0.05), although precision is superior
for the conductivity methodology. Performance of the sensor in terms of
frequency stability and selectivity was improved by the incorporation of
PVC, a plasticizer and a lipophilic salt in the coating composition. Limits
of detection found for potassium were 1.92 ppm, or 1.75 ppm for a crystal
with a frequency decrease due to coating of 2.9 kHz or 3.9 kHz,
respectively. Selectivity coefficients (fixed interference 80 ppm) for
potassium over Na, Ca, Al, Zn, Mg, and Fe ranged from 0.103 to 0.332.
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