Results obtained measuring the Zn solutions at different concentration
with the techniques previously described are summarised in
Table 1, which shows R2, sensitivity (for each 100 μg/L of Zn concentration)
and relative standard deviation (RSD) in order to demonstrate
the repeatability and reproducibility of the measurements.
Spectral responses for capacitance and resistance at the investigated
frequencies (20 Hz–20 kHz) for Zn concentrations 0–100 mg/L are illustrated
in Figs. S1a and S2a: it is notable how the capacitance and
resistance reflect an inverted behaviour. The capacitance increases how
the Zn concentration and the specific conductivity increase (Table S1),
conversely the resistance decreases. Moreover, the Cp decreases moving
towards higher frequencies, contrariwise the Rp remains stable. The
part of the spectrum that shows a higher relation between these two
parameters and Zn concentration was identified between 200 and
600 Hz (magnified in Figs. S1b and S2b). Two frequency were selected
as representative of this this frequency range for assessing the correlation
between Cp and Rp and Zn ion concentration (250 Hz and 500 Hz,
respectively).