Environmental Engineering Reference
In-Depth Information
Fig. 6.8
Schematic diagram of the calibration cell for DO MEA characterization. A picoammeter
was used to measure current directly with respect to a commercial Ag/AgCl reference electrode
Therefore, this characterization range represents all soluble water in the environ-
ment. For example, DO concentration in tap water measured using a commercial
DO electrode under normal laboratory conditions (25
◦
C) was ~17% O
2
or ~7 mg/L.
A cyclic voltammetry test was performed in two different DO concentrations of
saline solution at a scan rate of 10 mV/s (Fig. 6.9a). Pure nitrogen gas was used
to prepare the 0 mg/L (0% O
2
) in the saline solution, and the 8.7 mg/L (21%
O
2
) in the saline solution at ambient temperature was used in comparison. Initial
and switching potentials were 0.2 V and -1.2 V, which were applied to a recessed
DO working microelectrode. The cyclic voltammogram shows good oxidation and
reduction curves to verify the reliable working electrode and also confirms that the
applied bias of -750 mV was appropriate. The 0 mg/L saline solution curve is lower
due to the lack of oxygen. It is clear that the measured current is proportional to the
DO concentration.
Response of a sensor as a function of DO concentration is shown in Fig. 6.9b.
Calibration curve shows a linear relationship between the current response and the
DO concentration, with high correlation coefficients. The sensor performed linearly,
and exhibited a high sensitivity of ~200 pA/mg/L in saline. Standard deviations in
these measurements were very low, indicating low variability and high stability. The
time for 90% response (t
90
) was typically less than 20 s, which is much shorter than
that of macroscale commercial oxygen electrodes. Figure 6.9c illustrates response
of a sensor to a challenge with three different concentrations of DO. These charac-
teristics are a substantial improvement over the previously reported microelectrodes
constructed from pulled glass pipettes which exhibited similar response times but
