Environmental Engineering Reference
In-Depth Information
Fig. 6.10
Characterization of phosphate MEA sensor at pH 7.5: (
a
) calibration curve in various
concentrations of KH
2
PO
4
,(
b
) long-term stability test in 10
-3.9
MKH
2
PO
4
,(
c
) the oxygen effect
in 10
-3.9
MKH
2
PO
4
,and(
d
) the stirring effect in 10
-3.9
MKH
2
PO
4
[59]
Electrochemical characterization of the phosphate MEA showed a linear
response over a wide range of concentrations. The data is plotted in Fig. 6.10a. The
sensor exhibited a high sensitivity of about 96 mV per decade of KH
2
PO
4
concen-
tration in the 10
-5.1
to 10
-3
M range. Others have shown phosphate sensors based
on cobalt rods and wires to have a linear response in the 32-55 mV per decade
change of phosphate concentration [71-74]. A phosphate sensor recently demon-
strated using Co thin film in a planar microfluidic chip exhibited a lower sensitivity
of ~35 mV per decade change of concentration [77]. This significantly increased
sensitivity of the phosphate MEA is most likely due to the 3-D thin film structure of
the sensor and the simple electrical interface. The response time (
t
90
)oftheMEA
ranged from ~1 to 30 s as KH
2
PO
4
concentration was decreased from 10
-3.1
Mto
10
-5.1
MatpH7.5.
The long-term stability of the phosphate MEA was monitored by continuously
measuring potential of the 10
-3.9
MKH
2
PO
4
solution. The results are shown in
Fig. 6.10b. After 30 min, the average measured potential was -377.1 mV with a
standard deviation of 2.9 mV. The sampling frequency was 1 Hz. The phosphate
MEA produced a very stable response without a Faraday cage.
Oxygen has been reported to affect activity of phosphate ions [69, 70]. The
sensor response to H
2
PO
4
-
ions should decrease with increasing oxygen concen-
tration. The effect of dissolved oxygen was evaluated using 0 mg/L (0% DO) and
8.7 mg/L (21% DO) concentrations in a 10
-3.9
MKH
2
PO
4
solution. Nitrogen gas
and air were bubbled for more than 20 min to produce 0 and 21% DO solutions
