Environmental Engineering Reference
In-Depth Information
using the same experimental setup for DO MEA characterization as described pre-
viously. A commercial oxygen milli-electrode (MI-730, Microelectrodes Inc.) was
used to verify concentration of oxygen in the test solution and during calibration.
Figure 6.10c shows ~ 69 mV offset between 0 and 21% DO in the same 10
-3.9
MKH
2
PO
4
solution. This is a substantial change, significantly higher than the
measurement variations (standard deviations are <4.5 mV). Thus, oxygen calibra-
tion should be performed before or with phosphate measurements. In this work,
all phosphate measurements were performed in ambient conditions with ~17% DO
(measured using the commercial DO sensor).
The effect of stirring in 10
-3.9
MKH
2
PO
4
at pH 7.5 was also investigated to
verify the stability of the phosphate MEA. The experiment was performed by mea-
suring sensor output at four sequentially increasing stirring velocities. As shown
in Fig. 6.10d, the electrode potential profile exhibited an immediate change of
about 30 mV when 100 rpm agitation was applied, yielding Re ~ 6%. However,
higher velocities (200, 300, and 400 rpm) showed a stable potential with a slightly
increased Re ~ 11%, independent of stirring intensity. The electrode potential
returned to non-stirring potential immediately when stirring was turned off. Given
the demonstrated sensitivity to the dissolved oxygen, this stirring effect sensor
response is expected as agitation influences oxygen diffusion on the sensor surface.
Meruva and Meyerhoff suggested the mixed potential response mechanism
of cobalt electrode sensors toward inorganic phosphate. According to this
mechanism, a slow oxidation of cobalt occurs at the electrode surface cou-
pled with simultaneous reduction of oxygen and the formation of Co
3
(PO
4
)
2
.
Thus, the sensor detects mixed potential due to both oxidation and reduction.
This mechanism characteristic provides a reasonable explanation of the cobalt-
based phosphate sensor, and especially for the effects of dissolved oxygen and
stirring.
As discussed earlier, cobalt-based electrodes have been reported not to be suscep-
tible to interference from Cl
-
,NO
3
-
, and SO
4
2-
ions [68, 69]. In order to confirm it,
the phosphate MEA was used to measure phosphate ions while subjected to inter-
ference from several ions typical in wastewater. The ion concentration in each test
solution was 0.015 g/L of CH
3
COO
-
, 0.043 g/L of Cl
-
, 0.196 g/L of NO
3
-
, and
0.196 g/L of SO
4
2-
. The concentrations were based on those typical of an acti-
vated sludge sample [70]. The phosphate MEA sensor was used for the above tests
and stored in a dry desicator for one month prior to performing these interference
tests. Results in Fig. 6.11 show a good linear relationship between the electrode
potential and the phosphate concentrations for each interference ion tested. The
“no-ion” condition was the control test without presence of interference ions. The
exhibited sensitivities of the phosphate MEA per decade change of KH
2
PO
4
con-
centration were 67.3 mV for CH
3
COO
-
, 72.1 mV for Cl
-
, 75.8 mV for NO
3
-
,
65.4 mV for SO
4
2-
, and 70.1 mV for the “no-ion” control. Thus, the interference
tests yielded a sensitivity of 70.2
4.7 mV per decade of KH
2
PO
4
concentration,
which compares very well to the 70.1 mV per decade value measured for the control.
Overall the interference effect was not significant, but for accurate measurements,
pre-calibration should be performed.
±
