Environmental Engineering Reference
In-Depth Information
ORP MEA sensor produced a very stable response over the course of several days
without the need of a Faraday cage.
The stirring effect on ORP measurements was also investigated to verify the
sensitivity, the reliability, and the stability of the ORP MEA [55, 56]. For in
situ monitoring, convection of sample medium is a critical factor in measurement
error. The experiment was carried out bysequentially inserting the integrated micro-
electrode into standard ORP solutions using five different stirring velocities. As
shown in Fig. 6.7e, the redox potential profile exhibited a trend of a very grad-
ual decrease, as stirring intensity increased. The slightly unstable potential profile
between 300~500 rpm occurred when the stirring bar began bumping the beaker
wall. The stirring effect (Re) can be calculated as [67]:
( V
V 1 )
Re
=
×
100%
(6.1)
V
where V 1 is the potential measured from the unstirred sample, and V is the potential
measured from the stirred sample. Even with artificial turbulence at 300-500 rpm,
the measured ORP variability was less than 1 mV which is Re ~ 0.2%. Thus, it can
be concluded that the signal was not substantially influenced by stirring.
6.2.4 DO MEA Sensor
The DO sensor calibration set up is schematically illustrated in Fig. 6.8. The
DO MEA sensors were polarized and calibrated with a commercial Ag/AgCl
reference electrode (MI-401, Microelectrodes Inc.). The polarization voltage and
current were supplied by a Chemical Microsensors II potentiostat (Diamond General
Development Corp., Product No 1231). The -750 mV polarization voltage was
applied to the oxygen microelectrodes against the Ag/AgCl reference electrode for
at least several hours prior to calibration. Negative applied voltage can reduce the
amount of O 2 on the cathode of the microelectrode surface, and given sufficient
potential and time, O 2 concentration can be reduced to zero. Thus, the residual oxy-
gen which can cause measurement errors is removed by a polarization process to
permit measurement of more accurate DO values. Following polarization, a test
solution was prepared by aeration of 0.85% saline solution with pure nitrogen gas
(0% O 2 or 0 mg/L DO), a gas mixture containing 10% O 2 and 90% N 2 (10% O 2
or 4.1 mg/L DO), and air (21% O 2 or 8.7 mg/L DO). The aeration was applied
for at least 20 min to establish a stable concentration. A commercial oxygen milli-
electrode (MI-730, Microelectrodes Inc.) was used to verify the concentration of
oxygen in bulk solution and during calibration.
The electrolyte in saline solution plays an important role in an electrical connec-
tion between the working electrode and the reference electrode [64]. The maximum
solubility of dissolved oxygen in natural water at 25 C is 8.7 mg/L (or 21% O 2 ).
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