Environmental Engineering Reference
In-Depth Information
6.2.3 ORP MEA Sensor
ORP electrodes directly measure the potential of a solution. This is in contrast to
the ion selective electrodes that measure a potential that is proportional to the con-
centrations of the chemical species in a solution. Thus, an ORP electrode is not
specific, and measures the oxidized and reduced forms of all chemical species in
the solution. Consequently, it is standard practice to verify the performance of an
ORP electrode against standard and reference solutions. Three ORP reference solu-
tions of 450, 228, and 90 mV at 25
◦
C (Sensorex Corp., Garden Grove, CA) and
Orion ORP standard solution of 221 mV (Thermo Fisher Scientific, Beverly, MA)
were used to investigate performance of the OPR sensors. A commercial Ag/AgCl
milli-electrode (MI-401, Microelectrodes Inc.) was used as reference. The American
Society for Testing and Materials (ASTM) standard D1498 [66], recommends that
measured redox potentials should be within 10 mV of the nominal redox potentials
for a good redox electrode.
The standardization curve of the ORP sensor is shown in Fig. 6.7a. The MEA
sensor is compared with a commercially available millielectrode (COM) and a con-
ventional pulled-glass pipette microelectrode (ME) against the four redox standard
or reference solutions. The slopes of the three curves are very close to the theoretical
value of 1.00, which clearly indicates that the ORP MEA compares very well with
the conventional two electrode types and is deemed acceptable for measurements of
redox potentials.
The ORP values can be correlated to the logarithm of the hydrogen concentra-
tion (i.e., pH) with a linear relationship. This is illustrated in Fig. 6.7b. The ORP
MEA sensor showed a log-linear ORP response down to a hydrogen concentration
of 10
-10
M (pH 10). The sensitivity of the ORP MEA sensor (change in redox poten-
tial per pH unit) is calculated to be ~61.5 mV/pH, which is very close to the ideal
calculated slope of 59 mV/pH as reported by Pang and Zhang [45].
The response time of the MEA was substantially faster than that of the commer-
cial milli-electrode (COM) due to smaller tip size and simple thin film structure.
Figure 6.7c illustrates the representative results. Overall, the MEA reached 99%
of the final stable reading in less than 1 s for the ferrous-ferric standard solution,
in approximately 10 s for both the pH 4 quinhydrone reference and the Orion
ORP standard solutions, and in less than 30 s for the pH 7 quinhydrone refer-
ence solutions. Under the same conditions, the response times for the commercial
milli-electrode were 2 min for the ferrous-ferric standard solution, approximately
5 min for both the pH 4 quinhydrone reference and the Orion ORP standard solu-
tions, and more than 10 min for the pH 7 quinhydrone reference solution [55,
56]. In
in situ
monitoring, fast response time means lower power consumption for
the sensor system, especially if a power-down protocol between measurements is
used.
The MEAs proved to be extraordinarily stable. The stability of the MEA was
evaluated by continuously measuring redox potential of the Orion ORP standard
solution (Fig. 6.7d). Both MEA and an Ag/AgCl reference electrode were in the
standard solution for the duration of the experiment, while potential measurements
