Environmental Engineering Reference
In-Depth Information
Fig. 6.7
Characterization of ORP MEA sensor: (
a
) standardization curves of the three ORP
electrodes against four redox standard or reference solutions with respect to Ag/AgCl reference
electrode, (
b
)
pH sensitivity of the ORP MEA and the commercial milli-electrode in ferrous-ferric
standard solution
,(
c
) response times in ferrous-ferric standard solution at 23
◦
C, (
d
) redox potential
of Orion ORP standard solution measured at 23
◦
C, and (
e
) performance in ferrous-ferric standard
solution for changing mixing intensity. From [55, 56]
were recorded hourly. After four days, the average measured ORP was 215.5 mV
with a standard deviation of 1.7 mV, which is well within the
±
10 mV specification
of the ASTM [66].
To evaluate the long term stability, the MEA was rinsed with water, dried with
nitrogen, and stored on a shelf from several days to several months. Repeated
measurements following storage showed accurate and reproducible measurements
without special cleaning or reconditioning procedures. The most common problem
reported for aqueous samples is that readings can differ by a significant margin, as
much as 50-100 mV, even though the sensors are in the same solution. Here, the
