Ozone and the Reef Aquarium, Part 3: Changes in a Reef Aquarium upon Initiating Ozone by Randy Holmes-Farley - Reefkeeping.com
Here are some examples of apparent ORP measurement complications:
1. After using ozone for several weeks in my system, the ORP was running in the 300-330 mv range, depending on the pH and whether I had recently cleaned the skimmer. Removal of the probe for calibration with a commercial 400 mV fluid (American Marine/Pinpoint) always showed about 410 mV after 30-60 minutes, which I decided was close enough.
Then the probe was used to measure the highly oxidizing fluid exiting the ozone reaction chamber (680 mV) for 24 hours. Upon returning the probe to the aquarium, the ORP was reading only 276 mV, and stayed at about that level for several days.
Putting it back into another 400 mV calibration pack showed the ORP still to be about 410 mV, as before. Then returning the probe to the aquarium gave an ORP that slowly rose, and after 24 hours was again above 300 mV.
I do not believe that the aquarium's ORP happened to be unusually low right after exposing the probe to the highly oxidizing fluid. I do believe that ORP probes can retain a memory of what they have been exposed to, which is exerted through changes in the organic materials bound to the probe's surface. Exposure to different solutions, even calibration solutions, can alter the nature of these bound organics in a way that can impact the ORP that is measured for days after the initial exposure.
2. Many aquarists find that ORP rises over many days to weeks as the probe sits in their aquarium water. This rise may be due to algae growing on it, releasing O2
and other oxidizing species very near the metal tip. These aquarists find that gently cleaning the ORP probe often causes the value to drop back to earlier levels. I have not noticed this effect, but my ORP probe is kept in a totally dark sump.