Instantly is an exaggeration. More like over the course of an hour it drops from 225 to maybe 180.
I'm not sure what the issue is. Hydrogen peroxide also drops ORP when added, but that's presumably becasse it can reduce Cu++ (higher ORP) to Cu+ (lower ORP), but ozone should not do that.
When I started ozone, this was my observation:
ORP Changes
When ozone is first added to reef aquaria, ORP (oxidation reduction potential) rises. What exactly this means on a molecular level is complicated and not well understood, but it is the expected result of oxidizing the various redox active chemical species in the water. Some aquarists see a big rise in ORP - large enough that they need to control the ozone generator so that the ORP does not rise too high (>450-500 mV). Others see a relatively small rise, and still have ORP in the 300 mV range even after using the generally recommended amount of ozone. Some even find that it remains in the upper 200 mV range. This variability is important for aquarists to keep in mind, and they should not conclude that there is a problem if the ORP remains fairly "low."
In the absence of ozone, my aquarium seems to have an ORP that ranges in the mid- to upper 200 mV range. While I monitored ORP carefully when initiating ozone to be sure not to overdo the addition, such concern was unfounded. Even when adding ozone at full production (100 mg/h as claimed by Aqua Medic; equivalent to about 0.5 mg/h/gallon of display volume or 0.3 mg/h/gallon in the entire system), the ORP never rose above about 335 mV. Even with newly dried desiccant in the air dryer, a freshly cleaned skimmer and ozone added to any of the devices that I used (Coralife ozone reactor, straight into a skimmer, or my tubing reactor), the ORP was never higher than this. Even when used through the skimmer with no activated carbon treatment of the effluent, the ORP was no higher. Consequently, I did not need to use the ozone on a controller.
Was ozone being added? Clearly, yes. The water's ORP exiting my tubing reactor measured 680 mV. I could also detect an ozone/OPO residual of 0.1 - 0.24 ppm chlorine equivalents (details of such test methods are provided in air flow section of the previous article). So the ozone was having the desired effect on the water in the reactor.
Would the ORP have risen more with additional ozone? Certainly, yes. But using higher amounts of ozone causes more risk of harm from overdose, and it may not further improve the primary reason for using it: water clarity. More ozone might have resulted in more disinfection of the water in the ozone reaction chamber, and possibly a further reduction in dissolved organic levels, but I am not sure that either of those is necessarily beneficial.
ORP Measurement
Because many aquarists focus on ORP levels when using ozone, a final repeated caution about the inherent complications of ORP measurement is warranted, along with some examples. ORP is not a simple measurement like pH. The metal probe's tip is very sensitive to what is bound to it, and it can take days for it to become coated with organics and other materials in reef aquarium water. So, when the probe is first put into a reef system, the ORP can drift for quite some time as these processes slowly take place. Moreover, every time the probe is calibrated or moved into a different solution, it can again take days to re-establish itself in the aquarium.
For all measurements I used a Pinpoint ORP probe attached to an Orion 710A pH/ORP meter. Proper operation was checked using Pinpoint's 400 mV single-use calibration solutions.
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.
In summary, aquarists should be wary of over interpreting ORP. Small changes may be caused by factors unrelated to ozone use per se (such as pH or organics binding to the probe). All ORP measurements ought to be viewed as having a relatively large "uncertainty" when they are being compared to other aquaria, perhaps on the order of +/- 25 ppm or more. Changes observed in a single aquarium over time may be more useful and interpretable than comparisons between systems.
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