View Full Version : Kalk's relationship with ORP?

01/23/2006, 12:51 PM
I recently temporarily stopped dosing kalk to a 650g reef system. I immediately (within 48hrs) say my ORP reading rise from an average of 325 to 370. Since my calcium reactor has also been off-line my PH is nearly identical now to what it was when I was dosing. Any ideas why a systems' ORP would increase this much simply by not dosing Kalk?

Randy Holmes-Farley
01/23/2006, 12:57 PM
Did the pH drop? Dropping pH will show up as an ORP increase:

ORP and the Reef Aquarium

from it:

"The Empirical Relationship Between ORP and pH in Aquaria

While understanding the details of the theoretical relationship between pH and ORP is complicated, measuring it for a single aquarium is fairly easy. Figure 1 shows simultaneous plots of pH and ORP values over the course of several days in the aquarium of Simon Huntington. Clearly, the measured ORP and the pH are on exactly opposite cycles, as one would expect from a system where reactions involving oxygen are important (and as is shown by rH).

Does the reaction exactly follow the one H+/e- rule? Maybe not exactly. Figure 2 shows a plot of rH as a function of time using Simon's data. If the effects of pH on ORP were exactly removed by calculating rH using:

rH = mV / 29 + (2xpH) + 6.67

then one might expect rH to not have a diurnal cycle. In this figure, the data suggest that there is still a diurnal dependence to rH, possibly due to pH effects. I have seen data from other aquaria as well, and in those cases the same holds: that rH largely compensates for ORP changes with pH, but not perfectly. Since things other than pH (such as O2) may change during the night and day in aquaria, this experiment may be confounded by these other variables.

Simon also ran an additional experiment on his aquarium. He took a water sample, and added either sulfuric acid or sodium hydroxide to it to adjust pH. In this experiment, the other factors that might cycle diurnally in an aquarium are constant. The results are shown in Figures 3 and 4. The fact that the ORP goes almost exactly back to where it was at the start, despite the pH excursions, suggests that the acids and bases are not altering the "base" ORP, but are only impacting ORP through pH.

The ORP moves inversely to pH, as expected (Figure 3). But, the fact that the rH is generally not flat as the pH is changed (Figure 4), but rather tracks with pH changes, suggests that the mathematical conversion used (rH = mV / 29 + (2xpH) + 6.67) is overcorrecting for pH changes. That result in turn implies that the pH dependence of ORP may be less than predicted by the H+/e- ratio of 1.0. Perhaps this result indicates that in Simon's aquarium, some reactions with an H+/e- ratio below one are important in controlling ORP.

Overall, my suggestion for aquarists using ORP measurement devices is to be aware of how pH can influence ORP measurements, but to not overly emphasize specific pH corrections."


01/23/2006, 01:11 PM
PH is a little lower now. PH now ranges from 8.30 (day) to 8.16 (night). Old readings were 8.38 (day) to 8.23 (night). ORP now ranges from 378 (night) to 340 (day). Old readings 335 (night) to 315 (day). I understand and see the relationship between PH and ORP....I didn't expect to see such a large change. Thanks for the article.

Randy Holmes-Farley
01/23/2006, 02:05 PM
There could be other reasons as well, but I'd just be speculating. I've not heard of a strong correlation between ORP and the use of limewater.

01/23/2006, 02:25 PM
I have not seen that either since I started using limewater. No apparent affect on ORP. (other than the PH change which we already know affects ORP). One of the reasons ORP is lower during the day is because when the fish are more active, they use more oxygen lowering the ORP. When is the last time you cleaned your probe. Usually a little algea on the probe tip will raise the ORP numbers giving you a false reading. If your probe is clean then I suspect when you stopped dripping kalk, your Ph dropped lower than you think. I seem to recall reading that a .1 drop in PH raises ORP by about 59mv.

01/10/2007, 03:31 PM
I've noticed the same today. My kalk ran out and I put the ro/di line in to refill the brute container. So I had my litermeter turned off all day while filling/mixing the kalk. Here's a graph of what happened and you can see where I was finished the mixing and turned the litermeter back on:


Randy Holmes-Farley
01/11/2007, 04:45 AM

Thanks for posting that. :)

01/11/2007, 10:22 AM
Could it be that aerobic organic eating bacteria have eaten the waste reducing rh at the procedure, and ORP is up due to organic reduction?

Randy Holmes-Farley
01/11/2007, 02:29 PM
I'm not sure what you mean. What part of the experiment does that relate to?