ORP and Ph

rob safron · Jan 9, 2018

so from my graph below, my Ph only gets as high at 8.05-8.1 and usually hits a low of 7.8. You can see where I did a water change and the ORP dropped as well as my low Ph is now down in the low 7.7’s. Why would fresh salt water effect ORP but more importantly Ph like that. Even days after the water change.

DLHDesign · Jan 9, 2018

A few questions;
1) Is this RO/DI water?
2) Was the new water the same temp as the tank water?
3) How much of a water change (approx %) did you do?
4) Did your probes get exposed to air during the change?
5) How long did the new water mix before being used?

rob safron · Jan 9, 2018

Yes RODI.
Water was mixed and heated for 36 hours.
Probably less than 10% change.
Change the water out of the tank and no the probes did not get exposed to air in the sump.

Randy Holmes-Farley · Jan 9, 2018

ORP in a salt mix is controlled by certain trace metals.

ORP usually drops with a water change, likely because the new salt mix is formulated to have a low ORP by using certain trace elements in a more reduced form. Ferrous iron (Fe++) and Mn++, for example, instead of ferric iron (Fe+++) and Mn++++, the latter two of which are far less soluble. So they likely use the reduced forms (as reefers doe for trace element additions) to maintain solubility.

Randy Holmes-Farley · Jan 9, 2018

pH is determined only by alkalinity and the CO2 level in the water. Water changes do not usually have lasting effects on pH. If the probe moved around or was exposed during the water change, it may need recalibration.

rob safron · Jan 9, 2018

I did recently changed to aqua forest salt. I have two apex pH probes and both correlate the same. Can’t explain a sudden change in CO right at the same time as the water change. I do use CO2 scrubber but change the media before expired. Guess I always thought the higher ORP meant cleaner water and I assumed with a water change your OrP should go up versus down.