Orp drops with water change

[leepink23]

First I want to say I am not chasing Orp, just curious what makes it drop from 450 to 350 after a 15 gallon water change with red sea blue bucket. Tank is 150 gallons.

 

[ReeferCub]

Not 100, but mine drops 30-40 points each time I change water. Don't know the reason though. May be Randy can explain if he see this.

 

[rkpetersen]

Each salt has a different effect on ORP. The salt I currently prefer, Red Sea Coral Pro (black bucket), also causes the largest drop in ORP with my typical 5% water change, as much as 50-60 mV. Other salts I've used, Reef Crystals and RPM, had a lesser effect, although all 3 decrease ORP acutely to some degree. I assume it's due to certain ions being present in reduced state in freshly mixed salt, but I'm not sure which (iron? manganese?)

 

[leepink23]

Each salt has a different effect on ORP. The salt I currently prefer, Red Sea Coral Pro (black bucket), also causes the largest drop in ORP with my typical 5% water change, as much as 50-60 mV. Other salts I've used, Reef Crystals and RPM, had a lesser effect, although all 3 decrease ORP acutely to some degree. I assume it's due to certain ions being present in reduced state in freshly mixed salt, but I'm not sure which (iron? manganese?)

That's consistent, i get 100 mV change with 10%, you get 50 mV with 5% both with red sea, me blue bucket you black bucket.

 

[JVU]

I’m also curious about this and see it consistently. My ORP hangs out near the upper limits of what my apex considers normal, so I’m thinking it is just bringing it closer to a more neutral ORP, but I don’t understand the chemistry of it. Hoping to learn something!

 

[Randy Holmes-Farley]

The ORP of a fresh salt mix is determined by the relative numbers of certain redox active ions in it. For example, ferric (Fe+++) vs ferrous (Fe++) iron and Mn++++ vs Mn++. So the ORP of a salt mix depends on exactly which ingredients are used to make it. Some lower ORP species are more soluble in seawater than their higher ORP counterparts (iron and manganese, for example), and are hence more bioavailable, and that may be why many manufacturers produce a lowish ORP mix.

I have more here:

ORP and the Reef Aquarium - Reefkeeping.com
http://reefkeeping.com/issues/2003-12/rhf/feature/index.htm

 

[bigjgmac]

I was curious about this myself. I'm a chemist by education and profession and figured it (obviously) had something to do with the salt mix. I'm seeing a roughly 10% drop in ORP when performing a water change. All other measurables don't vary significantly.

I'm going to have to run an experiment to see if mixing longer has any effect on minimizing this. At the end of the day we all want stability in general and for those acros in particular. I'm a believer in water changes for many reasons, but this is certianly an argument, and a good one, for minimizing or eliminating them.

 

[Randy Holmes-Farley]

I was curious about this myself. I'm a chemist by education and profession and figured it (obviously) had something to do with the salt mix. I'm seeing a roughly 10% drop in ORP when performing a water change. All other measurables don't vary significantly.

I'm going to have to run an experiment to see if mixing longer has any effect on minimizing this. At the end of the day we all want stability in general and for those acros in particular. I'm a believer in water changes for many reasons, but this is certianly an argument, and a good one, for minimizing or eliminating them.

I would not presume mixing longer to raise ORP is necessarily a good thing, if what happens is oxidation of ferrous iron to less soluble ferric iron, etc. You may just be precipitating out certain trace elements.

 

[bigjgmac]

I would not presume mixing longer to raise ORP is necessarily a good thing, if what happens is oxidation of ferrous iron to less soluble ferric iron, etc. You may just be precipitating out certain trace elements.

You make a good point. That said, I'm going to mix salt for 48 hours for next week's 10% water change and see if the shift in ORP changes.

 

[Randy Holmes-Farley]

You make a good point. That said, I'm going to mix salt for 48 hours for next week's 10% water change and see if the shift in ORP changes.

OK, let us know what you find.

 

[bigjgmac]

Water change coming up later today. I was thinking about what was going on with a large ORP drop associated with a water change (and feel like a bit of an idiot as a result). Sometimes the most obvious answers are the hardest to see. I didn't turn off ALK dosing while I was doing my water change. As a result I got a small slug in the sump, which is where my probe is located as well as where my alkalinity is dosed. When I do the change today I'll make sure ALK dosing is off. I guess I now have two variables, salt that has mixed for 48 hours instead of 24 hours, and I'll have dosing off instead of on.

So...no mystery, just the obvious. It'll be interesting to see the change in ORP this time. I'll update the graph below after my water change and post the results.

 

[Randy Holmes-Farley]

Would also be interesting to track ORP in new salt water as a function of time with stirring.

 

[bigjgmac]

It would be interesting to track that. I have a handheld meter that will measure ORP, so it would have to be a series of "grab samples" as opposed to continuous monitoring. It looks like that may happen as well, because I got some unexpected results post water change.

I had another approximate 30 point ORP drop with this most recent water change. 48 hours of mixing. All dosing turned off. Tropic Marin Reef Pro mixed to a salinity of 35 ppt and heated to 78 F. I'm wondering if I need additional surface agitation in my mixing vessel - a 20 gallon Brute trash can with a 500 gph sump pump for mixing and agitation. Maybe I can point the discharge up so it mushrooms rather than sideways and create a whirlpool. Or maybe I don't need to worry about it. I was fine until I started looking at data! If this trend keeps up I'll be anareobic by mid January! On one hand I'm not really all that concerned. On the other hand OPR has gone from 337 to 292 in a week. As you can see from the graph, it didn't fully recover since my last water change, regaining about 50% of the 30 point drop. I welcome any thoughts.

 

[Randy Holmes-Farley]

Or maybe I don't need to worry about it. I was fine until I started looking at data!

I think that is the ticket. Higher ORP does not mean anything is better. Folks have been poorly taught over the years to assume higher ORP is better.