Iron and ORP question

[recess62]

I received my triton results which showed an Fe of 0. I dosed 10ml of Kent iron and manganese supplement. Over the next 4 hours I noted a dramatic drop in the ORP. Can someone explain the chemistry? Did I just create rust?? Lol

 

[Randy Holmes-Farley]

What did you dose? Assuming you dosed a ferrous salt (Fe++) rather than ferric (Fe+++) then that will lower ORP. That is no concern. It does not mean the iron converted into rust.

I discuss effects of metals and other compounds on ORP here:

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

FWIW, Triton cannot detect nearly low enough to say you need to dose iron, although dosing iron is often desirable.

here's my comment on it:

My Triton Testing Results: By Randy Holmes-Farley
http://www.reefedition.com/my-triton-testing-results-by-randy-holmes-farley/

Iron (Fe). The natural iron level varies a lot with depth, but surface seawater may have only 0.006 µg/L. The Triton LOD = 0.3 µg/L. I dose iron, and when I dose it I boost iron to roughly 1-2 µg/L, which would be detectable. This sample was taken more than a week after the last iron dosing, and none was detected as it gets depleted in the meanwhile. I’ve not yet seen a Triton test result for a real aquarium sample that had detectable iron, but that doesn’t mean these tanks are necessarily deficient. Iron is also a case where the form is critical, and ICP cannot distinguish form. Binding to organic matter, for example, can alter the bioavailability of iron.

 

[recess62]

Thanks for your reply Randy. I am using the Kent marine iron and manganese supplement. It appears the iron is provided by using FeEDTA

The rust comment was tongue in cheek

 

[Randy Holmes-Farley]

Manganese may actually be the most important metal for ORP in seawater. In any case, the ORP changes are no concern.