Whats the oxidation state of iron phosphate in aerated sea water

[UK_Pete]

Hi Randy, I'm trying to work out how much phosphate will be removed from an aquarium when you dose a particular amount of ferrous citrate, but I'm not sure which form the iron phosphate will take. Will the oxidation state of the iron change and form iron (III) phosphate, or will it stay as iron (II) and so form iron (II) phosphate? In your article you say that iron (III) is the most stable under aerobic conditions so I assume it changes to this one but I can't find a definite answer anywhere.

Thanks, Pete

 

[Randy Holmes-Farley]

If you dose iron II, it may precipitate before it converts to iron III, but most still in solution will become iron III. The conversion of precipitated iron II to III will be slower, but I'm not sure how slow.

But I doubt you can calculate anything useful since most of whatever you dose may simply end up as iron oxide.

 

[UK_Pete]

Thanks Randy. If it does turn to iron oxide before it meets a phosphate ion, will it bind phosphate in that state, like GFO does, or is it out of the game from that point?

Thanks, Pete

 

[Randy Holmes-Farley]

It will bind some phosphate in its surface, but in a typical crystal, 99.99999+% of the iron would be on the inside and not exposed.

 

[UK_Pete]

OK thanks Randy. I have 0.9 ppm phosphate after I must have misjudged both the density and drop size of phosphoric acid, and I have a 18.5 ppt iron solution, which I was hoping to dose either 4.7 ml of (if it was iron 3), or 7 ml of (if it was iron 2), in small amounts of course. But this is probably a useless calculation in this situation as you said, is that right? Or would it be fairly valid because of the quite high phosphate?

 

[Randy Holmes-Farley]

Yes, it is largely a useless calculation. I think if you want to do this route, trial and error is the only way.

If you keep track, however, we can determine how much phosphate a given amount of iron removed.

 

[UK_Pete]

Oh well. I will take note of the amounts though and graph them to see if I can see any pattern as you say. Thanks Randy.

 

[Randy Holmes-Farley]

You're welcome.

Good luck!

 

[UK_Pete]

Sorry to come back to this one Randy but I just remembered and checked one of your phosphate articles where you say the phosphate is only bound in an equilibrium with iron, and that it can be released back to the water. Does this mean that if I do remove lots of phosphate from the water with iron citrate, and the precipitate is not skimmed out, that the precipitate that remains in the tank on rocks etc will then release its phosphate back to the water once the levels fall? So that if I do use this method I might get a more permanent phosphate problem than if I used lots of GFO to remove the phosphate?

Thanks, Pete

 

[Randy Holmes-Farley]

My comment was probably in the context of GFO, where the binding is all on accessible surfaces of the GFO, allowing the phosphate to come on and off easily.

If the phosphate is buried inside an iron oxide precipitate, then it won't easily be released. I don't know if pure crystals of iron phosphate form, but if they do, that might slowly dissolve, but I'm not really certain of its properties or how low phosphate would have to get to allow dissolution. It might also get coated with calcium carbonate the way GFO seems to, which might hold the phosphate inside.

 

[UK_Pete]

OK thanks Randy. If it's an unknown area, I don't need any more of those at the moment! So I think I might being the levels down closer to the right range with GFO for a week or two before tuning it ongoing from then with the iron in that case.

BTW I was wondering if I could test organic phosphate by using the predigestion step (mentioned in another thread but I think it got buried). Normally I think a 30 minute reflux with ammonium persulfate is used, I have sodium persulfate, do you think this would serve the same function? I guess you can't be sure the predigestion would work with a hanna checker but I have ordered the chemicals to do the standard phosphate test (ascorbic acid) so I wondered if the sodium persulfate would be the same as the ammonium persulfate.

 

[Randy Holmes-Farley]

I'm not sure on the predigestion conditions and whether that substitution would work. Hach sells a total phosphate kit, so reading through their directions might give you a better idea what is needed.

 

[UK_Pete]

Hi Randy, just checked the hach test as you suggested and they use the same procedure with potassium persulphate and sulfuric acid, although the hach test neutralises the water with NaOH after the digestion. So I have one test using ammonium persulphate and sulfuric acid, another test with sulfuric acid but using potassium persulfate, and I would like to use sodium persulfate. The process is just acidify, add oxidiser, boil for 30 mins, neutralise (only in the hach test), and proceed as with the inorganic phosphate test. I imagine since the ammonium and sodium salts are OK, I can probably use the sodium persulfate instead in this case, would this give you any more confidence or would you advise to get one of the other salts?

Thanks, Pete

 

[Randy Holmes-Farley]

That sounds reasonable, but I'm not really familiar with the requirements of the process to say for sure.