# [Wet Thumb Forum]-Co-precipitation of ionic Fe and PO4



## wetmanNY (Feb 1, 2003)

My answer, which would have been "add chelation, whether in the form of natural tannins or EDTA, etc," is simplistic and skirts the issue.

Recently an AquaBotanic member posted: " I haven't come across any decent phosphate and iron test kits yet. They tend to always show zero. although I'm adding enough iron and phosphate so that the plants grow like weeds." Hmm. Very definite cause-and-effect here. This is not an unfamiliar situation, from what I'm overhearing...

What _about_ ionic iron and phosphate, anyway?

[This message was edited by wetmanNY on Sat February 01 2003 at 01:19 PM.]


----------



## wetmanNY (Feb 1, 2003)

My answer, which would have been "add chelation, whether in the form of natural tannins or EDTA, etc," is simplistic and skirts the issue.

Recently an AquaBotanic member posted: " I haven't come across any decent phosphate and iron test kits yet. They tend to always show zero. although I'm adding enough iron and phosphate so that the plants grow like weeds." Hmm. Very definite cause-and-effect here. This is not an unfamiliar situation, from what I'm overhearing...

What _about_ ionic iron and phosphate, anyway?

[This message was edited by wetmanNY on Sat February 01 2003 at 01:19 PM.]


----------



## Roger Miller (Jun 19, 2004)

I think the original poster's problem is just that the iron and phosphate test kits available in the hobby are mostly trash.

As far as coprecipitation is concerned; it happens. If you're following a Walstad-like system then it probably won't be very important because you aren't adding iron. For the rest of us who do add iron, coprecipitation may be one factor that removes phosphorus from solution. When phosphorus gets too low, add some.

I really don't know how significant coprecipitation is. I ran some simulations with coprecipitation included. At the concentrations reached in the simulation coprecipitation was not very important.


Roger Miler
back "in pocket"


----------



## wetmanNY (Feb 1, 2003)

Roger Miller, when you state "At the concentrations reached in the simulation coprecipitation was not very important." that suggests that you found that the _rate_ of co-precipitation went down as the concentration was diluted. Otherwise, if there's only a little iron and phosphate, than just a little co-precipitation might be important... perhaps even enough to give an impression that these fertilizers were actually being utilized.

Inaccurate test kits and our amateurish procedures are sort of a _pre-problem_ aren't they?


----------



## Roger Miller (Jun 19, 2004)

Reaction rates usually have a strong dependence on the concentration of reatancts. One would not expect a reaction to proceed quickly at low concentrations. At low enough iron concentrations (as when iron is not dosed) it won't happen at all.

Misleading results from crappy test kits cause more problems then they solve.


Roger Miller


----------



## Guest (Feb 6, 2003)

I looked into this a bit more for Daniel on another list a few days ago.

I think the effects as far as our tanks are concerned regarding precipitation are really insignificant.

You can try adding the PO4 one day and the Iron later. I've never seen any differences. 

Iron precipitates when there's dissolved O2 present and turns into rust essentially, no one complains about that and the same thing can happen with Calcium.
If it does drop out of solultion, that's fine, it's in the gravel now. It's not going anywhere. Roots will get it etc.

Softer more acidic water seems to help keep many traces in solution better than harder more alkaline waters. I still use the same amount of traces, I see no difference but I'd say I likely don't need quite as much. My target pH is about 6.3-6.4 vs 6.9-7.0 before(KH 3 vs KH 10). I keep the CO2 up pretty high.

Some of the binding agents like gluconate and ETDA, I'm not sure how they effect the whole mess.

Regards, 
Tom Barr


----------



## wetmanNY (Feb 1, 2003)

It seems-- well "precipitous"-- to declare that the effects of ionic iron on soluble reactive phosphate is "not very important" or "really insignificant," when at the same time the test kits for PO4 we use, and equally important but perhaps underestimated, our amateurish procedures are making the actual test results for phosphate utterly unreliable.

Tim Hovanec, with no particular axe to grind (he's selling test kits too), writes: "In a professional laboratory, glassware is set aside and used only for phosphate testing. It is washed with acid, never with a soap of any kind and when not in use, the glassware is kept full of deionized water and covered with foil."

"When testing for phosphate, make sure the vial you use has not been washed with soap. Never put fingers over the end of the vial to shake and dissolve the reagents, as oils, etc. from your skin will get into the sample. Rinse the vial well with phosphate-free water after you finish."(_that would be distilled or de-ionized water_)

"As described above, a significant amount of the phosphate in the aquarium may be the organic form which will not be measured by the typical test kit. To determine the amount of organic phosphate, the water sample must first be digested. Digestion is a process where organic material is subjected to a highly acidic or basic environment usually at a high temperature in an autoclave. This process causes any organic phosphate, either suspended in the water or attached to something organic, to become SRP. After cooling, the sample is measured for SRP which now equals total phosphorus. By subtracting the determined SRP of an undigested sample, one now knows the concentration of SRP, total phosphate and total organic phosphate. While certain laboratory test kit suppliers do sell the materials for doing digestions, it should only be done by those experienced in laboratory safety as hot acids and bases are extremely dangerous. They can burn you, and ruin your kitchen countertops and best cookware. I suggest this be left to a properly set up water quality laboratory." Hovanec's simple introductory phosphate article is at the Marineland Laboratories site www.marineland.com . These are procedures Tom Barr and Roger Miller are following in PO4 testing, but for the rest of us, I wonder how many of us let our phosphate-testing equipment dry after a quick rinse under the tap, in our polyphosphate-enriched tapwater?

Tom Barr rightly points out that "more acidic water seems to help keep many traces in solution better than harder more alkaline waters." But in normally oxygenated water insoluble ferric Fe(III) doesn't convert to soluble available ferrous Fe(II) until the pH drops to-- what? pH 4.0? so low that the number is forgettable for aquarium purposes.

What _do_ count, what do keep iron and orthophosphate from co-precipitating or dropping out as hydroxides, are the tannins and EDTA, the chelators-- both natural and artificial.

When iron is constantly dosed and phosphate tests are reading "undetectable" co-precipitation shouldn't be too swiftly dismissed. For example, in dosing ferric chloride to reduce phosphates (as in Tetra's "New Balance" magic elixir apparently), it's the co-precipitation that's making it work!

Indeed, iron does precipitate throughout our systems' oxic water column, where there's always dissolved O2 present, and turns into insoluble hydroxides, though not rust essentially. If no one complains about that, it may be because otherwise fertilizer iron dosing could quickly reach toxic levels. (Calcium, on the other hand, could only precipitate out at the pH levels of a Lake Tanganyika.) Like many of us, I've already got a lot of insoluble iron in my Flourite, slowly being released to roots in anoxic acidic lower substrate levels. I don't really need more parts-per-billion traces of iron. Does anyone need more traces of iron?


----------

