# Dosing phosphates and iron



## Anthony (Jan 11, 2005)

Why is it that iron and phosphates shouln't be dosed together? I've read that they "drop out" but why? I seems to me that plants would be better off with a continual supply of all nutrients instead of dosing alternate days.


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## e.lark (Feb 2, 2005)

KevinC said:


> Re: Mixing macro and micro nutrients:
> 
> The only likely (as I see it) precipitates on mixing them together:
> 
> ...


Huh? :tongue:


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## KevinC (May 24, 2004)

e.lark said:


> Huh? :tongue:


You really want to know??

For "insoluble" solids, we set up an equilibrium expression:

Fe3(PO4)2*8H2O(s) <----> 3 Fe+2(aq) + 2 PO4-3(aq) + 8 H2O(l)

Ksp = [Fe+2]^3 * [PO4-3]^2 = 1*10^-36

Let x be the amount of the solid that would dissolve (assuming excess solid in a solution), then the amount of Fe+2 = 3x, the amount of phosphate = 2x. These are the MAXIMUM amounts that would be in solution (though there can be an excess of ONE of the components).

(3x)^3 * (2x)^2 = 1*10^-36

so x = 2.4*10^-8M

This equals an iron concentration of 7.2*10^-8M = 0.004 ppm
and a phosphate concentration of 0.0045 ppm

Above these concentrations the iron phosphate precipitate would be expected to form.

HOWEVER - the above calculation assumes: Ion-free water except for the iron and phosphate, 25C temp, AND no EDTA or other complexing agent for the iron.

CAN you keep 0.1ppm iron and 1.0ppm phosphate in the water column? Sure! But we do know it is difficult to keep the iron there - this is probably one of the contributors (oxidation to Fe+3 - an even less soluble ion - is the another contributor). 

SHOULD you mix the micro solution with a phosphate stock solution? NO! The much higher concentrations of iron and phosphate in stock solutions just about guarantees a precipitate.

Kevin


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## malkore (Nov 3, 2003)

They can be dosed together, from different bottles. you can't mix the two together in one bottle, or you get 'fallout'.


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## scolley (Apr 4, 2004)

Wow. I'm glad to see this treated seriously. I don't understand a d*mn thing about Kevin's explanation. But I do understand the implications...

1) You _can't _ put iron and phosphates in the same dosing solution
- but - 
2) You _can _ dose them on the same day.​
Thinking that runs counter is this appears to be one of those "planted tank myths" that is in serious need of debunking.


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## Laith (Jul 7, 2004)

KevinC said:


> ...
> This equals an iron concentration of 7.4*10^-8M = 0.004 ppm
> and a phosphate concentration of 0.0045 ppm
> 
> ...


A question:

0.1 mg/l of Fe is 25 times the concentration of .004 and 1mg/l of PO4 is 222 times 0.0045.

Even with the additional ions in tank water and the chelate, don't these concentration multipliers imply that there is still a risk of precipitation?

And I dose more than that of both...

However, I would love to dose everything daily if at all possible! Less fluctuation of nutrients for the plants...


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## ringram (Jan 19, 2005)

looks like Greek to me.


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## Hypancistrus (Oct 28, 2004)

Seachem says that Flourish Excel will keep iron in a reduced state.


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## Urkevitz (Jan 12, 2004)

I keep my phosphates high, when I tried adding Flourish Iron a white cloud of precipitate formed. Since switching to Iron Chelate I haven't had any problems.


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## ThomE (Aug 26, 2004)

Next time when somebody asks "You really want to know??" ... Answer the damn question with a no!!!!!!! :hihi: 

Serous Kevin thanks for the answer. I knew you couldn't mix the two but I could supply an answer when somebody questions me.


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## KevinC (May 24, 2004)

Laith said:


> A question:
> 
> 0.1 mg/l of Fe is 25 times the concentration of .004 and 1mg/l of PO4 is 222 times 0.0045.
> 
> ...


Yes, there is still the distinct possibility. I hadn't thought about it until I did the calculation for the post above. I now wonder if this is another important pathway to removal of iron from the water column . . . I'm a big fan of EDTA for chelation!

The alternate day dosing doesn't make much sense to me (other than convenience) - if I dose 0.1ppm Fe on Monday and 1.0ppm phosphate on Tuesday, am I assuming most of the iron has already been used up??? That implies that I need to add more iron on Tuesday too! Otherwise, there is still the likelihood of precipitation.

To Hypancistrus: The calculation I did was for the reduced (+2) iron - IF there is oxidation, then precipitation is more likely since the +3 state is generally less soluble.

Kevin


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## FMZ (Jul 13, 2004)

From what I have read, Phosphate bonds with Iron, forming a Phosphate Iron which is harder for plants to absorb rather then Iron and Phosphates alone...this is why they say dose it on a different day.


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## scolley (Apr 4, 2004)

FMZ said:


> From what I have read, Phosphate bonds with Iron, forming a Phosphate Iron which is harder for plants to absorb rather then Iron and Phosphates alone...this is why they say dose it on a different day.


That sound's like a good precaution, but it assumes that on your day for iron dosing that phosphates have dropped to 0 ppm. Otherwise there is still the presumed possibility of negative things happening (precipitates and/or bonding) - based on this precautionary form of thinking.

The problem here is simple, we either take this very cautious approach, and drive the phosphates in our tank down to zero before iron dosing, or we accept the possibility that concentrations play a role, and seek to understand at what level the concentrations matter.

For myself, I'm not driving phosphates down to zero for anything. I've seen what happens when I do that. So I'm stuck hoping to find the levels at which concentrations do, and do not, matter.


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## bharada (Mar 5, 2004)

From my experience, dosing PO4 and Fe on the same day results in a white cloud that takes the better part of the day to dissipate. Alternate day dosing results in a far less dense cloud which dissipates in an hour or two. So there's still presipitation going on in alternate day dosing, just not as much.

To combat this you could dose after lights out which give the cloud all night to clear.


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## Laith (Jul 7, 2004)

KevinC said:


> ...
> The alternate day dosing doesn't make much sense to me (other than convenience) - if I dose 0.1ppm Fe on Monday and 1.0ppm phosphate on Tuesday, am I assuming most of the iron has already been used up??? That implies that I need to add more iron on Tuesday too! Otherwise, there is still the likelihood of precipitation....
> 
> Kevin


Yes, looked at that way, it doesn't sound very logical.

Before seeing your figures, I had thought that the risk of precipitation occured at much higher concentrations. More like the concentrations in a stock solution of the ferts.

And that in the tank we don't get anywhere near those concentrations so there was no risk...

But thinking about it now, if my assumption was correct, then there would be no issue of dosing both on the same day!


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## FMZ (Jul 13, 2004)

scolley said:


> That sound's like a good precaution, but it assumes that on your day for iron dosing that phosphates have dropped to 0 ppm. Otherwise there is still the presumed possibility of negative things happening (precipitates and/or bonding) - based on this precautionary form of thinking.
> 
> The problem here is simple, we either take this very cautious approach, and drive the phosphates in our tank down to zero before iron dosing, or we accept the possibility that concentrations play a role, and seek to understand at what level the concentrations matter.
> 
> For myself, I'm not driving phosphates down to zero for anything. I've seen what happens when I do that. So I'm stuck hoping to find the levels at which concentrations do, and do not, matter.


Your right actually, since I never have 0 ppm phophates.


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## scolley (Apr 4, 2004)

Yeah, it's the "I dropped Chemistry, but can still understand a problem" point of view! :icon_wink


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## spinjector (Mar 20, 2005)

Wow guys this is an awesome thread. This *exactly* explains something I've been pulling my hair out over for the past two weeks. I went to an LFS out of town which was really well stocked and they had the whole line of Flourish products. I picked up Flourish Phosphorous, a Hagen phosphorous test kit, and a Hagen Iron test kit (I already had the Iron and dosed it extra with the regular Phosphorous every day to keep the iron up).

I got home and started dosing P & Fe daily together with my other Flourish goodies. A few hours later I went to test for both and got a goose-egg zero on both tests! I tried testing a couple of times and still got nothing. I thought maybe I was not understanding the dosage, or my tank was so depleted that I needed a large initial dose to get it up, so I put like 5ml of the P in for two days and still got nothing on the test! I even created a pseudo-reference solution by putting three drops of each alone in a gallon of freshly dechlorinated water and testing that - and it worked! That proved wrong the thought I had that my test kits were bad, and then made me realize something was going on IN the tank.

So now I know. THANKS! Good explanations too... I was a chemistry whiz in high school, and although it's been a while and I've forgotten a lot, I mostly understood all those numbers...

* I have a question though... I did not get the white cloud of precipitate, even after doing 5ml of both Flourish Iron & Phosphorous in my 20 gallon tank for two days in a row. However it precipitated, I would assume it got sucked into the undergravel filter and is sitting there now. Can plants utilize Iron Phosphate...? Or does it have a super strong ionic bond like sodium chloride, making it difficult, if not impossible for the plants to metabolize...?


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## KevinC (May 24, 2004)

spinjector said:


> * I have a question though... I did not get the white cloud of precipitate, even after doing 5ml of both Flourish Iron & Phosphorous in my 20 gallon tank for two days in a row. However it precipitated, I would assume it got sucked into the undergravel filter and is sitting there now. Can plants utilize Iron Phosphate...? Or does it have a super strong ionic bond like sodium chloride, making it difficult, if not impossible for the plants to metabolize...?


I did try an experiment in the lab last week. My goal was to show that EDTA prevented the precipitation, but I wasn't able to get that far:

1. Prepared solutions - one of 1ppm Fe+2, one of 0.1ppm Fe+2, one of 10ppm PO4, one of 1ppm PO4. The iron came from ferrous ammonium sulfate (pH adjusted to near 7 with NaOH), while the phosphate came from NaH2PO4*2H2O adjusted to pH 7 with NaOH.

2. Starting with 20ml of 0.1ppm Fe+2, added 10ppm PO4 solution in 0.2ml portions, mixed, measured turbidity (to 0.01NTU). NO SIGNIFICANT CHANGE COMPARED TO A BLANK OF ION-FREE WATER AFTER 2.0ml!

3. Reversed the above: 20ml of 1ppm PO4, added 1ppm Fe+2 in 0.2ml portions. Same result as #2.

4. Now perturbed, I tried mixing 100ppm PO4 stock solution with 10ppm Fe+2 solution - VERY SLIGHT cloudiness (or my eyes were crossed by then).

My conclusion: turbidity is not adequate for measuring the precipitation OR it is not precipitating (quickly? at all?) I have another way to try and measure the precipitation that I will work on - capillary electrophoresis to measure the non-precipitated phosphate and/or Fe+2.

Kevin


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## Betowess (Dec 9, 2004)

Urkevitz said:


> I keep my phosphates high, when I tried adding Flourish Iron a white cloud of precipitate formed. Since switching to Iron Chelate I haven't had any problems.


Do you know if Watson carries the Iron Chelate, or what source do you get it from? Because I'm about out of Flourish Iron and I was thinking of ordering Greg's iron dry fert. 

Thanks, bob 
And yes, this is a super informative thread! But I really shouldn't be doing this at work...LOL


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## Urkevitz (Jan 12, 2004)

Yes Greg Watson sells Iron Chelate. That's where I purchased mine.


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## spinjector (Mar 20, 2005)

KevinC said:


> I have another way to try and measure the precipitation that I will work on - capillary electrophoresis to measure the non-precipitated phosphate and/or Fe+2.


Cool Kevin! This is awesome... Thanks.

Did you by any chance test for Iron or Phosphorous as you were doing your tests? I would be curious how consumer-type test kits would respond under your test conditions.


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