Dosing phosphates and iron - The Planted Tank Forum
 
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Old 04-06-2005, 07:58 AM Thread Starter
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Dosing phosphates and iron

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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How comes shoppings so stupid, looks at all this stuff I haves, what do I do's with it.
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Old 04-06-2005, 08:20 AM
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Quote:
Originally Posted by KevinC
Re: Mixing macro and micro nutrients:

The only likely (as I see it) precipitates on mixing them together:

Fe3(PO4)2*8H2O: Iron(II) and phosphate - Ksp = 1*10^-36
Zn3(PO4)2*4H2O: Zinc(II) and phosphate - Ksp = 5*10^-36

Even though the iron is complexed in micro mixtures, that complex does not look to be strong enough to prevent displacement by phosphate. This does not really apply to the aquarium since both Fe and phosphate are at such low concentrations by then.

All nitrate salts are soluble, so you can mix the nitrate in with the micros. Potassium salts are all soluble as well, and only a few sulfate salts are insoluble (Ca, Sr, Ba, Ag, Hg, Pb), so the K2SO4 should be fine (as long as it dissolves initially - 1g/8.3ml).

Mix it all up and see - if the concentrations in your dosing solution are low enough, it might still work.

Kevin
Huh?

ernest

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Old 04-06-2005, 01:59 PM
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Quote:
Originally Posted by e.lark
Huh?
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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Last edited by KevinC; 04-07-2005 at 06:52 PM.
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Old 04-06-2005, 03:37 PM
 
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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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Old 04-06-2005, 04:05 PM
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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.

Steve - 33g reef and a 180g planted in need of a re-scape.
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Old 04-06-2005, 06:08 PM
 
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Quote:
Originally Posted by KevinC
...
This equals an iron concentration of 7.4*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).

...

Kevin
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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Old 04-06-2005, 07:09 PM
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looks like Greek to me.

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Old 04-06-2005, 07:30 PM
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Seachem says that Flourish Excel will keep iron in a reduced state.
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Old 04-06-2005, 08:38 PM
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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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Old 04-06-2005, 09:32 PM
 
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Next time when somebody asks "You really want to know??" ... Answer the damn question with a no!!!!!!!

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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Old 04-07-2005, 02:38 AM
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Quote:
Originally Posted by Laith
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...
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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Old 04-07-2005, 06:05 AM
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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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Old 04-07-2005, 12:00 PM
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Quote:
Originally Posted by FMZ
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.

Steve - 33g reef and a 180g planted in need of a re-scape.
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Old 04-07-2005, 04:50 PM
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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.

óBill

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Old 04-07-2005, 04:53 PM
 
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Quote:
Originally Posted by KevinC
...
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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