Calculating with DTPA Fe 8%

Hufsa

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Hi everyone, I purchased some Fe DTPA 8% from APC but I am unsure how to go about calculating my dosage, since the calculator only has DTPA 10%, 11% and 7%.
For 180 Liters, DTPA Fe% calculator gives me;
"To reach your target of 0.05ppm Fe you will need to add 129 milligrams (equivalent to 1/64 tsp ) DTPA Fe (7%) to your 180L aquarium to yield:
Element ppm/degree
Fe 0.05"

Can someone help me out with how the formulation would be for 8%?

I set 0.05 ppm as a target since thats how much I want each pump of the bottle to dispense.
If I know this value for 8% Fe I can calculate my way to the total amount needed in a bottle of a certain size.
Im aware the calculator also assists with this but I figured it might be be easier to get help with the less complicated equation.

Thank you very much for any help :)
 

dw1305

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Hi all,
For 180 Liters, DTPA Fe% calculator gives me;
"To reach your target of 0.05ppm Fe you will need to add 129 milligrams (equivalent to 1/64 tsp ) DTPA Fe (7%) to your 180L aquarium to yield:
Element ppm/degree Fe 0.05"
You can use that one, the difference between 7% Fe and 8% Fe isn't really relevant.

cheers Darrel
 

Hufsa

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Thanks for your quick reply dw1305, I understand the practical difference is more or less nil but I'm a bit of a stickler for detail so I would really prefer to have the calculations just right, for nothing other than to make the itch in my brain go away, if that makes sense?
 

dw1305

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Hi all,
so I would really prefer to have the calculations just right
I can go through the workings, it is quite straightforward normally for most simple salts (like KNO3), but a bit messy for a large organic acid chelator like DTPA. In fact it is so messy that an accurate calculation won't be any more accurate <"than a bit of guestimation">, but here goes.

You need the percentage of the element you are interested in, in the compound you are using, in this case FeDTPA and 8% Fe. If you need to work out the percentage, you need to know the Relative Molecular Mass (RMM) of the compound, and the Relative Atomic Mass (RAM) of the element. This is a bit tricky, because the formula is C14H18N3O10Fe(NH4)2, the RAM of iron is ~56, and adding up all the other RAM (14*12 for carbon (C) etc) gets us somewhere near 480 as the RMM for FeDTPA, which would make it 12% Fe (you need a RMM of 700 to get 8% Fe). In this case I assume that there is another metal chelated as well.

When you add 1g of FeDTPA to a litre of water, you've added 0.08 g of Fe, 0.08g is equivalent to 80 milligram, and milligrams per litre is equivalent to ppm. You have a solution of 80 mg/L Fe. You have a tank of 180 litres,so you can divide 80/180 and that gives you 0.44 ppm, but if you had added 0.09g of Fe you would have the much more useful 0.5 ppm.

To get to our magic number of 0.5ppm you need to start with 1.125g of FeDTPA, we can then divide that by 10 to get to 0.05 and that gives us 112.5 mg of FeDTPA.

cheers Darrel
 

Hufsa

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Thank you again Darrel, you are too kind :)

In this case I assume that there is another metal chelated as well.
Im not sure I understand this part, are you saying that the Fe 8% is not "pure" and that I am adding some other unknown metal along with it?
 

dw1305

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Hi all,
are you saying that the Fe 8% is not "pure" and that I am adding some other unknown metal along with it?
Probably, you know that all the chelated cations aren't Fe++(+), because you don't have a high enough iron content.

The FeDTPA (or micro-element mix etc) you buy for use in the aquarium will be an industrially produced product re-branded, something like <"Yara Rexolin FeDTPA-L">.

The DTPA may be partially chelated, (some of the exchange sites don't have a cation) or there maybe another cation present. I don't know which it is for DTPA, but for EDTA you start with another cation chelated, and I'll assume it is the same for DTPA and that sodium (Na) is the most likely cation (it is for disodium EDTA). It will definitely be a monovalent cation (Na+, K+).

<"When you make the FeEDTA"> you use ferric sulphate heptahydrate (FeSO4.7H2O) as your iron ion donor solution and the sodium ions Na+ are exchanged for Fe+++ in solution, via <"the lyotropic series">. You can end up with either FeNaEDTA, or FeEDTA, or a mixture of both.

If you have salt like potassium nitrate (KNO3) you have a known composition and it is a bit more straightforward:

K = 39 and NO3 = 14 + (3*16) = 72, therefore KNO3 has an RMM = 101 and K ~29% and NO3 is ~ 71%. It is always going to be exactly this. The only thing you need to take into account with some salts is their degree of hydration, the <"water of crystallization">.This means that "Epsom Salts" are not MgSO4, but MgSO4.7H2O, and only ~10% Mg.

cheers Darrel
 
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