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post #1 of 7 (permalink) Old 06-26-2015, 08:04 AM Thread Starter
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Iron precipitation?

I recently received some DTPA 11% Fe and an Fe test kit.

Previously I was dosing rexolin APN micro mix [http://www.duralite.com.au/rexolin-apn-micro-mix/] and while I couldn't test Fe levels, there didn't appear to be anything strange happening. With this DTPA 11% Fe, when I dose the tank my ORP drops very rapidly and then slowly returns to previous levels.

The tank holds around 140 liters of water, and I have two 140 liter tubs I use as sumps. The first tub holds nearly the full 140 liters and the second tub with the pump holds 90 liters (on average). I dose into the first tub, which the water then goes into the second tub, then pumped into the tank. I guestimate the flow rate to be around 900 liters per hour.

The first time I dosed (0.3ppm), the pH of the tank was 6.22 and ORP 565. I have trouble reading the results of the Fe test, but I guess somewhere near 0.3ppm of chelated Fe (0.0ppm free Fe). After 15 mins ORP had dropped to 498, another 10 mins and down to 494, thereafter it began to slowly rise back to original levels. I tested Fe again about 20mins after dosing and the results were almost identical, maybe a slight shade darker.

I had not long dosed 3ppm KNO3, 0.3ppm KH2PO4, my own GH booster which was 1ppm CaCl2 and 0.3ppm MgSO4, and 4 grams NaCO3 (around 6ppm). So four hours later I dosed 0.2ppm. This time pH was 6.15 and ORP was 555. I got busy, but came back in 30mins and ORP was down to 480.

Figuring the pH might be playing some role, I then dosed 0.2ppm the next morning before the CO2 turned on. pH 7.48, ORP 488 (ORP rises and falls with pH). 10mins later ORP had dropped 478, before again rising slowly. So a smaller drop in ORP, but a lower starting ORP.

I tested Fe again, and the results look identical to the previous day. Previous day on right, and today on left.
https://dl.dropboxusercontent.com/u/...626_094041.jpg
https://dl.dropboxusercontent.com/u/...626_094229.jpg


I have dosed 0.3ppm of the rexolin APN and ORP doesn't change at all. I've dosed 0.2ppm of the APN 1 hour after dosing 0.9ppm of KH2PO4 and nothing exploded.

My head hurts reading about REDOX, but it looks to me like this Fe is being precipitated immediately.

TDS 150ppm
Nitrate 5-8ppm
PO4 5ppm
GH 5dKH
KH 3dKH

Can anyone shed some light?
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post #2 of 7 (permalink) Old 06-26-2015, 08:32 AM
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It is mentioned on here too many times to count, that you dose the KH2PO4 on one day then on the next day the iron. I don't know how you dose these but just in case it has something to do/w it...

The shortest distance between any two points is a straight line...in the opposite direction...
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post #3 of 7 (permalink) Old 06-26-2015, 04:22 PM
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I would suspect (guess) the DTPA is to blame rather than the precipitation of iron. DTPA is a much stronger chelate than EDTA so I would expect to see a much stronger reaction. Typical iron chelates are not "fully complexed" with iron. This means that the DTPA still has "room" to react with other metal cations potentially reducing redox.

Is this what's going on? I have no idea. I'm just spit-balling here. I'm certainly no chemist. Heck, I still have trouble mixing Kool-aid.
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post #4 of 7 (permalink) Old 06-26-2015, 10:30 PM Thread Starter
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Thanks for the responses.

If it's (just) the chelate, wouldn't the Fe levels rise? APN is supposed to be DTPA also.
I guess since APN is a trace mix, there are more metals for the DTPA to bind to.
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post #5 of 7 (permalink) Old 06-26-2015, 10:53 PM
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My theory (and it's just that, a theory) is that there is "free DTPA" to bind other ions in the water column. My thinking is that if you add nothing except DTPA you would see a decrease in ORP. So the iron previously bound by DTPA would remain bound and not increase any significant amount. I would not expect to see an iron increase in the short term.

I didn't realize APN was chelated with DTPA. It's possible the amount of chelate is less for APN than the iron supplement you are dosing. Again, this is all pure speculation on my part. Hopefully, we will hear from someone with a solid chemistry background.
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post #6 of 7 (permalink) Old 06-27-2015, 10:09 AM Thread Starter
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The Nutrafin Fe test kit is described as measuring "free iron" and "chelated iron".
Based on that description, I would expect to see an increase in Fe levels. *shrug*

For clarity, Rexolin APN has DTPA Fe, the other metals are bound with EDTA.
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post #7 of 7 (permalink) Old 06-28-2015, 07:41 AM Thread Starter
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I'm willing to assume the Fe was simply being precipitated, and the chelate was driving down ORP.

I waited 48 hours since dosing PO4, and while ORP still lowered (not as much as previously), this time I seen an increase in Fe levels that match the dosage rate.

There's probably a reason why you don't dose Fe close to PO4.
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