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.
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.
Can anyone shed some light?