M
Marcel G
Guest
Sometimes we are making wrong conclusions from what we see (or think we see) in our tanks. And this appears to be the case.
How did you come to the conclusion that adding trace elements the same day as macroelements causes problems with iron availability? What did you based this conclusion on? Any facts? Any scientific (= rigorous) experiments? Many scientists I know add trace elements with macroelements into their test tanks (in hydroponics) ... without any problems! So why are we advised to do otherwise? To back this advise up on our subjective observations is very poor reason. Why are we repeating this myths again and again, without making any effort to verify if it's true?
Some facts about (iron) chelates:
Chelating agents (special organic compounds) "wrap" iron ions, thereby making them invisible for other anions in solution (with whom they would otherwise form a stable salts), and at the same time available to plants.
Chelates are composed of a negatively charged organic complex and a positively charged cation (such as Fe+2 or Fe+3). Chelate caughts iron "into claws."
As long as the cation (e.g. iron) is locked in the chelate, it resists the attacks of various anions (e.g. hydroxides or phosphates), which would otherwise (if it was outside the chelate) readily and promptly combine with it to form insoluble compounds (precipitates).
How much "invisible" the iron in the chelate is, depends primarily on the:
→ strength of a particular chelate
→ pH of the solution (typically associated with alkalinity)
→ temperature
→ irradiation (light intensity)
Types of chelates:
→ Fe-EDDHA and Fe-EDDHMA discolorate water to pink or brown at the concentrations as low as 0.2 ppm Fe!
In each commercial mixture of microelements some of the above iron chelates are usually used.
Unfortunately, not all manufacturers state on the product labels what chelates are used in their stock solutions.
As I mention above, usability (stability) of particular chelates depends not only on pH, but also on the temperature and light intensity (irradiation). For this reason the real usability of chelates is usually lower than commonly stated (e.g. Fe-DTPA should be theoretically stable up to pH 7.5, although under normal conditions it may start to degrade even at a lower pH). If we put the chelate solution into the refrigerator (where there is dark and very low temperature), the chelate may stay in the solution even for weeks or months. But as soon as we put the chelate into the tank, where there is higher temperature and relatively high intensity of light, it immediately begins to degrade and release the iron ions into the water, that without protection of chelates awaits they rapid oxidation and precipitation (see the picture). Therefore, one-time application of the chelated iron does not by itself guarantee continuous supply of iron, and logically it won't protect us against the formation of precipitates – a process which is essentially inevitable in the aquarium regardless of the chelate we use.
Picture: Concentrated stock solution of fertilizer with Fe-EDTA chelate irradiated with 500 µmol.m-2.s-1.
Picture: Concentrated stock solution of fertilizer with Fe-DTPA chelate irradiated with 250 and 500 µmol.m-2.s-1.
Regarding the susceptibility of iron to bind to other anions in solution (e.g. PO4-- or OH-) forming insoluble precipitates (e.g. Fe2(PO4)3 or Fe(OH)3), this applies only to oxidized iron (Fe+3), that is freely available in water or is being released from the degrading chelate. As long as the iron is locked in the chelate, it won't bind to other compounds.
If thus iron precipitates in our water, it may be caused by:
- gradual degradation of chelates by light, temperature and pH (or possibly other biotic factors)
- the use of too weak chelates in solution with higher pH
- by using as low pH as possible (which is not always possible in the aquarium)
- by using a sufficiently strong chelates (ideally DTPA)
- by continuous dosing regime (rather smaller amounts but more often)
Last edited by a moderator:
