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There is certain amount of micronutrients in waterc that plants can uptake by roots and leafs for health grow. Some of elements , especially speeking of iron (Fe ), which don't last long in water as free cationt, and thanks very common phenomena, such elements get forms which are not no longer soluble as insoluble crystals which most likely will be unavailable to the plant. Adding these forms micronutrients to water is therefore inappropriate. I have to mention that some elements are toxic to fishes and invertebrates copper, zinc, cobalt ...). Fortunately, there are substances , called chelates , which can micronutrients keep in soluble form for a long time.

The name "chelate" comes from the Greek "chela" - "crab claw" . Chelators or chelating agents are ligands capable to form a molecular ring ( rings see picture below) with metal ions. Rings of such ligand "protects" the metal atom from creation of strong ties and the transition to water-insoluble forms not accessible to the palnts - for example Fe 3+ . Chelates generally have excellent solubility in water.
Basically metals (not all ) are available in the aquarium (problematic is mainly iron) only after some time ( dependent on many factors like pH , protein content hardness of the water ... ) and then transite to a state where they are poorly accessible for plants and for some species of plants are not accessible at all. Chelate is a large molecule that "wrapps and hold" metal atom and protect it from reacting with other ions. This will also prevent the toxicity of heavy metals such as zinc and copper.

Chelators are not used only as a fertilizers. You may meet it in the food industry , where maintaining the taste of food (from beer to sauces at McDonalds) . In medicine, they are used for curing of heavy metals poisoning - chelate is dissolved in blood and then easily filtered out by liver or kidneys. Another important use is in chemistry.

Figure 1 : chelate (circles ) wrapping (protecting ) metal ( yellow )

Figure 2: The model EDTA

Plants require metals for proper growth. These metals are received from surrounding of the plants in the chelated forms. Plants can produce acids themselves with similar effects as chelate agents(eg citric acid) and only used to dissolve the metal and its transport to the plant body. Already chelated elements are easily accessible for the plant. Proccess of releasing of metal from chelate is not known.

Stability of chelates

Stability of chelates is quite complicated. Each chelate "holds" (is stable) only under certain conditions.
  • radiation
  • The problem of all of the large molecules, which include chelates, is sensitivity to radiation of high frequency (energy) - UV light is a good example . UV light can destroy chelate as well as is able to damage your DNA and cause cancer.

    Figure 3 : Stability of Fe -DTPA and its temperature and lighting dependency

  • pH
  • Each chelate is stable only in a certain pH range, depending on which chelating agent has been used.

  • competition
  • Figure 4 shows the stability of chelates in clean water. The problem is if the solution is other metals (calcium and magnesium are also metals!) , Unbound chelated , in part , depends on the concentration of the winner of the iron and iron chelate displaced. If there are other metals (not bounded in chelate) ions (from sulfates or chlorides) in solution, they can displace original bounded metal in chalate.It shows Figure 5 Stir the micro elements in PMDD or Estimative Index always separately!

Figure 4: stability of chelates
red = unstable, blue = stable
Figure 4: stability of chelates red = unstable, blue = stable

figure 5 Substitution of Fe in Fe-DTPA by free metals

From the tables of stability can be inferred suitability of chelated mixtures of micronutrients:

  • Canadian Plantex CSM+B (only Fe-EDTA)is suitability for water up to pH 6,5
  • Dutch Tenso coctail (Fe EDTA+DTPA)is suitability for water up to pH 7,5
  • Czech MicroMix plus (FeEDTA, DTPA, EDDHMA, gluconate)is suitability for water up to pH 9
However, when daily dosed all "irons" can be used. Stabilty chelate is determined by the size and the quantity of circles of ligand. Effect of chelatation is strongest if the circles "protecting" metal are formed of 5 or 6 atoms.

The most commonly used chelating agents are :
  • EDTA - ethylenediaminetetraacetic acid
  • DTPA - diethylene triamine pentaacetic acid
  • EDDHA - ethylendiamindihydroxyphenylacetic acid
  • EDDHMA -
  • EDDHSA - ethylendiaminedisulfohydroxyphenylacetic acid


Fe - EDDHMA    Fe -DTPA Fe -EDTA   

EDDHA , EDDHMA and EDDHSA chealtes only iron. EDTA and DTPA chelates iron and copper, zinc, manganese. Iron ( Fe 2 + also Fe 3 + ) . EDTA and DTPA are not very stable at high pH (see figure). Because Fe EDDHMA is much more stable in alkaline environment and should therefore be preferred over FeEDTA and FeDTPA waters with a high content of calcium carbonate . Fe - EDDHA is of 2.5 x expensive than EDTA. Its use is very effective and usually just part does the same effect effect in comparison with Fe - EDTA. Fe - EDTA is the least stable . EDTA is often used because it is inexpensive. There is known effect of Fe- EDTA and Fe -DTPA mixture where resultant mixture is as stable as Fe -DTPA itself .
The bond strength Fe - EDDHA is 100 billion times stronger than the Fe - DTPA , and that is 1000 times stronger than Fe - EDTA.

If you decide to buy chelates in your store be careful what you are buying, because there is more then 5 different compound sold as "Fe EDTA". Some of them contain amonia which your fish might not fully appreciate. So if you do not know what you are doing, do not do it.
Next complication when buying chelate like EDDHMA are isomers. Isomers are compounds with the same chemical formula but have different physically 3D arrangment. It also gives them different properties. When producing EDDHMA - Fe always look at ortho and para isomers ration. While the ortho isomer has excellent properties, phara izomer is is not considered as a chelate. Fe-Eddhma 6% , in fact, usually contains only 70% of chelated iron (4.8%) Links :



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Tomáš Lízner

Ahoj, velice zajímavý článek. Měl bych dotaz ke stabilitě vázané na světlo a teplotu. V grafu uvažuješ situace s vysokými teplotami 20 a 40°C. Mohl bys prosím přidat (nebo alespoň popsat) graf zohledňující stabilitu při skladování roztoku v lednici v neprůhledné lahvi? Tedy tma a teplota kolem 6-8 stupňů. Nepovedlo se mi ani v zahraničních zdrojích dopátrat zpracování této závislosti a i když předpokládám, že stabilita bude vyšší, je určitě velký rozdíl mezi 14 dny a třeba třemi měsíci... Díky. p.s. neškodilo by vymazat ten post níže od zyf.. :-/

Vaughn I would be very grateful if you could translate your graphs. Article is very interesting. Thank you.
LexLutor Parádní článek, Konečně vím, co to tam děla. Diky!
MRP Lepší bych nerekl, ale můze byt vhodnejsi pri vyssim pH vody. Samozrejme je potreba brat v uvahu, ze ma uplne jine slozeni respektive pomery mikroprvku.
Tomáš Dobrý den, znamená to, že Tenso Coctail je lepší než Plantex, protože obsahuje DTPA-FE?
Burian Vážení - chtěl bych se zeptat jestli existuje chelát na vyvázání rtuti a hliníku z lidského těla. Odpověď zašlete na e-mail: děkuji.B.
Peter Dobrý text.