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Iron is an essential nutrient for plant growth. However, if you put a simple iron compound such as iron nitrate in your solution, it will form a precipitate with other chemicals in the solution such as phosphate. To avoid this, you must use chelated iron. A chelating agent is a molecule that grabs onto an ion such as iron and holds it tightly so that it cannot precipitate. However, plants still have ways of extracting the iron they need from these compounds. EDTA iron is one type of chelated iron that you can use in a nutrient solution.
Source: Barak, Philip. "SoilScience/Horticulture/Agronomy 326 Class Notes: 15 Feb 2001." http://www.soils.wisc.edu/~barak/soilscience326/feb15_01.htm. Last visited: August 2001.
You may not be able to find EDTA iron or other chelated iron in your chemistry lab, and it is pretty expensive to buy from a chemical supplier. However, there is a way to make the chelated iron in the lab if you have EDTA acid, which is somewhat easier to find.
The following process was invented by Josh Porter in the North Hunterdon High School chemistry lab. Although it seemed to work pretty well, it is still considered experimental, and the product should be tested after the reaction is complete to make sure the process worked correctly.
Like the acid-base reaction above, this reaction requires some stoichiometry to calculate the correct amounts of reactants to use. Note that the molar mass of the EDTA acid is 292.24 g and the molar mass of the EDTA iron is 367.05 g. For example, if you need 7.9 g of EDTA iron:
You can substitute however much EDTA iron you need into the above equations to calculate how much of each reactant you need.
Materials needed:EDTA acid
iron (III) sulfate
large flask (1-Liter or larger)
stirring rod or magnetic stirrer
hot plate or Bunsen burner
pH paper or pH meter
another beaker or flask, 1-Liter or larger
very retentive filter paper
Fill the flask partway with deionized water; again, tap water usually contains ions that can mess up the reaction. Measure out the appropriate amounts of EDTA acid, iron (III) sulfate, and sodium sulfate on a balance with filter paper and dissolve them in the flask. If they do not dissolve right away, add some more deionized water and stir the mixture until the reactants are mostly dissolved.
Put the flask on a hot plate or above a Bunsen burner and heat it until the solution is boiling or nearly boiling. Stir the solution often or use a hot plate with a built-in magnetic stirrer. Keep the solution hot for about an hour, then turn off the heat and let the solution cool.
When the solution is cool, measure out about 80% of the calculated amount of hydrated barium hydroxide and add it to the solution. If you are using barium hydroxide that is not hydrated, you will need to recalculate the amount needed with the correct molar mass for plain barium hydroxide. A precipitate (barium sulfate) should form in the solution. At this point the pH of the solution should be very low. Slowly add more barium hydroxide, measuring the pH as you go with pH paper or a pH meter, until the pH reaches about 7.
To extract the solution of EDTA iron, you must filter the barium sulfate out of the solution. This is done best by placing a piece of very retentive filter paper in a funnel, setting the funnel in the top of another flask or beaker, and pouring the solution through the filter paper into the flask or beaker. This may take several hours to complete, but it works very well if the filter paper is good enough.
Do not try filtering the solution through a Buchner funnel with a vacuum pump; that technique did not work very well when we tried it. The simple gravity-filter system is the best choice for this procedure.
The solution that collects from the bottom of the funnel, if it does not contain a precipitate, is the EDTA iron you need. To test and make sure it is good, take a very small sample of it and add a little bit of potassium dihydrogen phosphate. If no precipitate forms, the EDTA iron is good. You can add it to the bottle of concentrated solution where you would normally add the chemical.
© 2001 S.H.A.R.P.: The Super Hydroponic Awesome Radish Project. All rights reserved. Photographs from this page may not be used without permission.