Recipe for Ferric Salts of Ethylenediaminetetraacetic Acid

Abstract

The supply of iron to plant roots was one of the main problems in water culture until about 1950. The main source of iron was originally ferrous sulfate, already used by Sachs (9). It had to be supplied almost daily to the nutrient solution as it precipitates gradually. Some improvement was obtained with organic com-pounds, mainly ferric citrate, introduced by Gile and Carrero (4). The favorable effect of natural humates on plants in water culture was noticed by Bottomley (1). Olsen (8) interpreted this as a natural formation of iron humate complexes, resulting in experi-ments with iron humate. Horner et al. (6) developed a synthetic humate, capable of holding iron in solution. Unfortunately, this humate is difficult to prepare, besides which all humates are of indeterminate composition. Iron humates and other sources such as iron lignin sulfonate (2) and iron metaphosphate (3) gave satisfactory results for certain plants, but none of these com-pounds has been generally accepted. Alter a publication by Jacobson (7), ferric salts of ethylene-diaminetetraacetic acid soon met with general acceptance in water culture. One addition of FeEDTA proved to be sufficient for many weeks. Thanks to Jacobson, FeEDTA became the main source of iron for the greater part of water culture. Jacobson's original recipe is as follows: It is convenient to prepare the complex by dissolving 26.1 gm ethyl-ene diamine tetra-acetic acid in 268 ml of 1.0 N KOH, then adding 24.9 gm FeSO4-7 H2O and diluting to one litre. After aerating overnight to produce the stable ferric complex, the pH should be about 5.5. One ml of this solution provides 5 ppm to one litre nutrient solution. When this recipe is followed, the pH is 2.4 to 2.6 directly after preparation, and 3.5 to 3.6 after 12 hr of intensive aeration, in-stead of the required pH about 5.5. It was evident that not all the ferrous iron changed into chelated ferric form. A good criterion for a complete reaction is the color of the solution, which should be dark yellow-brown (No. 156, Senf color chart). Jacobson's recipe has been repeated in many other publications, including Hewitt's standard work on water culture methods (5). In a personal communication Hewitt concluded that no