The complexes of ethylendiamine-tetraacetic acid with heavy metals
Baker, Ann Eldora
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The research presented here was concerned with the determination of the number and composition of the complexes of the heavy metals, nickel, cobalt, copper, ferric iron and aluminum, with ethylenediamine-tetraacetic acid. Potentiometric titrations of ethylenediamine-tetraacetic acid plus a heavy metal with sodium hydroxide showed a lowering of pH when compared with the titration of the acid alone. This proved that complexes were being formed and that during this formation protons were given off, thus lowering the pH. Different curves, each with definite level areas where reactions were taking place, were obtained when different ratios of metal to acid were used. Thus complexes were formed containing different numbers of acid molecules and metal atoms. When an ion of ethylenediamine-tetraacetic acid was titrated potentiometrically against a metal ion, pH jumps and leveling of the pH occurred at definite ratios of metal to acid. In these cases one complex formed, which, upon addition of more metal ion, formed another complex differing in both metal to acid ratio and in hydrogen ion or hydroxyl ion content. Vihen the second reaction was complet,ed the pH leveled off. Thus these pH curves, in which pH was plotted against the molar metal to acid ratio, determined the composition of each complex as far as the number of metal atoms and acid molecules were concerned. The curves also indicated that hydrogen or hydroxyl groups were contained in the complexes. Conductimetric titrations of each acid ion against each metal ion were carried out. A break in the linear curve of the conductivity plotted against the molar ratio of metal ion concentration to acid ion concentration showed the number of metal atoms and acid molecules present in each complex. These results corroborated those of the previously mentioned potentiometric titrations. Solutions of acid ion and metal ion were then mixed so that the molar metal to acid ratio was that of the complex between the two ions, thus the ratio found above. The resulting solutions were then titrated with base or acid. If in the case of titration with base a normal acid base curve was obtained when pH was plotted against the molar ratio of added hydroxyl ion to the total ethylenediaminetetraacetic acid in the solution, then the protons reacting with the base were freed when the complex was formed. If the curve was not a normal one, the hydroxyl ion reacted with the complex directly. When hydrochloric acid was used instead of base in the titration, if a pH jump occurred, protons were being taken up by the complex. In each case the pH jumps occurred at definite molar ratios of hydroxyl ion to total ethylenediaruine-tetraacetic acid or of hydrogen ion to total tetraacetic acid, thus showing how many hydrogens were given off in the formation of the complex, hydrogens were given off in the formation of the complex, and how many hydroxyl or hydrogen ions were taken up by any complex to form a new complex. Equations were then written, which agreed with the combined results obtained above. From these equations the composition of each complex was known.
Thesis (M.A.)--Boston University