The complexes of ethylendiamine-tetraacetic acid with heavy metals
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Abstract
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.
Description
Thesis (M.A.)--Boston University
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