- New alloys containing refractory metals have been developed
by the aero-space industry to meet the demand for thermally resistant
materials. The chemical analysis of these alloys has been
handled successfully by the use of ion exchange. Hydrofluoric acid,
either alone or combined with hydrochloric acid, has here-to-fore
been used to dissolve these alloys and complex their constituents,
thus forming metal-fluoride anionic species which are preferentially
adsorbed by the resin. Large volumes of eluents, made up of
rather concentrated hydrofluoric-hydrochloric acid solutions, are
necessary to separate the adsorbed species.
The anion exchange behavior of titanium, zirconium, hafnium,
molybdenum, tantalum and niobium, in the aqueous and mixed solvent
sulfuric-oxalic acid system, was studied for three reasons:
1. The lack of methods designed for the separation and determination of small amounts of titanium, zirconium or hafnium
in molybdenum, tantalum, and niobium base alloys.
2. The inconveniences associated with the use of hydrofluoric
3. The recent findings in ion exchange chromatography that the
introduction of an organic solvent into the eluent enhances the chance
of finding the proper conditions of separation.
The distribution coefficient concept and the plate theory were
utilized to find the proper conditions of separation. The effect of
oxalic acid concentration, sulfuric acid percentage and methanol percentage
on the elution character of the six metal ions was investigated.
It was found that greater adsorption is favored at low percentage of
sulfuric acid and high concentration of oxalic acid. In the presence
of both acids, methanol decreases the adsorption of molybdenum,
titanium, tantalum and niobium, and increases the adsorption of
zirconium and hafnium.
As a result of the previous studies, 17 separation procedures,
involving different combinations of the six metals investigated, were
developed using the concept of the minimum height column. These
procedures were tested on synthetic metal mixtures and found adequate.
The range in which some of these procedures could be used
is wide. As low as 0.05% titanium and 0.05% zirconium could be
separated from a molybdenum base alloy and determined accurately.
Large amounts of constituents can be separated and determined as well. In addition to designing these separation procedures, some conclusions
were drawn regarding the complex formation involved:
1. Titanium does not form anionic complexes in the presence
of sulfuric acid and needs high level of sulfate ion to form such complexes.
2. Zirconium sulfate complexes are more stable than the hafnium
3. Molybdenum forms sulfato complexes which seem to be in
slow equilibrium with some unadsorbable species.
4. All six metal oxalato complexes are stable at low hydrogen
ion concentration. At high acidities these complexes break down
due to the repression of the oxalic acid dissociation.
Methanol tends to increase the adsorption of metal ions from
strong acid solutions. In the presence of oxalic acid, however, the
adsorption decreases. This is, probably, due to the suppression of
the dissociation of oxalic acid.