A Polarographic Study of the Titanium-Ethylenediaminetetraacetate Complexes^1,2a

Abstract

Titanium(II1) and (IV) in solutions of ethylenediaminetetraacetic acid yield well-formed, reversible polarographic waves.Below pH 2, the half-wave potential is independent of pH. Above pH 2.5 the dependence is linear and indicates that two hydrogen ions are involved in the electrode reaction. As the pH is increased the waves become more irreversible, the diffusion currents are diminished, and the solutions become colloidal. The diffusion current of the wave is a direct measure of the concentration of the complex in the presence of either an excess of titanium or of the complex former. Measurements of pH, total titanium, total complexing agent and diffusion currents indicate that, below pH 2, the TiY complex (H4Y = ethylenediaminetetraacetic acid) is reduced to the TiY-complex. The TiY complex is in equilibrium with the TiO++ ion. Above pH 2.5, the TiOY-complex is the predominant species and is likewise reduced to the TiY-ion. As the pH is increased above 3, the TiO++ becomes increasingly hydrolyzed to colloidal titanium dioxide. In spite of the complications arising from numerous titanium sDecies. in moderate concentrations of ethplenediaminetetraacetate, pH 2 to 5 , the titanium haves are reproducible and analytically useful.The natural abundance of titanium and the increasing use of this light metal in high temperature alloys has encouraged further studies of its chemical properties and a search for improved methods for its determination. Since a major difficulty in working with titanium is to keep it in solution, further information concerning its complex ions is very desirable.The polarography of titanium(II1) and (IV) in ethylenediaminetetraacetic acid has been briefly studied by Blumer and Kolthoff . 3 They conclude that between pH 1 and 2.5 the electrode reaction is probably TiY-= TiY -+-eand calculated an approximate value of the dissociation constant for TiU of 2 X (For convenience, the formula H4kT is used for ethylenediaminetetraacetic acid and "Versene" refers to its anions.) They also stated that above pH 2.5 the half-wave potential of the complex was dependent upon both pH and Versene concentration, and that the waves were irreversible. The possibility of the existence of hydroxy complexes and polynuclear complexes was suggested. The waves of titanium in tartaric citric acid,6.6 and oxalic acid' solutions have also been studied.This paper summarizes the results of a systematic study of the polarography of titanium(II1) and (IV) in solutions of ethylenediaminetetraacetic acid and its salts. Experimental All polarograms were taken using a calibrated Sargent Model X X I Polarograph and an H-cell with an external saturated calomel electroode. A grounded water-bath, maintained a t 25.0 + 0.1 , was employed. KO damping was used, and the internal resistance of the cell (about 200 ohms) was considered negligible. The capillary used has an m value of 1.925 mg./sec., and a drop time of 4.55 sec. a t 0.0 v. i's. S.C.E. and 4.99 sec. a t -0.4 v. r s . S.C.E. Residual currents were sometimes measured, but in mos...

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