An investigation of the absorption spectra of EGTA metal complexes and the development of spectrophotometric methods for the analyses of copper, erbium, europium, holmium, neodymium, and praseodymium Public Deposited

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  • The reactions of EGTA, [ethylene bis (oxyethylenenitrilo)] tetraacetic acid, with 51 metal ions have been studied spectrophotometrically in aqueous solutions. The absorption spectra of the chelates formed were measured with a recording spectrophotometer between 200 and 700 mμ. The pH of each solution was varied and a spectrum was scanned at different acidity levels. EGTA stock solutions were prepared by dissolving the acid in two or more equivalents of sodium hydroxide followed by aqueous dilution. The dibasic EGTA ion existed below pH 8 and the tetrabasic ion existed above pH 10. The reagent solutions possessed outstanding stability and with few exceptions retained their stability when complexed with metal ions. The spectra of 25 metal ions showed measurable differences in the presence of EGTA. The majority of the solutions that formed characteristic spectra were stable. Many of the complexes formed were colorless in the visible region but did have characteristic spectra in the ultraviolet. The most pronounced spectral changes in the ultraviolet region resulted from EGTA chelation with the following ions: iron (II), copper (II), palladium (II), europium (III), and thallium (I). In the visible region praseodymium (III), neodymium, holmium, erbium, iridium (III), gold (III), and uranyl ions had significant spectral differences resulting from the addition of EGTA. The variety of spectral changes induced in various rare earth ions by complexation with EGTA made possible a specific qualitative identification of praseodymium in the presence of much greater concentrations of erbium, holmium, and neodymium. The combining ratio of metal ion to EGTA was determined by a spectrophotometric method of continuous variations for copper (II), neodymium, holmium, and erbium. All ions formed 1:1 chelates. The conformity of the EGTA-metal ion solutions to Beer's Law was determined for copper, neodymium, europium, holmium, and erbium complexes. A study was made of the quantitative use of the copper (II)- EGTA absorption maximum at 298 mμ and the holmium-EGTA absorption peak at 450 Though the spectral absorption sensitivity of the holmium-EGTA complex was not large, the complex did have a molar absorptivity of over twice that of holmium ion. The absorption of the copper (II)-EGTA complex at 298 mμ, with a molar absorptivity of 3065, made possible routine analysis of copper ion in concentrations above one part per million. The results of a series of analyses of solutions with copper concentrations from 4.00 x 10⁻⁵ M to 4.00 x 10⁻⁴ M showed good agreement with an average error of 1.4% of the known concentration.
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