Ultraviolet Spectrophotometric Determination of Iron with Ethylenediaminetetraacetic Acid

Abstract

Abstract A dilute solution of Fe3+—EDTA complex has a characteristic absorption peak at 260 mμ which is available for ultraviolet spectrophotometric determination of iron. An excess of disodium salt of EDTA was added to 0.1 m hydrochloric acid solution containing various amounts of iron as Fe3+ and the absorbance was measured with a Beckman DU spectrophotometer. A calibration curve for iron at 260 mμ was obtained and conformity to Beer’s law was found for concentrations up to 17p.p.m. of iron. The sensitivity of the method is 0.0065 γ/cm2. The absorbance of Fe3+—EDTA solution decreases when irradiated by light, while it recovers when placed in the dark. Fe3+—EDTA complex is stable in acidic medium, although its absorbance decreases rapidly below pH 0.5. The absorbance is also reduced on the alkaline side. Hydrochloric, sulfuric, perchloric or acetic acid can be used in the determination. Nitric acid is used only when the concentration is kept in a certain limited value, this acid having the absorption maximum at 300 mμ and the minimum at 260 mμ. Influence of the coexisting substances on the absorption measurement of Fe3+—EDTA complex was examined. Among the cations examined, alkalies and alkaline earths do not interfere. Cu2+, Hg2+ and Ti4+ interfere seriously when combined with EDTA, because their EDTA complexes show remarkable absorption in the ultraviolet region, which may suggest a new ultraviolet spectrophotometric determination of the metals. After the work reported herein had been completed, the authors were informed of the work of Hill-Cottingham, who reports a somewhat similar study on iron complexes of EDTA and other chelating agents.