Yankuang ceshi (Mar 2020)
Determination of Fe(Ⅱ) and Total Iron in Red Sandstone by o-phenanthroline Spectrophotometry
Abstract
BACKGROUND Red bed sandstone is a set of continental or shallow lake clastic rocks in the Danxia landform. Accurate determination of Fe(Ⅱ) and Fe(Ⅲ) in red bed sandstone that indicates the redox relationship during weathering can serve as the dividing basis of weathering degree and depth of the block. During the determination of iron content by phenanthroline spectrophotometry, Fe(Ⅱ) is easily oxidized and the result is a lower value than it should be. Effective prevention of oxidation is the focus of accurate measurement of Fe(Ⅱ) and Fe(Ⅲ) content. OBJECTIVES To explore the preservation method of Fe(Ⅱ) during dissolution and establish a spectrophotometric method for the determination of Fe(Ⅱ) and total iron in red sandstone. METHODS Color reagents were added in the samples, and the samples were dissolved by hydrofluoric acid and dilute sulfuric acid, in a constant temperature water bath at 90℃. Hydroxylamine hydrochloride was used as a reducing agent, o-phenanthroline was used as a chromogenic agent, pH was adjusted by acetate-ammonium acetate buffer, and trace Fe(Ⅱ) and total iron were determined by spectrophotometer. RESULTS The detection limit of Fe(Ⅱ) was 0.002%, relative standard deviation (RSD, n=8) was less than 2%, and the standard recovery was between 92.6% and 94.7%. The Fe(Ⅱ) content of red sandstone in the Danxia landform area of Chishui was determined by this analysis method. The content of Fe(Ⅱ) ranged from 0.01% to 0.1%, the total iron content was 0.7%-1.5%, and the content of Fe(Ⅲ) calculated by subtraction was 0.7%-1.5%. The measurement results were consistent with those of X-ray fluorescence spectrometry and potassium complexate titration method. CONCLUSIONS This method uses developer and hydrofluoric acid-dilute sulfuric acid to dissolve the samples, which can effectively preserve the original Fe(Ⅱ) in sandstone. The same solution can be used for the determination of both total iron and Fe(Ⅱ) which provides a simple and accurate analysis method for various types of iron content in red bed sandstone.
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