Speciation of Fe (II) and Fe (III) in Geological Samples by Solvent Extraction and Flame Atomic Absorption Spectrometry (FAAS)

A procedure has been devised for the selective determination of ferrous and ferric oxides in geological samples such as rocks, soils, cores, stream sediments, etc. This is based on the selective dissolution of both oxides with an ammonium bi-fluoride and sulfuric acid mixture, followed by solvent extraction separation of Fe(III) into ethyl acetate using thiocyanate. The extracted Fe(III) was stripped off into dilute hydrochloric acid solution and determined by flame atomic absorption spectrometry (FAAS). Fe(II) in the aqueous solution was determined by FAAS separately. The method was applied to a number of geological samples including certified reference materials (CRMs) and synthetic mixtures. The results obtained by the proposed method for the CRMs were found to be in excellent agreement with the reported values. Similarly, the results of a set of geological samples were comparable with those obtained by the colorimetric method involving 1,10-orthophenanthrolene and the standard titrimetric (Pratt's) method.