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A precise, accurate and rapid method for the sequential determination of FeO and Fe 2 O 3 in rocks, soils and some non-refractory minerals by 1,10-phenanthroline spectrophotometry is described. Fe(II) and Fe(III) were leached from the sample (-200 mesh) using a mixture of NH 4 HF 2 and H 2 SO 4 at 40-80°C for 10 min on a hot plate. Both Fe(II) and Fe(III) could be conveniently estimated sequentially from the same reaction mixture at the lg g -1 to percentage level. The method is better than the existing wet chemical methods, including the commonly used Pratt's titrimetric redox method, for Fe(II) and Fe(III) determinations in rock and soil samples in terms of precision, accuracy and rapidity. The throughput of the method was very high; at least forty to fifty samples could be estimated easily in a day. The results obtained compare favourably with those obtained by Pratt's method, as well as for certified/recommended values of a set of eleven certified reference materials having FeO and Fe 2 O 3 contents in the range 0.21-14.63% and 0.58-8.48%, respectively. The optimised 1,10 phenanthroline method was found to be accurate to within 0.21% m/m FeO and 0.30% m/m Fe 2 O 3 compared with the literature values of the certified reference materials studied.

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