The extraction ofnaturally occurring free iron oxides from soils and clays with oxalate solutions is distinctly photosensitive. Apparently wave-lengths around 366 m~, the near ultra-violet region, are the most efficient; this radiation is predominant in the emission of a medium-pressure mercury arc lamp. A qualitative consideration f the dissolution process indicates that in all probability electron transfer f om solute to solid via intermediary C.zO~- is involved. Such a mechanism ay explain the attack of near-neutral oxalate solutions upon natural ferric oxides and the decomposition f the quite resistant hematite inslightly acid media, when irradiated. The reaction appears to be autocatalytic. Electron transfer utilizing thermal electrons may also explain the attack of dithio-nite ions upon ferric oxides, which was shown to proceed in media bove neutrality. Finally, a detailed escription of the method is given, in-cluding the application for estimating free iron oxides in particle-size fractions obtained uring the course of mechanical nalysis.
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Andrew S. de Endredy (1963) studied this question.