The determination of ferrous and ferric iron in rocks and minerals; and a note on sulphosalicylic acid as a reagent for Fe and Ti

Abstract

SynopsisInstrumental methods of chemical analysis, including electron and ion-probes, X-ray fluorescence, atomic absorption, and plasma-source spectrometry, have largely replaced classical wet-chemical analysis except for ‘referee’ analyses, but have not so far proved able to determine the net state of oxidation of a rock or mineral with reasonable accuracy. Mössbauer spectroscopy can indeed give fairly accurate Fe2+and Fe3+estimates in favourable circumstances, but fails when the Fe2+: Fe3+ratio is very small or very large or when the iron is present in several different lattice positions with different surroundings; and it cannot arrive at the net state of oxidation when other elements of variable valency are present. The term ‘net state of oxidation’ calls for some comment, and is dealt with in the full paper; for the moment we note as an example that Mössbauer spectroscopy shows that much of the iron in ludlockite is ferric, whereas a ‘ferrous iron’ determination corresponds to an arsenate of lead and ferrous iron, from which it follows that part of the arsenic must be trivalent, a result that could not have been arrived at from either the ‘ferrous iron’ or the Mössbauer data along. Thus in the present state of the art a wet-chemical method for net state of oxidation is a necessity.