DETERMINATION OF IRON(II) AND IRON(III) BY
MÖSSBAUER SPECTROSCOPY
Joseph W. Stucki
University of Illinois
Sample Preparation
Samples intended for Fe(II) and Fe(III) analysis by Mössbauer spectroscopy should be regarded as being susceptible to oxidation by atmospheric oxygen and, thus, should be protected. The most convenient method for protecting the sample from oxidation is to place it inside the glove box with the Mössbauer sample holder and perform the sample loading there. If the sample is a suspension, it may be analyzed in either the gel or freeze-dried state. The guidelines for amount of sample inside the holder should be followed even if the sample is wet. This implies a knowledge of the water content of the sample.
If freeze drying is desired, this can be accomplished inside the glove box using the thermoelectric cold plate to freeze the sample, then the sample tube can be connected to the internal vacuum valve in the glove box to evacuate and sublime the sample. Once the freeze-dried sample is loaded into the sample holder, it should be removed from the glove box and placed directly into the Mössbauer sample positioner and inserted into the cryostat. The He atmosphere in the cryostat will protect it from reoxidation. The brief exposure of a freeze-dried sample to the atmosphere while transferring it to the positioner and cryostat offers little risk of oxidation. If a gel sample is to be analyzed, the same procedure can be used, except every effort must be made to minimize the transfer time to the cryostat. Being wet, the gel sample has greater risk of oxidation; but, with it covered inside the holder the exposure should be minimal if the transfer is made rapidly.
Obtaining Spectra
If using the WEB/Janis Model SHI-805 closed-cycle refrigerated Mössbauer spectrometer, go to Operation of Mössbauer Spectrometer for directions in acquiring the spectra. The temperature of operation should normally be 77 K. If Fe oxides are present, this temperature only permits a ratio of Fe(II) to total Fe, not the ratio of Fe(II) to total silicate Fe; so, if the ratio just in the silicate is desired, the spectrum should be acquired at 4 K. This should enable estimates of Fe(III) in oxides, Fe(III) in the silicate, and Fe(II) in the silicate. For complex mixtures of magnetically ordered phases, however, these estimates become rather difficult, especially if magnetic ordering begins within the silicate.
Interpreting Spectra
The spectrum must be fit with an appropriate number of Fe(II) and Fe(III) sites and components. Once a satisfactory fit is obtained, the relative areas, A2 and A3, of component peaks due to Fe(II) and Fe(III), respectively, are used to calculate the ratio, taking into account the recoil-free fractions, f2 and f3, of the di- and tri-valent oxidation states, respectively. The formula is simply
Fe(II):Fe(III) = (A2 x f2)/(A3 x f3)
The recoil-free fractions are not necessarily equal and they vary with temperature. Below 120 K, however, we found that they change little with temperature. So, as a first approximation, they may be assumed to be equal and unchanging at 77 K or below. This reduces the calculation to just the area ratio. For further discussion about recoil-free fractions, refer to (Goodman, 1980) and (De Grave and Van Alboom, 1991).
De Grave, E. and A. Van Alboom. 1991. Evaluation of ferrous and ferric mössbauer fractions. Physics and Chemistry of Minerals. 18:337-342.
Goodman, B.A. 1980. Mössbauer spectroscopy. Pp. 1-92 In Stucki, J.W., and W.L. Banwart (Eds.), Advanced Chemical Methods for Soil and Clay Mineral Research. D. Reidel Publishing Co., Dordrecht.