Analytical Methods:Ferrous Iron

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Questions
In anaerobic conditions iron is converted from its usual Fe3+ (ferric) form to the more mobile Fe2+ (ferrous) form. This can result in the development of greyish gley soil colours. In situations where there is seasonal waterlogging and anaerobic conditions interspersed with drier periods when more oxygen is present redox conditions may develop.

Samples
Samples must be fresh as interaction with the atmosphere can lead to oxidation, and analysis should be carried out on freshly fractured surfaces. If the soil naturally falls apart into structural units (peds) these should be broken in two to reveal their inner core.

Oxidation and reduction processes can be quite localised for example associated with pores, roots, or areas of compaction. It is a good idea to take a number of samples in order to get a better feel for conditions generally across a site.

Chemicals and materials

 * alpha-alpha-Dipyridil1 (also called 2-2-Dipyridil).
 * Table salt
 * amber or opaque dispensing bottle with a pipette or dropper.
 * disposable gloves for handling alpha-alpha-dipyridil.

Alpha-alpha dipyridil has a short shelf-life. Store in opaque or amber bottles away from direct sunlight and preferably refrigerated. ensure that the solution is colourless (not pink) before use and test its efficacy on a shiny portion of a spade or trowel; a bright pink colour should devlop within a few seconds.

Method

 * 1) Take a fresh, uncontaminated aggregate or cube of soil from the section and break it open.
 * 2) Place a few drops of alpha-alpha-dipyridil on the freshly broken surface.
 * 3) Wait a few seconds then inspect the surface for evidence of the development of a pink or red colouration.

In very organic soils, it can be hard to see the reaction so instead a small amount of soil should be placed in a small beaker or tube with alpha=alpha dipyridil solution and a small amount of table salt. The mixture should be shaken or stirred thoroughly. The solution will turn pink if reducing conditions are present.

Interpretation
A positive reaction (i.e. pink staining) indicates the presence of ferrous iron (Fe2+). This is indicative of reducing conditions which result from waterlogging and the development of anaerobic conditions. Such conditions are conducive to the preservation of sensitive organic and palaeoenvironmental remains. However, the reaction only indicates reduction at the time of sampling. If waterlogging is seasonal there may be periods of the year when the soils are dry and more oxygen is available resulting in periods of alternating reducing and oxidising conditions. This can be very damaging for organic preservation.

Some soils, particularly sands and organic soils contain very little iron and hence even when there are reducing conditions there may not be enough ferrous iron (Fe2+) to give a positive reaction.

Safety
1 Always consult chemical safety data sheets and follow appropriate health and safety protocols before handling or using chemicals.

Many chemical safety data sheets are available here from this Oxford University based database.

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