Redox and redox equilibria
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Why are some iron(II) salts less suitable for use as a primary standard in volumetric work than others?
A primary standard has to be available in a pure state. It also has to be stable in air (so not hygroscopic or deliquescent and not liable to decompose).
In the case of iron(II) salts it has to be possible to dissolve the weighed specimen of salt without it being oxidised by the air to iron(III).
Because of the problems of oxidation, iron(II) salts do not make good primary standards. The best choice is probably iroin(II) ammonium sulphate made up into solution, under acid conditions, in purified water that has been boiled, then cooled to remove dissolved oxygen.
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updated: 28 February 2005
