Any old iron?

The previous trail¹ looked at compounds of iron. But a brief excursion can also introduce some of iron's interesting redox and historical chemistry. 

Iron(II) sulphate - copperas 

Surely, your walk will take you past some concrete. This is made from cement² which, perhaps surprisingly, may contain traces of chromium(VI) compounds (chromates). Contact with skin can result in dermatitis. Since January 2005 it has been illegal to sell cement containing more than 0.0002 per cent by weight of chromium(VI). To prevent this, iron(II) sulphate is used to reduce chromium(VI) to harmless chromium(III): 

3Fe²⁺(aq) + CrO₄^2-(aq) + 8H⁺(aq) → 3Fe³⁺(aq) + Cr³⁺(aq) + 4H₂O(aq) 

Iron(II) sulphate has many other uses and has long been an important industrial chemical. The compound has been made in Britain since ca 1570. Confusingly, it was known as copperas or, better, as green copperas (to distinguish it from blue and white copperas, ie copper(II) sulphate and zinc sulphate respectively). At the time the chemistry and composition of these related compounds was not well understood. Historically, copperas was used as a mordant for wool (ie to help the dyeing process), in black ink, and in tanning leather. The compound was also used in various patent medicines and the Royal Navy even tried it as a cure for scurvy. Today, iron tablets, typically containing 200mg FeSO₄, are sold in most pharmacies as a cure for anaemia.