Properties of arsenic IV intermediate

The reactions discussed above show that arsenic(fV) is of redox amphoteric character and a stronger reducing agent than arsenic(in), but at the same time it is a stronger oxidant than arsenic(V). Partners of the oxidation-reduction reactions of arsenic(fV) known so far can be seen in Table 13. It follows from the redox amphoteric character that the oxidation potentials of couples involving arsenic species are in the order 

This order of the potentials indicates that arsenic(IV) is an intermediate in the oxidation of arsenicflll) and reduction of arsenic(V) which is unstable from thermodynamic point of view and disproportionates easily according to 

An estimate of the oxidation potential of couples involving arsenic(IV) can be given, based on the reaction 

It is striking that reactions of arsenic(III) with 1-equivalent oxidizing agents take place fairly slowly, while reactions with 2-equivalent partners e.g. with iodine, are fast. To interpret this behaviour it was suggested by Waters that arsenious acid has the tautomeric forms 

On the other hand, reactive radicals that are capable of abstracting hydrogen atoms from other molecules could react homolytically, e.g. 

---