Reduces arsenate [As(V)] to arsenite [As(III)] using glutathione and glutaredoxin as sources of reducing equivalents. GrxA is the most effective electron donor in vivo compared to other glutaredoxins. Constitutes the major arsenate reductase compared to ArsI1 and ArsI2. Also shows weak phosphatase activity toward p-nitrophenyl phosphate. H2O + O-phospho-L-tyrosyl-[protein] = L-tyrosyl-[protein] + phosphate [glutaredoxin]-dithiol + arsenate + glutathione + H(+) = arsenite + glutathionyl-S-S-[glutaredoxin] + H2O Homodimer. Cys-8 probably provides the initial functional group for covalent attachment of the electron-accepting substrate, arsenate, to ArsC. Belongs to the low molecular weight phosphotyrosine protein phosphatase family.