Catalyzes the reduction of arsenate [As(V)] to arsenite [As(III)] (PubMed:16797027). In vitro, can dephosphorylate para-nitrophenyl phosphate (pNPP) (PubMed:11698660). [thioredoxin]-dithiol + arsenate + H(+) = [thioredoxin]-disulfide + arsenite + H2O Activity is potassium and sulfate-independent. kcat is 96 min(-1) in the presence of KCl (PubMed:16797027). kcat is 80 min(-1) in the presence of NaCl (PubMed:16797027). kcat is 120 min(-1) in the presence of Na(2)SO(4) (PubMed:16797027). kcat is 95 min(-1) in the presence of K(2)SO(4) (PubMed:16797027). Monomer. Induced by arsenate, arsenite, and antimonite. Mutant is sensitive to arsenate but is still resistant to arsenite. Belongs to the low molecular weight phosphotyrosine protein phosphatase family. Thioredoxin-coupled ArsC subfamily.