Catalyzes the reduction of arsenate [As(V)] to arsenite [As(III)] (PubMed:1409657, PubMed:8003493, PubMed:10606519, PubMed:11862551, PubMed:12072565, PubMed:12682056, PubMed:16797027). In vitro, has also low phosphatase activity with para-nitrophenyl phosphate (pNPP) as substrate (PubMed:11573087). [thioredoxin]-dithiol + arsenate + H(+) = [thioredoxin]-disulfide + arsenite + H2O Potassium binding stabilizes the enzyme and increases its specific activity (PubMed:12682056, PubMed:16797027). Activity is also stimulated by sulfate (PubMed:16797027). Inhibited by arsenite (mixed inhibitor) (PubMed:11862551). kcat is 4.5 min(-1) with arsenate as substrate (at 2 uM arsenate) and kcat is 9.9 min(-1) with arsenate as substrate (at 10 mM arsenate) (PubMed:10606519). kcat is 54 min(-1) in the presence of KCl (PubMed:12682056, PubMed:16797027). kcat is 35 min(-1) in the presence of NaCl (PubMed:12682056, PubMed:16797027). kcat is 172 min(-1) in the presence of Na(2)SO(4) (PubMed:16797027). kcat is 219 min(-1) in the presence of K(2)SO(4) (PubMed:16797027). kcat is 0.53 min(-1) for the phosphatase activity with para-nitrophenyl phosphate as substrate (PubMed:11573087). Optimum pH is 8. Monomer. Induced by arsenate [As(V)], arsenite [As(III)], and antimonite [Sb(III)]. Cys-89 performs a nucleophilic attack on a Cys-10-Cys-82 intermediate, forming another disulfide intermediate with Cys-82. Deletion of the gene leads to a complete loss of arsenate resistance, but does not affect arsenite or antimony resistance. Belongs to the low molecular weight phosphotyrosine protein phosphatase family. Thioredoxin-coupled ArsC subfamily.