The most important and familiar oxyacid of sulfur. Recognized oxyacids of sulfur include those represented by formulas H₂SO_x and H₂S₂O_y with x = 1–5 and y = 2–8. Of these, H₂SO₄ (sulfuric), H₂SO₅ (peroxysulfuric or Caro's acid), H₂S₂O₇ (pyrosulfuric), and H₂S₂O₈ (peroxydisulfuric) have been characterized.