2H-Naphtho(1,2-d)triazole-6,8-disulfonic acid, 2,2'-(azoxybis((2-sulfo-4,1-phenylene)-2,1-ethenediyl(3-sulfo-4,1-phenylene)-NNO-azoxy-4,1-phenylene))bis-, tetrasodium salt 74398-75-7