An organometallic compound containing arsenic. In clinical-biochemical practice it is used to measure the concentration of calcium - it forms a colored chelate with it.
2,2′-(1,8-Dihydroxy-3,6-disulfonaphthylene-2,7-bisazo)bisbenzenearsonic acid, 2,7-Bis(2-arsonophenylazo)chromotropic acid