M. KEYVANFARD and H. POURETEDAL, “Catalytic Spectrophotometric Determination of Trace Amounts of Ag(I) using the Oxidation of 1,8-Dihydroxy-2-(4-sulfophenylazo)naphthalene-3,6- disulfonic Acid Trisodium Salt with Peroxodisulfate”, ajc, vol. 19, no. 5, pp. 3747–3754, Mar. 2010.