Abstract: 1-(2-pyridylazo)-2-naphthol-sulfonic acid (PAN-S) was obtained simply and effectively by sulphonation of PAN.The acidity constant of the reagent is reported.The investigation of the reaction of PAN-Swith metal ions shows that the characteristic of PAN-Sas a spectrophotometric reagent is similar to that of PAN.The analytical procedure would be simpler and more convenient as PAN-Sis water-soluble.