GUPTA and Chatterjee1B8 introduced ac peak potential (E,) in place of the half-wave potential into the Kolthoff-Lingane equations and the DeFord-Hume equation4 for the study of complexes in solution.
Their study involved ligands which either did not exhibit dc waves or yielded waves whose E,,, were more negative than those of the metal ions. The present communication deals with the determination of the dissociation constant of the complex of Fe(II1) with 7-nitro-oxine-5-sulphonic acid (NOSA) by ac polarography.
NOSA was prepared and purified as described by Welcher.6 The stock solution of Fe(II1) was prepared by dissolving ferric nitrate (Merck) in distilled water and the iron content was estimated by the usual methods.
pH was adjusted to 2.5 by HCl, and NaClO, was used to maintain the ionic strength at 1.0. The dme had the following characteristics : m -4.564 mg/s and t = 1.8 s in 0.1 M KC1 (open circuit). Temperature was maintained at 30 f O.l"C by a Townson and Mercer unit. Mercury used for the dme was first purified chemically and subsequently distilled under reduced pressure. The see was used as reference.
The dc polarographic reduction6 of NOSA at the dme and spectrophotometric studies' on the Fe-NOSA complex have been reported by us earlier ;' a stable complex is formed in the pH region 2-O to 3.0. In this pH region NOSA yielded a dc polarographic wave6 which seriously interfered with the Fe(II1) wave, thereby rendering the measurement of the shift of Ella very difficult.
In ac polarographic studies, however, it was found that NOSA did not give any peak, thus enabling the measurement of the shift of EP of the complex to be made more reliably.
The fact that E, became more negative with increasing ligand concentration suggested complex formation.
The plot of (-E,), DS log (concentration of NOSA) yielded a straight line having a slope -0.1822.
From this value the co-ordination number was calculated to be 3.083, ca 3, with n = 1. The formula Fe(NOSA), may thus be assigned to the complex, in agreement with the value obtained spectrophotometrically.'
The extrapolated value of (E,),, was found to be -0.800 V, corresponding to l=O M NOSA.
The dissociation constant, calculated by using the