Syntheses and Fe(II)/Fe(III) equilibria of the new multidentate ligands pyridine-2-phosphonic-6-carboxylic acid and 2,6-pyridinediphosphonic acid for the use of their iron chelates as catalysts for the oxidation of H2S to S8 by air

The syntheses of two terdentate chelating agents, pyridine-2-phosphonic-6-carboxylic acid (2PP6C) and 2,6-pyridinediphosphonic acid (2,6PDPA), are described. The stepwise stability constants for the ferric complexes at 25.0 degrees C and mu=,0.100 M (KNO3) are log K-ML = 15.97 and log K-ML2 = 9.50 for 2PP6C, and log K-ML = 20.87 and log K(ML)2 = 7.81 for 2,6PDPA. Under the same conditions the stepwise formation constants for the ferrous chelates are log K-ML = 8.70 and log K-ML2 = 5.10 for 2PP6C, and log K-ML = 10.12 and log K-ML2 = 5.33 for 2,6PDPA. The stabilities of the Fe(III) and Fe(II) chelates are high enough to prevent precipitation of Fe(OH)(3) and FeS, respectively, from solutions having pH as high as 10.0. The rates of oxidative degradation of these ligands are very slow when the iron chelates are used as redox catalysts for the oxidation of H2S to S-8 by air. The rates of oxidative degradation are immeasurably slow when the iron chelates are used with sodium thiosulfate as a radical scavenger.