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The rate of anaerobic bleaching of red ferric thioglycolate in mixed thioglycolate (0.2 Af)-acetate (0.1 Af)-phosphate (Af/15) buffers pH 4.6-5.S at 37°( system I) has been measured spectrophotometrically and shown to follow second-order kinetics, whereas the anaerobic bleaching of the labile blue ferric thioglycolate in 0.07 M thioglycolic acid in 60% ethanol at -35°follows first-order kinetics. In the air saturated system I the rate of oxidation, catalyzed by 1.4 X 10 ~5 M iron salt, was measured, as well as the steady-state concentration of red ferric thioglycolate. The oxidation rate was maximal at pH 4.9. The iron catalysis of thioglycolate (RSH~) oxidation by oxygen has sometimes been assumed to proceed as follows:(a) formation of ferrous complex, Fe(II)RS-complex, (b) oxidation: Fe(II)RS-complex + Os -Fe(III)RS-complex, and (c) autoóxidation-reduction: Fe(III)RS-complex -*• Fe(II)RS-complex + RSSR . It is shown that the oxidation rate calculated for this scheme can account for only 1-3% of the experimentally observed oxidation rate. A modification of this reaction scheme involving free radicals is discussed.

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