RESULTS AND DISCUSSION Pyrogallol autoxidizes rapidly in aqueous solution : the faster the higher the pH, and several intermediate products are apparently formed. Thus the solution first becomes yellow-brown with a spectrum showing a shoulder between 400 and 425 nm. After a number of minutes the colour begins to turn green and finally, after a few hours, a yellow colour appears. In the present investigation, the autoxidation was studied essentially during the first step(s) and the rate was taken from the linear increase in absorbance at 420 nm which is seen for a number of minutes after an induction period of some 10 s. The validity of this procedure is demonstrated in Fig. 1 by the constant relationship between increase in absorbance at 420 nm per min and oxygen consumption. However, this relationship was not tested at the higher pH values because of too slow a response of the oxygen electrode. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBAEffect of zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBAEDTA In the absence of EDTA the autoxidation is faster than in its presence, and it tends to vary in rate and is less affected by superoxide dismutase. In the presence of EDTA the rate is independent of the con- centration of the chelator (tested in the range 0.1- 2 mM), which indicates that the effect of EDTA is only due to its binding of traces of metal ions. Most experiments in the present report were performed in the presence of 1 mM EDTA. Effect of pH zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBAon the Autoxidation Fig. 1 shows the effect of pH on a number of parameters of the autoxidation. The rate of autoxida- tion, taken as the initial rate of increase in absorbance at 420 nm, is increased by a factor of 2 per 0.3 pH units around pH 8. When pH 9 is approached, the slope of the curve is decreased and a minimum is found around pH 9.5. At higher pH values, the slope again increases and finally corresponds to a doubling of the rate per 0.3 pH units. At pH 7.9 the autoxida- tion is inhibited to 99% by superoxide dismutase. The sensitivity to superoxide dismutase decreases when the pH is increased, but still amounts to 93% at pH 9. At higher pH values there is a strong decrease in the sensitivity to superoxide dismutase and at pH 10.6 the autoxidation is inhibited to only 15% by superoxide dismutase. The decrease in inhibition as the pH is raised can hardly be attributed to inactiva- tion of the superoxide dismutase, as it was added in a large excess and addition of more superoxide dismutase had no further effect. The results indicate that there are at least two different mechanisms of autoxidation in the pH interval investigated. One mechanism, predominant at pH < 9.5 involves 02.- as a chain-propagating species. At higher pH values, another mechanism, not depending on 02.-, becomes predominant.