Kinetics and mechanism of pyrogallol autoxidation: Calibration of the dynamic response of an oxygen electrode

The kinetics of the reaction between 1,2,3‐trihydroxybenzene (pyrogallol) and O 2 (autoxidation) have been determined by monitoring the concentration of dissolved dioxygen with a polarographic oxygen electrode. The reaction is carried out in pseudo‐first‐order excess pyrogallol, 25°C, 0.08 M NaCl, and 0.04 M phosphate buffer in the pH range 6.9–10.5. Data collection precedes reaction initiation, but only the data recorded after the estimated 3.2 s dead time are used in kinetics calculations. Observed rate constants are corrected for incomplete mixing, which is treated as a first‐order process that has an experimentally determined mixing rate constant of 4.0 s −1 . The rate law for the reaction is − d [O 2 ]/ dt = k app [PYR] tot [O 2 ], in which [PYR] tot is the total stoichiometric pyrogallol concentration. A mechanism is presented which explains the increase in rate with increasing [OH − ] by postulating that H 2 PYR − ( k 2 ) has greater reactivity with dissolved dioxygen than does H 3 PYR ( k 1 ). The data best fit the equation k app =( k 1 + k 2 K H [OH − ])/(1 + K H [OH − ]) when the value of the hydrolysis constant K H (the quotient of the pyrogallol acid dissociation and water autoprotolysis constants) for this medium equals 3.1×10 4 M −1 . The resulting values of k 1 and k 2 , respectively, equal (0.13 + 0.01) M −1 s −1 and (3.5 plusmn; 0.1) M −1 s −1 . This reaction is recommended as a test reaction for calibrating the dynamic response of an O 2 ‐electrode. © 1993 John Wiley & Sons, Inc.