Investigation of the consumption of diphenylpicrylhydrazyl in solution in the absence and presence of ultrasound

The reaction by which diphenylpicrylhydrazyl (DPPH) is decolorised in trichlorobenzene, toluene and benzene solution in the absence of ultrasound and at elevated temperatures has been demonstrated to conform to first-order kinetics. This reaction is best rationalised as resulting from an abstraction reaction between DPPH and the solvent. The activation energy for this reaction is in good agreement with those deduced for the abstraction reactions between a polystyrene macro-radical and either toluene or benzene.

The reaction of DPPH in the presence of ultrasound has also been shown to occur by an abstraction reaction and not by reaction with radicals resulting from the extremes of temperature and pressure generated as the cavitation bubbles collapse. The rate constants for this reaction, determined over a wide range of sonochemical conditions, appear to be dependent on the power dissipated by the ultrasonic horn, the diameter of the irradiating horn and the vapour pressure of the solvent.