In extracting cadmium from aqueous solution of sulfate with bis(2-ethylhexyl)dithiophosphoric acid-trioctylamine in heptane, a reduction in pH could cause the decrease of extraction rate, then its increase. There were the transitions among several mechanisms in the extraction. On the basis of an extreme hypothesis, the effects of three mechanisms on extraction were discussed separately, with a preliminary equation of the extraction got. The action of such three mechanisms changing with pH was analyzed, and the rough and preliminary equation was corrected into a comparatively precise equation. The complex relationship among the three extraction mechanisms could be revealed by the mechanism contribution curves with a direct vision method. In certain pH ranges, the three mechanisms affected extraction result by the overlapping ways. The extreme hypothesis can be described below: in the system of several mechanisms, it could be considered extremely that under some conditions a certain mechanism takes key effect and others can be neglected.

Key words: extreme hypothesis, correction, mechanism, solvent extraction, bis(2-ethylhexyl)dithiophosphoric acid-trioctylamine, cadmium