In this paper

water-oxalic acid-TOA(octanol) is used as the experimental system

and TOA

octanal as complexing agent

diluent respectively.The extraction mechanism of oxalic acid by TOA and the influence of the concentration of TOA ( S0) and solute on extraction equilibrium are studied.The structure of complex in organic phase is measured by infrared spectroscopy

and it is obtained that there are two types of complexates in organic phase

1∶1 and 1∶2 (acid∶TOA).The distribution coefficient increases with S0.Loading of complexing agent shows an increasing tendency with S0 and the equilibrium concentration of acid in aqueous phase( x ).The saturated loading is achieved at higher x and increases with S0.The methematical model with two type complexes is proposed based.The model parameters

K11 and K12 are obtained by fitting the experimental data of extraction equilibrium

and K11 is much larger than K12 .