Abstract: The extraction mechanism of Th(Ⅳ)with bis(2,4,4-trimethylpentyl)phosphinicacid(HA)in,n-octane from HCl medium with different ionic strength is studied,When theionic strength of the aqueous phase is not controlled,the extraction equilibrium is Th⁴⁺+Cl+3(HA)₂₍₀₎⇔ThClA₃·(HA)₃₍₀₎+3H⁺ And when the ionic strength is 1.0mol/L, the extraction equilibrium is Th⁴⁺+2Cl^-+3(HA)₂₍₀₎⇔ThCl₂A₂·(HA)₄₍₀₎+2H⁺ The composition of the extracted complex determined by the saturation method is ThClA3.The effect of temperature on the extraction of Th(Ⅳ)indicates that it is an endothermal re-action. The stability constants and thermal mechanical functions of the extraction reactionare calculated.The IR and NMR spectra of the saturated complex are studied.

Key words: bis(trimethlpentyl)phosphinic acid, extraction mechanism, Th(Ⅳ), IR, NMR