N-Methyl-N'- [ο- (2-ethylhexrylthio) phenyl] thiourea (=MEPT) was synthesized to examine the extraction properties of metal ions. MEPT selectively extractedd copper (II), palladium (II), gold (III) and silver (I) over base metals in 1 mol dm^-3 aqueous ammonium nitrate solution. The extraction of copper (II), palladium (II) and gold (III) was dependent on pH. The stoichiometry of the extraction of copper (II) with MEPT was elucidated by examining the effects of hydrogen ion, extractant and nitrate ion concentrations on its extractability. It was found that copper (II) was extracted as a complex of copper (II) : MEPT with a mole ratio of 1: 3. The copper (II) extraction with MEPT (=HR) was represented by the following reaction:
Cu²⁺+3 (HR) _??_CuR₂⋅ (HR) +2H⁺; K
The extraction equilibrium constant was determined to be K=0.60 (mol dm^-3) ^-1.