Abstract: Primaquine (P) has long been used as an antimalarial drug. The following formulas are derived for determination of the ionization constants of PH₃³⁺ by pH-titration method:in which K_w is the ionic product of water, a is the mole ratio of HClO₄ to primaquine and C_p is the total concentration of rimaquine.The ionization constants of primaquine in 50% (v/v)ethanol in water determined at 25℃ in the ionic strength range of 5×10_-3～ 5× 10^-2 mol/L are :Ka₁=( 3. 84±2. 35)× 10^-2(attributed to the secondary ammonium group of primaquine) ;Ka₂= (1.50±1.17 )× 10^-8 (attributed to the tertiary ammonium group ) ;Ka₃= (2.07± 0.27 )× 10^-10 (attributed to the primary ammonium group).The coordination ratio of primaquine to vitamin C in the above solvent is determined by continuous variation and mole ratio methods based on pH and conductance measurements to be 1 : 1, indicating that the coordination compound formed in the solution is mainly a 1 : 1 compound.