Potentiometric Study of the Copper(II) Ions Complexation Process with Thiourea in Acidic Medium

Potentiometric titration method was used to study the copper(II) ions complexation process with thiourea in medium containing 1 mol/l of hydrochloric acid at a temperature of 298 K. It was found out that mononuclear (CuTM_n) and binuclear (Cu₂TM_n) complexes were observed in the copper(II) --- thiourea system depending on the copper ions concentration. It was determined that in acid medium with copper(II) ions concentration less than 5 · 10^--4 mol/l and mononuclear complexes of the composition are generated in the system, which constant is equal to lg β₃ = 11.9. With an increase in the copper(II) ions concentration (C_Cu⁺² > 5 · 10^--4 mol/l), binuclear thiourea complex dominates in the system. Stability constant logarithm of the Cu₂TM₆ composition binuclear complex is equal to 27.5 and was calculated using the modified Leden's method. Based on the constants, complexes existence regions were determined depending on the ratio C_TM/C_Cu⁺². With the C_TM/C_Cu⁺² ≥ 5 relations, existence of a dominant complex significantly depends on the copper(II) ions concentration. It turned out that fractions of all complex particles were growing with an increase in the copper(II) ions concentration in the system. It was found that stability of thiourea complexes in the CuL_i²⁺, AgL_i⁺, AuL_i⁺ rows was increasing

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