Moscow State Pedagogical University, Moscow, Russia:

A. S. Samadov, Graduate Student of the Chair for General Chemistry, e-mail: s.s.rasul@mail.ru
I. G. Gorichev, Professor of the Chair for General Chemistry, Doctor of Chemistry Sciences , e-mail: gorichevig@gmail.com

Bauman MSTU, Moscow, Russia:
E. A. Eliseeva, Associate Professor of the Chair for Chemistry, Candidate of Chemistry Sciences, e-mail: yakusheva@bmstu.ru
V. S. Boldyrev, Associate Professor of the Chair for Chemistry, Candidate of Technical Sciences, e-mail: boldyrev.v.s@bmstu.ru

The process of complexation of silver(I) ions with thiourea (Tu) in the temperature range 287.16–318.16 K at an ionic strength of 0.11 M (I = 0.1 NaNO₃ + 0.01 HNO₃) was studied by the potentiometric method using a silver electrode in aqueous solution. The study was performed at a concentration of silver(I) ions C_Ag+ = 1 · 10^–5 and C_Ag+ = 1 · 10^–3 M. The values of the stability constants of AgTui + complexes (i = 1–2) were estimated at CAg+ = 1 · 10^–5 M. The AgTu₃⁺ complex constant was determined at C_Ag+ = 1 · 10^–3 M and C_Tu ≥ 0.01 M. The stability constants of complexes were calculated using the Leden method. The values of the complex constants at 298.16 K are equal to: lgβ₁ = 5.59 ± 0.10 (AgTu⁺), lgβ₂ = 10.62 ± 0.03 (AgTu₂⁺) and lgβ₃ = 13.05±0.11 (AgTu₃⁺). It is established that the temperature effect does not affect the number of formed particles in the solution; an increase in temperature leads to a decrease in the stability of complexes. The analysis of the values of the stability constants by degrees shows that the step constants AgTu₃⁺ (AgTu₂⁺ + Tu = AgTu₃⁺ + lgβ₃ = 2.4) are much smaller than the constants for mono- and bis-coordinated complexes. The complexes thermodynamic parameters (ΔH⁰, ΔG⁰, ΔS⁰) were calculated by the temperature coefficient method. In the system under study, all complexation reactions are exothermic. The largest increase in the value of enthalpy (ΔH⁰) is observed in the formation of a complex containing two molecules of thiourea. The change in entropy (ΔS⁰) at the stage of formation of bis- and tris-coordinated complexes is negative, which is most likely due to a decrease in the number of particles in the system under study. It is determined that enthalpy-entropy dependences show a monotonic character. The spontaneous course of complexation reactions is determined by the value of the free Gibbs energy (ΔG⁰). In degrees G⁰ decreases (ΔG⁰)1 < (ΔG⁰)2 < (ΔG⁰)3) and has a negative value, which indicates a decrease in the affinity of the complexing agent to the ligand.

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