Potentiometer study of the process of complexation of silver (I) ions with thiourea

Abstract

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 CAg+ = 1 · 10–5 and CAg+ = 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 CAg+ = 1 · 10–3 M and CTu ≥ 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.