COMPLEX FORMATION OF GOLD (I) WITH 2-METHYLIMIDAZOLE

Abstract

The complex formation process of Au (I) with 2-methylimidazole was studied by potentiometric titration. When 2-methylimidazole is added to the [AuCl2]-/Au system, the potential gradually decreases without any jumps. The nonlinearity of the dependence of E on –lg[2MI] indicates the stepwise nature of the complex formation between [AuCl2]- and 2-methylimidazole. The general and thermodynamic stability constants of the complexes were determined in the temperature range of 278-318 K (at T=298 K, lgβ0[AuСl(Н2МИ)]+=13.84±0.03; lgβ10[Au2МИCl]=5.38±0.05; lgβ20[Au(2МИ)2]+=10.60±0.03). The reliability of the determination of the constants is proved by the satisfactory agreement between the experimentally found values of the potentials (Eex.) with theoretical ones (Etheor.). As the temperature increases, the stability of the complexes increases. A change in the ionic strength of the solution has little effect on the stability of the complexes. Comparison of the process of complexation of gold (I) and gold (III) with 2-methylimidazole shows that the stability of the complexes of trivalent gold is superior to similar complexes of gold (I). The thermodynamic functions of the complex formation reaction were calculated by the temperature coefficient method. In this case, the value of ΔrН0 was determined from the tangent of the slope of the direct dependence lgβ0=f(1/T), and the value of ΔrS0 was determined from the segment cut off by this straight line on the y-axis. The change in the isobaric-isothermal potential was calculated by the equation ΔrG0= ΔrН0-TΔrS0. The substitution of chloride ions for a 2MI molecule is accompanied by a positive change in ΔН, which has a negative contribution to the spontaneous course of reactions. For the [AuCl2]- - 2MI system, the number of particles participating in the reaction remains unchanged and the positive change in ΔS is most likely due to the release of water molecules from the hydration shell of the initial substances and reaction products.