Estudo crítico de equilíbrios no sistema cobalto (II)/tiocianato / Critical study of the equilibrium on the cobalt(II)/thiocianate system

AUTOR(ES)

Nina Coichev

DATA DE PUBLICAÇÃO

1984

RESUMO

The cobalt (II) /thiocyanate system in aqueous medium was studied with the purpose to obtain reliables data of overall stepwise formation constants to complexes. An indirect potentiometric method and a spectrophotometric one were used at 2,0 M ionic strength (NaCl04), at 25,0 ± O,l°oC. The cadmium(II)/thiocyanate system was used as indicator in the indirect potentiometric method with hanging drop electrode with cadmium amalgam. The following equilibrium constants of the cadmium(II)/thiocyanate complexes were obtained at 25°C and 2,O M ionic strenght (NaCl04: β1 = 21.51 ± 0.09 M-1, β2 = 123.3 ± 1,2 M-2, β3 = 129.9 ± 3.1 M-3 and β4 = 173.5 ± 1.2 M-4. Average 1igands number data, ñ, were obtained for several thiocyanate concentrations, up to 2 M, by adding Co(Cl04)2 (0.1 M) to the working solution of cadmium (II)/thiocyanate. The integration of the formation curve and treatment of Ledens functions by a self developed least square method, lead to the following overall formation constants of the cobalt(II)/thiocyanate system: β1 = 11.0 M-1, β2 = 22.0 M-2, β3 = 11.2 M-3 and β4 = 1.63 M-4. A study of the zinc(II)/thiocyanate system was performed with the purpose to check the indirect potentiometric method inasmuch as this system was formerly studied by a re1iable direct method. The formation curve presented a good agreement until the average ligand number 1. After this point a negative desviation is observed with regard the direct method by not well understood causes. Severa1 curves of average molar absorbtivity, ε vs the analytical ligand concentration were obtained at three wave lengths (515, 590 and 615 nm) and three differents cobalt(II) cation concentrations: 0.01, 0.05 and 0.1 M. The analysis on base of corresponding solutions permited to obtain ñ vs free thiocyanate ion concentrations, with the following equilibrium constants: β1 = 6.9 M-1, β2 = 28.9 M-2, β3 = 12.1 M-3

ASSUNTO(S)

thiocianate cobalt(ii) tiocianato cobalto(ii) constante de estabilidade stability constant

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