A. Eluard, B. Trémillon
Feb 1, 1967
Citations
0
Influential Citations
29
Citations
Journal
Journal of Electroanalytical Chemistry
Abstract
Abstract Molten potassium thiocyanate can be used as a solvent between its melting point, 173°, and its decomposition temperature, about 275°. An electrochemical study (polarography and anodic voltammetry) has been carried out in that medium, at 195°. The elements of column Ib, dissolved in molten KSCN, can exist only at the +1 oxidation state. Those of column IIb exist only at the +2 oxidation state; the disproportionation of mercurous ions has been reported. Fe 2+ , Co 2+ , and Mn 2+ ions have been observed and are the only stable oxidation states. The higher oxidation states, Cu 2+ , Fe 3+ oxidize thiocyanate ions into sulfur and cyanogen; therefore they cannot exist. Reciprocally, the metals are corroded by SCN- ions with the formation of cyanide and metallic sulfide. Furthermore, concentrated stable metallic cations provoke the thermal decomposition of SCN-, with the formation of metallic sulfide and cyanogen. Only complexes more stable than thiocyanate complexes can be formed in that medium: these are mainly sulfides (insoluble) and cyanides. The following cyanide complexes have been reported. Ib column. Complexes of the M(CN) 2 − type: Cu(CN) 2 − , Ag(CN) 2 − , Au(CN) 2 − , (p K 2 -values are 5.70, 3.20, and 11.2, respectively). IIb column. Only one complex, Hg(CN) 2 (p K 2 =6.5), in the case of mercury, and several complexes in the case of zinc or cadmium: Zn(CN) 2 , Zn(CN) 3 − , Zn(CN) 4 2− (the p K -values are 5.1, 6.1, and 8.6 respectively), CdCN + , Cd(CN) 2 , Cd(CN) 3 − , (1.8, 3.9, and 5.1, respectively). Cobalt(II) enters into a more complicated reaction: Co 3+ ions reduce the solvent in the presence of cyanide ions and lead to the formation of Co(CN) 6 3− complex. At the same time, there is a precipitation of cobalt sulfide and iron(II) is precipitated from the solution as ferrous ferrocyanide. A comparison is made with the stabilities of the same complexes in aqueous solutions and it is shown that the order of stability for complexes of the same type is preserved. The stabilities are, nevertheless, much lowered because of the strong complexing power of thiocyanate ions towards metallic ions.