Etude par spectroscopie Raman de l'influence de la pression sur l'association de l'ion thiocyanate avec divers cations metalliques en solution aqueuse

Using Raman spectroscopy we have investigated the effect of pressure on the association between the thiocyanate ion and some metallic cations: Zn 2 + , Mn 2 + , Al 3 + , In 3 + , Th 4 + , Hg 2 + . The ν(CS) stretching vibration of the anion has been analysed because of its sensitivity to the complexation; new ν(CS) lines appear at frequencies well distinct from that of the free ν(CS). The quantity of free SCN − has been determined from the intensity of the free ν(CS) line and has been used to calculate the formation constants for the equilibria M x + + n SCN − ⇄M(SCN)   n (x‐n) + . Values have been obtained at normal pressure and at 3000 atmospheres. Finally, the mean value of the change of the partial molal volume Δ V for the complexation reaction between the two extreme pressures has been calculated. The dissociation of the complexes is generally favoured by an increase of the pressure, and the Δ V values are positive. We think that the largest measured values of Δ V are due to a change of the coordination number of the cation on complexation.