Triple ion formation in acetonitrile

Abstract

The effect of halide ions on the cathodic waves of cations was examined by means of dc polarography in acetonitrile. The formation constants of Li+(Cl-), (n = 1-4), NH4+(C1-), (n = 2 and 3), Et3NH+(X-), (X = C1 or Br, n = 1 and 2), etc., were obtained. The present and the previous polarographic studies indicated the symmetrical formation of triple ions from trialkylammonium halides, Le., Kz = K3, in the following reactions: M+ + X- s M+X- (KJ, 2M+ + X- s (M+)zX- (Kz), and M+ + 2X- is M+(X-), (K3), where M+ stands for R3NH+ (R = Me, Et, and n-Bu) and X- is CI-, Br-, or I-. The conductivity data of R3NH+X- in acetonitrile were quantitatively explained by the triple ions in addition to the ion-pair formation. Assuming that the limiting equivalent conductivity of the triple ions, AT, is one-third of that of the simple ions, Ao, the calculated A values of Et3NHCl ((0.4-6.0) X M) with K, = 2.8 X lo4 and Kz = K3 = 3.0 X lo6 fitted the observed values within 0.9% error, while the error was more than 9% at higher concentrations when only ion pairing was considered. Me3NHCl and n-Bu3NHCl gave similar results. In the cases of bromide salts, less than 0.9% error in A values was given by the assumption of AT = &/2.