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The rates of dissociation of a variety of alkali metal cryptates have been measured in methanol. These have been combined with measured stability constants to give the corresponding formation rates. The pronounced selectivity of the cryptands for alkali metal cations is found to be reflected entirely in the dissociation rates, with the formation rates increasing monotonically with increasing cation size. The specific size-dependent interactions between the metal ions and the cryptands must then occur subsequent to the formation of the transition state (in the formation reaction). For a given metal ion, the formation rates increase with increasing cryptand size, and for 2,2,2 are similar to the rates of solvent exchange in the inner sphere of the cations. This suggests that during complex formation, particularly for the larger cryptands, interactions between the cryptand and the incoming cation can compensate very effectively for the loss of solvation of the cations.

High affinity of thallium͑I͒ ions with respect to a copper hexacyanoferrate ͑CuHCF͒ film was found. Interaction between the CuHCF film and thallium͑I͒ ions was investigated using the electrochemical quartz crystal microbalance and cyclic voltammetric techniques. In 0.5 M KNO 3 solution, at submillimolar concentrations of Tl͑I͒ ions, the CuHCF film reversibly transforms, during electrochemical experiments, from the potassium into the thallium form. For slightly higher ͑several millimoles͒ Tl͑I͒ concentrations, the electrochemical and gravimetric responses prove that the CuHCF film behaves as the thallium form only. Its formal potential (E f 0 ), calculated from the dependence of the E f 0 on ͓Tl͑I͔͒, is for ͓Tl͑I͔͒ ϭ 1 M, 0.28 V more positive than that found for the potassium form. Experimental results obtained suggest that in both cases, potassium͑I͒ and thallium͑I͒ ions, exchange in interstitial positions takes place. Since the ionic radii and the hydration parameters of both ions are similar, we concluded that this high affinity of thallium͑I͒ ions with respect to the CuHCF film, 250-100 times higher than that of potassium͑I͒ ion, results from chemical interactions. In consequence, the solubility product of a thallium analogue of copper hexacyanoferrate is much smaller than that of the potassium form. For different forms they are equal to ͑pK values͒: 37.8 (K 2 Cu 3 ͓Fe͑CN͒ 6 ͔ 2 ), 46.3 (Tl 2 Cu 3 ͓Fe͑CN͒ 6 ͔ 2 ), and 17.2 (Cu 3 ͓Fe͑CN͒ 6 ͔ 2 ).