Letter | Published:

Possible Discovery of Ion-Pairs by Interferometry

Naturevolume 187pages935936 (1960) | Download Citation



IN the course of an investigation on electro-deposition to determine the cause of what Ibl and Muller1 called an “anomalous maximum” near the cathode, and which O'Brien and Axon2 independently noted and named the “second wave” in a complete, electrode-to-electrode interference pattern, it was found that this second wave does not develop in electrolyte solutions in which ion-pairs do not form. Such a solution is nickel sulphate3, in which it has been shown that, presumably because of the strength of the bonds and lack of lability in the octahedral nickel aquo-complex, the closest approach of the sulphate ion cannot be less than the diameter of one water molecule (see Figs. 1 and 2). Electrolysis of a silver perchlorate solution gave the faintest traces of a second wave, which may be considered to be additional supporting evidence. Since Fajans and Luhdemann4 have shown that free ions in solution may be expected to give higher refractive indices than ion pairs, and indeed a plot of concentrations versus refractive index of values taken from Landolt-Bornstein shows no deviation from linearity for nickel sulphate (which deviation is usually attributed to ion-pairs) until concentrations in the neighbourhood of 1 M, it follows that the cathode maximum may be explained by the dissociation of ion-pairs.

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  1. 1

    Ibl, N., and Muller, R. H., Z. Elektrochem., 59, 671 (1955).

  2. 2

    O'Brien, R. N., and Axon, H. J., Trans. Inst. Met. Finishing, 34, 4 (1957).

  3. 3

    Duncan, J. F., and Keppert, D. L., in “The Structure of Electrolyte Solutions”, edit. by Hamer, 398 (John Wiley and Sons, New York, 1959).

  4. 4

    Fajans, K., and Luhdemann, R., Z. phys. Chem., B, 29, 150 (1935).

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  1. Department of Chemistry, University of Alberta, Edmonton

    • R. N. O'BRIEN
    •  & C. ROSENFIELD


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