There is no detailed electrochemical investigation of the nickel-mercury system. Though the presence of two solid compounds NiHg 3 and NiHg 4 has been shown already by Lihl 1 in his X-ray investigations and by Bates and Prentice 2 in their magnetic study, some authors 3"4 evidently assume the deposition of pure nickel during the electroreduction of nickel(II) ions at mercury cathodes.
With respect to nickel solubility in mercury the view is accepted that the solubility of this metal is similar to that of cobalt and iron and is equal to 2.1 x 10 -s atom~o at 25°C. However Galus and Kemula 5 have shown that using the hanging mercury drop electrode (HMDE) one may electrolytically prepare a homogeneous unstable nickel amalgam of concentration up to 10 -3 M. The time of stability of such an amalgam was dependent to a large extent on the amalgam concentration.
Dowgird and Galus 6 have found that the diffusion coefficient of nickel in mercury is equal to 6.5 x 10 -6 cm 2 s -1. This value is low in comparison with the diffusion coefficients of other metals in their amalgams; for instance, the diffusion coefficient of zinc in mercury is 2 x 10-5 cm 2 s-1.
The low value of the diffusion coefficient of nickel was explained by Dowgird and Galus as due to the interaction of nickel and mercury also in the homogeneous amalgam.
These and other investigations not reported here indicate interesting properties of the nickel amalgam which are not well understood and should be further investigated.
The results of electrochemical investigations of the kinetics and equilibria of transformations occurring in this system may be relatively simply interpreted under conditions of a fast electron transfer in the process Ni 2 ÷ + 2e + Hg ~-Ni(Hg) This electrode reaction is slow in a number of background electrolytes. Only in some solutions such as concentrated lithium, calcium and magnesium halides is it practically reversible under polarographic conditions.
In the present paper a part of our investigations of the nickel-mercury system is described. We concentrate here on the exact determination of conditions of stability of the homogeneous nickel amalgam and on the identification and * Dedicated to Professor Wiktor Kemula on the occasion of his 70th birthday.