Abstract

The significance of the application of the concepts “outer sphere” and “outer shell” to basic chromium complex solutions, has been examined.

These terms have arisen from observations on changes in the ultra violet absorption spectra of certain cobalt complexes which have been attributed to “ion pair” association. Extensions of this work to chromium complexes cannot be regarded as having proved their existence in these cases.

Results obtained on chromium nitrate complexes in the presence of both N/l and saturated KCI solutions showed no evidence of the existence of “ion pairs” at the upper limits of chromium concentration capable of measurement in a Beckman DB spectrophotometer using 1 cm cells.

It is concluded that while “ion pairs” may well exist in concentrated solution, they should not be regarded as specific chemical entities which would be stable to dilution, exchange resin separation or electrophoresis. Under these circumstances it would seem to be advisable to drop the rather misleading terms “outer sphere” and “outer shell” chromium complexes and to refer, with proper caution, to the possibility of “ion pair” association in concentrated solution.

References to “outer sphere tanning” should also be regarded with caution. The existence of “ion pairs” in the form oF salt links have long been recognised, together with the ease of rupture of such links by water or electrolytes.