214 PROPERTIES OF ELECTRICALLY CONDUCTING SYSTEMS 



The existence of definite anion-complexes comparable, for example, 

 with those of the cobalt and chromium salts is not indicated by the proper- 

 ties of electrolytes, either in solution or in the solid and liquid state. The 

 anions often consist of definite groups containing one or more electro- 

 negative atoms. Among these we have, for example, the nitrates, chlo- 

 rates, sulphates and other common anions, as well as many anions of 

 organic acids. So far as the degree of ionization is concerned, salts of 

 compound anions exhibit properties similar to those of salts of simple 

 anions. The ionization of the hydrogen derivatives, namely the acids, 

 of such anions, however, is largely dependent upon the nature of the 

 atoms occurring in the anion complex. The introduction of strongly 

 electronegative elements into the anion complex increases the strength, 

 that is, the ionization of the acid. This behavior of the acids is so well 

 known that details need not be introduced here. 



Certain elementary ions form complex anions with the same or other 

 elements, a more or less complex equilibrium existing among the various 

 complex anions formed in solution. The most common example of a 

 complex anion of this type is the complex iodide ion which is formed 

 by the direct interaction of the iodide ion with iodine, forming the ion 

 I-.I 2 . The equilibrium in the case of the tri-iodide ion has been exten- 

 sively studied by a number of investigators. The mean composition of 

 the solution in such cases depends upon the concentration, since the 

 equilibrium between the simple and the complex ion is a function of the 

 concentration. 21 



It is well known that the halogen salts form various complexes with 

 other halogens, thus indicating that complex anions are formed between 

 a halogen ion of one element with other elements of the halogen group. 

 The chlor-iodides are familiar examples of this type. The equilibrium 

 in the case of these complex anions has not been extensively studied. 



The work of Klister 22 indicates that the normal sulphide ion reacts 

 with excess sulphur to form a series of complex sulphur anions. These 

 anions appear to be comparable with the complex iodide ion, the charge 

 being associated with the original sulphide anion. The mean composition 

 of a solution of sodium sulphide in equilibrium with free sulphur varies 

 as a function of the concentration. The problem in the case of aqueous 

 solutions is complicated owing to hydrolysis. The behavior of aqueous 

 solutions of the alkali selenides and tellurides indicates that these metals 

 also form complex anions in the presence of excess of these metals. 23 

 They have not, however, been extensively investigated. 



Bray and MacKay, J. Am. Chem. Soc. 32, 915 (1910). 



"Ktister and Heberlein, Ztachr. f. Anorg. Chem. 43, 53 (1905) ; Kiister, ibid., A4. 431 

 (1905). Tibbals, J. Am. Chem. Soc. 31, 902 (1909). 



