164 
different type, resembling those of bivalent copper, whereas the cobalt 
cyanides are analogous to the iron eyanides. In the classic method of 
Liebig* for detecting nickel in cobalt salts, the inferior stability of nickelo- 
cyanide ion together with the ready oxidizability of cobaltoeyanide to cobalti- 
cyanide ion has long been used to effect a separation. For a solution con- 
taining cobalticyanide, nickelocyanide and eyanide ions the following equilib- 
ria are involved: 
[C6] x [CN’]® = Kimst. x [Co(CN) , ] and 
[Ni] x [CN’]4 = K inst. x [Ni(CN) , ]. 
The values of the instability constants are not accurately known, but it is 
certain that that of cobalticyanide ion is extremely small and that of nickelo- 
eyanide 1on much larger. Any reduction of the concentration of cyani de 
ion in the solution must result in decomposition of the nickeloeyanide ion 
and considerable increase of nickel ion concentration while the much more 
stable cobalticyanide ion is less affected. In Liebig’s method as modified 
by Gauhe,® cyanide ion is removed by oxidation with alkaline hypobromite 
or hypochlorite, the nickelous ion being simultaneously oxidized and pre- 
cipitated as Ni (OH);. This method is not altogether satisfactory, first, 
because, Owing to the necessity of adding an excess of the oxidizing agent, 
cobaltic hydroxde is also precipitated invariably so that the appearance 
of a brown precipitate is not per se, proof of the presence of nickel; second, 
because the manipulation, particularly the amounts of reagents, requires 
experience and care. 
Nickel glyoximine is decomposed by cyanide ion. Our problem, then, 
was to remove the cyanide ion so gradually that the cobalticyanide ion should 
remain practically unaffected. For this purpose we made use of the great 
stability of complex silver cyanide ions, together with the high insolubility 
of silver argenticyanide, Ag Ag(CN)», 0.0004 g. per liter® at 20°. For 
argenticyanide ion, [Ag] x [CN]? = ,»-** x [Ag(CN). |. The comparative 
insolubility of silver cobalticyanide, Ag;Co(CN);, accurate data for which 
are lacking, should also tend to prevent decomposition of cobalticyanide ion. 
When dimethylglyoxime is added to very dilute solutions of nickel salts, 
4Ann., 65, 244 (1848); 87, 128 (1853). 
5Z. analyt. Chem., 5, 75 (1866). 
6Bredig, Z. physik. Chem., 46, 602. 
