164 



dififerenl type, resembling those of bivalent copper, whereas the ooball 

 cyanides are analogous to the iron cyanides. In the classic method of 

 Liebig 4 for detecting nickel in cobalt salts, the inferior stability of oiekelo- 

 cyanide ion together with the ready oxidizability of cobaltocyanide to cobalti- 



cyanide ion has long been used to effect a separation. K<>r a solution con- 

 taining eobaltieyanide, nickelocyanide and cyanide ions the following equilib- 

 ria are involved: 



[Co " ] x fCN' ] 6 = K inst. x [Co(CN) ' 6 ' ] and 

 [Xi] x [CN'] 4 = K inst. x [Ni(CN) V ]. 



The values of the instability constants are not accurately known, but it is 

 certain that that of eobaltieyanide ion is extremely small and that of nickelo- 

 cyanide ion much larger. Any reduction of the concentration of cyani de 

 ion in the solution must result in decomposition of the nickelocyanide ion 

 and considerable increase of nickel ion concentration while the much more 

 stable eobaltieyanide 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 Xi (OH) 3 . This method is not altogether satisfactory, first, 

 because, owing to the necessity of adding an excess of the oxidizing agent, 

 cobaltic hydroxide 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. 



Xickel glyoximine is decomposed by cyanide ion. Our problem, then, 

 was to remove the cyanide ion so gradually that the eobaltieyanide 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(CX) 2 , 0.0004 g. per liter 6 at 20°. For 

 argenticyanide ion, [Ag] x [CX] 2 = VJ - 11 x [AgfCX)/]. The comparative 

 insolubility of silver eobaltieyanide, Ag 3 Co(CX) ; , accurate data for which 

 are lacking, should also tend to prevent decomposition of eobaltieyanide ion. 

 When dimethvlglvoxime is added to very dilute solutions of nickel salts. 



'Ann., 65, 244 (1848); 87, 128 (1853). 

 -Z. analyt. Chem.. .5. 75 (1866). 

 6 Bredig, Z. physik. Chem., J { 6, 602. 



