VogeVs Reaction for Cobalt 147 



A STUDY OF, AND A MODIFIED METHOD FOFv, VOGEL'S 

 REACTION FOR COBALT. 



F. J. Allen and A. R. Middleton, Purdue University. 



Among the more sensitive tests for cobaltous ion and one particu- 

 larly suitable for detection of traces of cobalt in nickel salts is the 

 reaction with thiocyanate, usually called Vogel's reaction. When an 

 a?|ueous solution of a nickel salt, to which excess of ammonium thiocyan- 

 ate has been added, is shaken with a mixture of equal parts of amyl 

 alcohol and ethyl ether, even minute amounts of cobalt form a blue 

 compound which dissolves in the ether-alcohol layer. Nickel remains 

 entirely in the aqueous layer. Iron, if present in traces only, causes 

 the color to appear purple but the interference of this element can be 

 overcome as shown below. Although easily performed and requiring 

 only reagents ordinarily available, especially in the modified procedure 

 to be described, this test is mentioned in few textbooks of chemical 

 analysis and seems to be not very widely known. 



HISTORICAL. In 1877 MorrelP noted that cobalt salts, when 

 added to a neutral or acid solution of ammonium thiocyanate in alcohol, 

 give a blue color becoming pink on dilution. He proposed the use of 

 'this reaction for determining alcohol. Two years later Zimmerman- 

 proposed a separation of iron from cobalt and nickel by excess of 

 ammonium thiocyanate and precipitation of the iron as hydroxide by 

 sodium carbonate solution added to the point of discharge of the red 

 color of ferric thiocyanate. About the same time Wolff"' studied the 

 absorption spectra of cobalt thiocyanate solutions. Slightly later Voger 

 proposed the reaction as a test for cobalt, removing iron by Zimmerman's 

 method and dissolving the blue compound into a mixture of equal parts 

 of amyl alcohol and ether. He stated that by the characteristic absorp- 

 tion spectrum of the blue ether-alcohol layer 1 part of CoCL in 400 parts 

 of FeCl; or in 200 parts of NiCh cculd be detected easily. 



Vogel's work remained practically unnoticed until 1900 when Tread- 

 welP called attention to it and showed that the reaction is capable of 

 detecting 0.02 mg. of cobalt in presence of large amounts of nickel. He 

 also isolated the blue compound dissolved in the ether-alcohol layer and 

 found its composition to be that of a double salt, Co(SCN). .2NH4SCN. 

 At about the same time Rosenheim and Cohn" by migration experiments 

 showed that the blue compound contains a complex anion since cobalt 

 migrated to the anode. They prepared nickel double thiocyanates in 

 crystalline form having the general formula, Ni(SCN)2 .4RSCN, and 

 found that in their solutions complex anions were not present, as nickel 

 migrated only to the cathode. In Treadwell's Analytical Chemistry this 

 reaction is included among the tests for cobalt. He prescribes neutral 



' Z. analyt. Chem.. Hi. 251. 



- Ber., 12, 2254. 



= Ber., 12, 2314. 



*Z. analyt. Chem.. 18, .38. 



^Z. anorg. Chem., 26, lOS. 



« Z. anorg. Chem., 27, 280. 



"Proc. 38th Meeting. 1922 .(1923)." 



