Vogel's Reaction for Cobalt 151 



DiscuHsioti of Ke.^Hlts. Table 1 shows that the maximum color is 

 developed when the solution is at least 0.8 normal in acid and that 

 increase cf acid concentration up to 1.5 normal is without effect. 

 Weakening- or disappearance of the blue color at acid concentrations 

 above 3 normal is to be expected if the blue compound is a complex 

 anion. By forming little ionized HSCN high concentration of hydrogen 

 ion should tend to decompose the blue complex and this decomposition 

 should be proportional to the concentration of mineral acid. 



Table 2 shows that the thiocyanate concentration should be at least 

 1 normal and that concentrations above this do not increase the sensitive- 

 ness of the test. 



Table 3 shows that when the aqueous layer contains only 4 per cent 

 ethyl alcohol the solubility of the blue compound in ether reaches a 

 maximum and that increase of alcohol concentration up to the point 

 where an ether layer no longer separates does not cause any increase 

 of sensitiveness. 



Table 4 shows that in acid solution the three alkali thiocyanates 

 used are equally effective as would be expected. Under the optimum 

 conditions of concentration 0.001 mg. of cobalt is detectable in 5 cc. 

 of solution. In neutral solution, however, as appears from table 5, the 

 ammonium complex salt is distinctly more soluble in ether than the 

 sodium or potassium salts. That addition of a little mineral acid to 

 the blue aqueous layer drives all the blue compound into the ether layer 

 is evidence that the ether-soluble compound is a complex acid and not 

 a salt, such as Co(SCN). or Na,Co(SCN)4. Additional evidence upon 

 this point will be presented in another paper. 



Table 6, which compares the effectiveness of amyl alcohol-ether, 

 1:1, used with neutral aqueous solution of ammonium thiocyanate, with 

 1:4 ethyl alcohol-ether, used with aqueous solution of any alkali thiocyan- 

 ate 1 normal in mineral acid, shows that the greater solubility in ether 

 of the free complex acid as compared with that of the ammonium salt 

 makes the test slightly more delicate when performed in acid solution. 

 In presence of large quantities of nickel salts this advantage becomes 

 still more marked. In acid solution the interference of ferric iron is 

 considerably greater but this is quickly and easily reduced by shaking 

 with a few crystals of sodium thiosulfate which does not affect at al] 

 the reaction with cobalt. 



Table 7 shows that in water-alcohol mixtures fairly high in alcohol 

 quite small amounts of cobalt are able to give a distinct color. As the 

 solutions here were neutral, the sodium complex salt must be assumed 

 present. Under these conditions, however, the green color of nickel 

 salts would interfere with the detection of small amounts of cobalt. 



Summary. 



1. Treadwell's method of carrying out the Vogel test for cobalt in 

 neutral solution using equal parts of ether and amyl alcohol was studied 

 by varying the components of the reaction mixture one by one while 

 keeping the others constant. The optimum conditions were found to 

 be high concentration of thiocyanate, small aqueous volume and small 



