COLOKIMETRIC ESTIMATION OF NICKEL AND COBALT. 407 



Dr. Winkler used solutions of various strengths and 

 various salts, and assumed in all cases that the colour 

 imparted by one part by weight of cobalt neutralizes that 

 given by three parts by weight of nickel. His maximum 

 error is '0052 gram of either metal in '34 gram of the mixed 

 metals or about 1*5°/°, but his experiments were not very 

 numerous. 



The method described in the following pages, which 

 differs somewhat from that employed by Dr. Winkler, was 

 adopted as the result of a number of preliminary experi- 

 ments and observations, some of which will be mentioned 

 in order to better follow its development. The tubes used 

 for comparing the colours of the solutions (except those 

 with MacMillan's apparatus) were Nessler tubes of 50 cc. 

 capacity, the height from to 50 cc. mark being 10*9 cm., 

 placed side by side on a glass plate over a white paper 

 reflector inclined at an angle of 45°. The colours were 

 observed by looking down through the solutions. 



Preliminary Experiments. 

 To observe the effect of dilution on the colours of the 

 nickel and cobalt solutions with the object of finding the 

 most suitable strengths to use ; two solutions, one twice 

 the strength of the other, were placed side by side in tubes 

 and viewed vertically, so that the colour seen was due to 

 the same weight of metal in each tube. A number of 

 experiments showed that in each case the more dilute 

 solution appeared if anything a little lighter in colour than 

 the other, (i.e. with solutions containing about *1 gm. to 

 '3 gm. of metal in 50 cc.) 



In another series, two solutions containing the same 

 weight of metal in each were used ; to one water was added 

 5 cc. at a time till it was 10 times more dilute than the 

 other, the two being compared after each addition of water, 

 with the result that the dilute solution always appeared 

 a trifle paler in tint than the stronger one. 



