422 Mr. Armit and Dr. Harden. Estimation of [Dec. 5, 



contains small traces of iron. The nickel is thus precipitated in the form of 

 the hydrate, and this is converted into nickel sesqnioxide hy the addition of 

 1 or 2 c.c. of bromine to the cold mixture. The nickel oxide is then collected 

 by filtration, and after having been well washed, is dissolved in hydrochloric 

 acid, and the solution evaporated to dryness to remove the excess of acid, and 

 the residue re-dissolved in water with a faint trace of acid, in order to prevent 

 the formation of basic salts. The solution is finally made up to a definite 

 Aolume. 



In the process of separation, no especial difficulties save the management of 

 the voluminous iron precipitate, are met with as a rule. At times an 

 insoluble residue is found on the filter paper when the oxide is dissolved. 

 This is a trace of a sulphide of copper or another metal of this group, which 

 has escaped precipitation by sulphuretted hydrogen in acid solution. 



3. Estimation. — The usual method of quantitative estimation of nickel 

 colorimetrically is carried out with ammonium sulphide, but it has been found 

 that sharper results can be obtained by employing a-dimethylgiyoxime 

 CH 3 .C(N.OH).C(KOH).CH 3 , which was recently shown by Tschugaeff to form 

 a scarlet red compound with nickel in the presence of ammonia.* 



For this purpose, a saturated solution of the reagent in absolute alcohol is 

 prepared, this is diluted with water until a little of the compound separates 

 out, and alcohol is then added until complete solution takes place. The fluid 

 to be tested and a standard solution of nickel sulphate are placed in burettes. 

 A measured quantity of the fluid is then run into a Nessler tube and to this 

 0*5 c.c. of a 10-per-cent. solution of ammonia and the same quantity 

 of the dimethylglyoxime solution are added and the whole made up to 

 30 c.c. It is better first to add the ammonia to the nickel solution, then the 

 dimethylglyoxime, and then allow the colour to develop before diluting up to 

 the 30 c.c. mark. All the solutions must be cold. The fluid becomes 

 coloured pinkish red, the depth of the coloration depending on the quantity 

 of nickel present. The most convenient quantity to work with is about 

 - 08 to O'Ol milligramme. The colour is then compared with that produced 

 by varying quantities of nickel from the standard solution. The determination 

 is not complete until a quantity has been found, which gives a colour which is 

 just too pink, and a second quantity the colour of which is just appreciably 

 less pink, than the fluid to be tested. The quantity of nickel contained is 

 then calculated as the amount midway between the two tubes. With a little 

 practice, it is quite easy quickly to determine very small differences of colour. 

 The estimation should be concluded as rapidly as is compatible with accuracy, 



* 'Deut. Chem. Ges. Ber.,' 1905, vol. 38, p. 2520. 



