TITRATION OF THE SOLUTION OF URANIUM 143 



cubic centimeters with a little hot distilled water, and one 

 or two drops of the solution of uranium are allowed to flow into 

 the flask from a graduated burette previously filled exactly to 

 zero. After each drop of the solution of uranium, the flask is 

 shaken and the liquid tried upon a drop of potassium ferrocyanid, 

 as has been previously indicated. For a skilled eye, four to six 

 drops are generally necessary to obtain the characteristic colora- 

 tion, that is, from two-tenths to three-tenths of a cubic centi- 

 meter. Beginners often use from five-tenths to six-tenths, and 

 sometimes even more. 



The sole important point is to arrest the operation as soon as 

 the reddish tint is surely seen, for afterwards the intensity of the 

 coloration does not increase proportionally to the quantity of 

 liquor employed. 



It is well to note that at the end of some time the coloration 

 becomes more intense than at the moment when the solutions 

 are mixed, so that care must be taken not to pass the saturation- 

 point. This slowness of the reaction is the more marked as there 

 is more sodium or ammonium acetate in the standard solutions. 

 This is the reason that it is important to introduce always the 

 same quantity, namely, five cubic centimeters. This is also the 

 reason why the uranium acetate should not be employed in pre- 

 paring the standard solution of uranium which ought to contain 

 the least possible amount of acetate in order that the necessary 

 quantity which is carried into each test should be as small as 

 possible and remain without appreciable influence. If it were 

 otherwise, the sensibility of the reaction would be diminished 

 ir proportion as a larger quantity of uranium solution was em- 

 ployed, giving rise to errors which would be as much more im- 

 portant as the quantities of phosphoric acid to be determined were 

 greater. The correction for the uranium solution having been 

 determined, it is written upon the label of the bottle containing it. 



Causes of Error. In the work which has just been described, 

 some causes of error may occur to which the attention of analysts 

 should be called. 



The first is the error which may arise from the consumption 

 of the small quantity of uranium phosphate which is taken with 



