TITRATION OF SAMPLE 145 



Suppose the titration has been made upon 10 cubic centime- 

 ters of the normal solution of phosphoric acid in the conditions 

 which we have just indicated, and the figure for the uranium 

 obtained is 10.2 cubic centimeters; if now the correction, which 

 may be supposed to amount to two-tenths cubic centimeter, be 

 subtracted there will remain 10 cubic centimeters of the uranium 

 solution which w r ould have precipitated exactly 50 milligrams 

 of phosphoric acid. 



The quantity of phosphoric acid which precipitates one cubic 

 centimeter of the solution will be consequently expressed by the 

 proportion 50-^-10^:5 milligrams, which is exactly the strength re- 

 quired. In the example which has just been given, the inscrip- 

 tion upon the flask holding the standard solution would be as 

 follows: Solution of uranium, one cubic centimeter equals five 

 milligrams of phosphorus pentoxid; correction, two-tenths cubic 

 centimeter. 



131. Titration of the Sample. The strength of the solution of 

 uranium having been exactly determined, by means of this solution 

 the strength of the sample in which the phosphoric acid has been 

 previously prepared as ammonium magnesium phosphate is as- 

 certained. In this case the quantity of phosphoric acid being un- 

 known, it is necessary to proceed slowly and to duplicate the tests 

 in order not to pass beyond the point of saturation. From this 

 there necessarily results a certain error in consequence of the 

 removal of quite a number of drops of the solution of the sample 

 before the saturation is complete. It is, therefore, necessary to 

 make a second determination in which there is at once added 

 almost the quantity of the solution of uranium determined by 

 the first analysis. Afterwards the analysis is finished by addi- 

 tions of very small quantities of uranium until saturation is 

 reached. Suppose, for instance, that the sample was that of a 

 mineral phosphate, five grams of which were dissolved in 100 

 cubic centimeters, and of which 10 cubic centimeters of the solu- 

 tion prepared as above, required 15.3 cubic centimeters of the 

 standard solution of uranium. We then would have the following 

 data : 



