144 AGRICULTURAL ANALYSIS 



a stirring rod when the liquid is tested with potassium ferro- 

 cyanid. It is very easy to be assured that the end of the reaction 

 has really been reached. For this purpose it is only necessary to 

 note the quantity of the solution already employed and to add to 

 it afterwards four drops ; shake, and make a new test with a drop 

 of the potassium ferrocyanid placed near the spot which the last 

 one occupied. If a decidedly reddish tint does not appear at the 

 moment of removing the glass rod, it is to be concluded that the 

 first appearance was an illusion, and the addition of uranium is 

 to be continued. If, on the contrary, the coloration appear of a 

 decided tint, the preceding number may be taken for exact. It 

 is then always beneficial to close the titration by this test of four 

 supplementary drops which will exaggerate the coloration and 

 confirm the figure found. 



The second cause of error, and one, moreover, which is the most 

 frequently met with, consists in passing the end of the reaction 

 by adding the uranium too rapidly. In place of giving then a 

 coloration scarcely perceptible, the test with the drop of potas- 

 sium ferrocyanid gives a very marked coloration. In this case 

 the analysis can still be saved. For this purpose the analyst has 

 at his disposal, a tenth-normal solution prepared with 100 cubic 

 centimeters of the standard solution of phosphoric acid diluted to 

 one liter with distilled water. Ten cubic centimeters of this tenth- 

 normal solution are added, and the titration continued. At the 

 end, the amount of additional phosphoric acid used is subtracted 

 from the total. 



A third cause of error is found in the foam which is often found 

 in the liquid, due to shaking. This foam may retain a por- 

 tion of the last drops of the solution of uranium which fall upon 

 its surface and prevent its mixture with the rest of the liquid. 

 If the glass stirring rod in being removed from the vessel, pass 

 through this froth charged with uranium, the characteristic col- 

 oration is obtained before real saturation is reached. Conse- 

 quently it is necessary to avoid, as much as possible, the forma- 

 tion of the foam, and especially to take care never to take the 

 drop for test after agitation except in the middle of the liquid, 

 where the foam does not exist. 



