142 AGRICULTURAL ANALYSIS 



solution of potassium ferrocyanid upon a greased white plate. 

 Since the quantity of the uranium solution present will be very 

 nearly 10 cubic centimeters at first, nine cubic centimeters can 

 be run in without testing. Afterwards, the operation is continued 

 by adding two or three drops at a time until the test upon the 

 white plate with the potassium ferrocyanid shows the end of the 

 reaction. When there is observed in the final test a slight change 

 of tint, the flask is filled up to the mark with boiling distilled 

 water and the process tried anew. If in the first part of the opera- 

 tion the point of saturation has not been passed, usually the ad- 

 dition of a drop or two of the uranium solution is required in order 

 to produce the characteristic reddish coloration, and this increase 

 is rendered necessary by the increase in the volume of the liquid. 

 Proceeding in this manner two or three times assures the attain- 

 ment of extreme precision, inasmuch as the analyst knows just 

 when to look for the point of saturation. 



Correction. The result of the preceding operation is not abso- 

 lutely exact. It is evident, indeed, that in addition to the quantity 

 of uranium required for the exact precipitation of the phosphoric 

 acid, it has been necessary to add an excess sufficient to produce 

 the reaction upon the potassium ferrocyanid. 



This excess is rendered constant by the precaution of operating 

 always upon the same volume, namely, 75 cubic centimeters. It 

 can be determined then, once for all, by making a blank deter- 

 mination under the same conditions but without using the phos- 

 phoric acid. 



The result of this determination is that it renders possible the 

 correction, which it is necessary to make, by subtracting the quan- 

 tity used in the blank titration from the preceding result in 

 order to obtain the exact strength of the uranium solution. 



The operation is carried on as follows: In a flat-bottomed 

 flask of about 150 cubic centimeters capacity and marked at 

 75 cubic centimeters, by means of a pipette, are placed 

 five cubic centimeters of the solution of sodium acetate ; some 

 hot distilled water is added until the flask is filled to the mark, 

 and it is then placed upon a sand-bath and heated to the boiling- 

 point. It is taken from the fire, the volume made up to 75 



