61. PIIOSPIIORIC ACID. 87 



tilled water. One cubic centimetre of such a solution con- 

 tains 0.005 grm. of phosphoric anhydride, P^O^. 



2. Dissolve 100 grms. of sodic acetate in 900 c.c. of 

 water, and add 100 c.c. of concentrated acetic acid. 



3. Dissolve about 33 grms. of uranic acetate in about 

 1 litre of water, and proceed to titrate this solution witli 

 reference to the standard solution of phosphate so that 1 

 cubic centimetre of it shall exactly precipitate 0.005 grm. 

 of phosphoric anhydride, as follows. 



Put 25 c.c. of the standard phosphatic solution in a 

 small flask, add 5 c.c. of the solution of sodic acetate, 

 heat to about 50° C, add 5 or 10 c.c. of the uranic solution 

 from a burette or graduated pipette, heat to boiling, and 

 let the mixture stand a few minutes ; the precipitate wi.l 

 settle quickly, and a drop of the clear supernatant liquid 

 can be taken out on the end of a small glass rod, and 

 tested with the solution of potassic ferrocyanide for ex- 

 cess of uranic oxide ; this test is best made by letting the 

 drop fall gently in the middle of a small shallow pool of 

 the solution of ferrocyanide, on a white porcelain plate, 

 when the slightest excess of the uranic oxide in the solu- 

 tion will be manifested by the formation of a brown zone 

 where the two liquids come in contact; the color soon 

 spreads throughout the entire liquid. If no color appears, 

 add 5 c.c. more of the uranic solution, boil again, let set- 

 tle, and test a drop of the supernatant liquid in another 

 little pool of the ferrocyanide, and so proceed until a 

 brown color is produced in the test drop. Suppose that 

 this brown color was obtained after adding 20 c.c. of the 

 uranic solution, but not after adding 15 ; repeat the trial 

 now with a fresh quantity of the standard phosphatic so- 

 lution, adding 16 c.c. of the uranic solution at once, be- 

 fore making the test, and repeating the test after each 

 addition of a cubic centimetre at a time. If, in this trial, 

 we find that a brown color is obtained with IT c.c. but 

 not with 16, we may make a third trial with another por- 



