208 QUANTITATIVE ANALYSIS. 



a dry filter into a dry vessel. Measure off 100 cc. of the clear 

 filtrate (on standing the filtrate becomes cloudy from the 

 formation of barium carbonate), acidify very faintly with 

 hydrochloric acid, then add 10 cc. more of hydrochloric acid 

 and proceed as directed in the determination of the total 

 sulphuric acid, only with the difference that the further 

 addition of barium chloride solution is unnecessary. 



XV. DETERMINATION OF PHOSPHORIC ACID. 



Phosphoric acid is generally determined by titration with 

 a solution of uranium acetate. If a solution of uranium (or 

 uranyl) nitrate or acetate be added to a solution of secondary 

 sodium phosphate acidified with acetic acid and containing 

 sodium acetate, a yellowish-white precipitate of uranyl phos- 

 phate is formed according to the equation l 



UO 2 (N0 3 ) 2 + Na 2 HP0 4 - U0 2 HP0 4 + 2NaN0 3 . 



Any excess of uranium is readily detected : a drop of the 

 mixture then gives, with a drop of potassium ferrocyanide 

 solution, a brownish-red precipitate of uranyl ferrocyanide. 

 This is the so-called end-reaction ; it appears only when the 

 phosphoric acid is completely precipitated and a slight excess 

 of uranium is present. Instead of this some cochineal solu- 

 tion may be added to the solution of the phosphates- an 

 excess of uranium produces a green color, but only when the 

 fluid contains no free nitric acid. There is nothing to pre- 

 vent using both end-reactions at the same time. 



Preparation of the Uranium Solution. Dissolve about 

 33 g. of commercial sodium uranate by warming with about 

 200 cc. of water and the smallest possible quantity of nitric 



1 As the urine contains monosodium phosphate the equation is 



UO 2 (NO 3 ) 2 + NaH 2 PO 4 = UO 2 HPO 4 + NaNO 3 + HNO 3 . 



The sodium acetate is added to get rid of the nitric acid set free in the 

 reaction. 



