I4O AGRICULTURAL ANALYSIS 



the phosphoric acid which it contains, a volume of the solution 

 of uranium almost exactly equal to its own, in order that the 

 expansions or contractions which the two liquors undergo, by rea- 

 son of changes in the temperature of the laboratory, should be 

 without influence upon the results. 



The solution of uranium, prepared as has been indicated above, 

 precipitates almost exactly five milligrams of phosphoric acid per 

 cubic centimeter; the typical solution of phosphoric acid is pre- 

 pared with eight and one-tenth grams of acid ammonium phos- 

 phate pure and dry, which is dissolved in a sufficient quantity of 

 distilled water to make one liter. 



Since the acid ammonium phosphate contains 61.74 per cent, 

 of anhydrous phosphoric acid, the quantity above gives exactly 

 five grams of that acid in a liter, or five milligrams in a cubic 

 centimeter. 



Instead of this solution the following is also recommended : 

 Dissolve 3.087 grams of pure ammonium phosphate in water and 

 make the volume up to one liter. Each 20 cubic centimeters of 

 this solution corresponds to 0.04 gram of phosphoric anhydrid. 



128. Salt for Setting the Uranium Solution. In determining the 

 strength of the uranium solution the crystallized sodium salt, 

 Na,HPO 4 .H 2 O, has been used. This salt easily loses water and 

 for this reason its weight is not constant. Miiller proposes to 

 use in its place, the acid sodium-ammonium salt, NaHNH 4 PO 4 . 

 4H 2 O. Even better results are secured by using the crystallized 

 dicalcium salt, CaHPO 4 .2H 2 O, which in the free air or even 

 over phosphoric anhydrid does not vary at ordinary tempera- 

 tures. 11 



In the preparation of this salt a solution of disodium phos- 

 phate, Na 2 HPO 4 , is added little by little to a dilute solution of cal- 

 cium chlorid until the lime is almost completely precipitated. 

 The gelatinous precipitate at first formed soon becomes crystalline, 

 and it is easy to wash it thoroughly. The washed salt is placed 

 on plates and dried at 70. Theoretically, the salt thus obtained 

 contains 41.27 per cent, of P 2 O 5 . 



129. Verification of the Strength of the Standard Solution of 

 Phosphoric Acid. The strength of the standard solution of phos- 



11 Bulletin de la Socit chimique de Paris, 1901, [3], 25 : 1000. 



