PHOSPHATES. 171 



by adding magnesia mixture. (This is magnesium sul- 

 phate made alkaline with ammonia and enough ammonium 

 chlorid to dissolve the precipitate first formed.) With 

 phosphoric acid or its salts it gives a white crystalline 

 precipitate. 



369. Form triple phosphate by making urine faintly 

 alkaline with ammonia and allowing it to stand until the 

 precipitate settles. Examine under the microscope for the 

 "coffin-lid" crystals. They can be more abundantly formed 

 for microscopic examination by adding to the urine a little 

 of a solution of magnesium sulphate before making it alka- 

 line with ammonia. 



370. DETERMINATION OF AMOUNT OF PHOSPHORIC ACID. 

 Prepare the following solutions: 



1. Uranium acetate: Dissolve about 34 grammes of crystal- 

 lized uranium acetate in water and dilute to one liter. This solu- 

 tion will be a little too strong. Its exact strength must be found 

 by the method to be described later. 



2. A solution of Na 2 HP0 4 , 12H 2 (crystallized disodium phos- 

 phate), one liter of which shall contain 10.085 grammes of the pure 

 crystallized salt. This salt gives up its water of crystallization 

 when exposed to the air, and cannot then be used. The crystals 

 must be perfectly bright. Fifty cubic centimeters of the solution 

 contain 0.1 gramme of P 2 O B (phosphoric anhydrid). 



3. Solution of sodium acetate of which one liter contains 100 

 cubic centimeters of 30-per-cent. acetic acid and 100 grammes of 

 sodium acetate. 



4. Solution of cochineal made by digesting for some time 1 

 gramme of powdered cochineal in a mixture of 20 volumes of 

 alcohol \vith 60 volumes of water. Filter or decant the liquid. 



Operation. First ascertain the strength of the uranium solu- 

 tion. To accomplish this, measure with a pipette 50 cubic centi- 

 meters of the sodium phosphate solution into a beaker; add 5 

 cubic centimeters of the sodium acetate solution and a few drops 

 of cochineal. Heat to boiling, and then from a burette run in the 



