38, 39] Estimation of Phosphoric Acid 27 



Before starting this analysis, a stock of magnesia mixture 

 should be prepared as follows : 



Preparation of magnesia mixture. — Dissolve 60 grams 

 MgCl 2 and 80 grams NH 4 C1 in 600 c.c. of distilled water. 

 Add 400 c.c. strong ammonia (Sp.G. = '88o), and allow to 

 stand twenty-four hours. Filter off any sediment. 



MgS0 4 is sometimes used instead of MgCl 2 , but is objec- 

 tionable, as a basic sulphate of magnesia is often precipitated 

 with the magnesium ammonium phosphate. 



Weigh out about 1 gram of pure Na 2 HP0 4 .i2H 2 0. 

 Transfer to a 12-oz. beaker, and dissolve in about 100 c.c. 

 of distilled water. 



38. Precipitation. — Measure off 30 c.c. of the magnesia 

 mixture and pour it into the beaker. Stir well for a few 

 seconds. Add 20 c.c. strong ammonia and stir again. Now 

 cover the beaker with a clock glass and allow it to stand in a 

 cool place for 2\ hours, stirring it once every fifteen minutes. 

 At the end of this time precipitation will be complete. Should 

 it be found more convenient to allow the beaker to stand over- 

 night, it will be advisable to add 10 c.c. strong ammonia next 

 morning and stir well. When the precipitate has subsided it 

 will be ready for filtering. 



39. Filtration and Washing. — The precipitate in this 

 case must not be washed by decantation, but must be got on 

 to the filter paper at once. It is much quicker, however, to 

 decant the clear liquid through the filter than to filter the turbid 

 liquid. When all the clear portion has passed through, wash 

 the pasty precipitate on to the filter with dilute ammonia (1 of 

 •880 NH 3 to 3 of H 2 0), using the special form of wash 

 bottle described in paragraph 14. Wash the precipitate six 

 times with ammonia, allowing each portion to run through 

 before adding the next. Test a few drops of the filtrate with 

 excess of dilute HN0 3 and a drop of AgN0 3 . No cloudiness 



