24 METHODS OF ANALYSIS [Chap. 



PHOSPHORIC ACID. 



17 Gravimetric Method. — Official. 



Concentrate 100-200 cc. of A, under 11 , to about 25-30 cc, neutralize with ammo- 

 nium hydroxid and add about 10 cc. additional. Dissolve the precipitate by the slow 

 addition of dilute nitric acid, stirring constantly and avoiding a large excess, add 

 gradually about 20 cc. of molybdate solution [I, 4 (a) ], and allow to stand for 1 or 2 

 hours in a water bath at a temperature of 40°C. After an hour determine if the 

 precipitation is complete, as follows: Pipette about 5 cc. of the clear liquid into 5 cc. 

 of warm molybdate solution. If any precipitate is produced, return the test liquid 

 to the main portion, add more molybdate solution, and repeat the operation until 

 all the phosphoric acid is precipitated. Then allow to stand for several hours at 

 room temperature, preferably overnight. Filter off the ammonium phosphomolyb- 

 date, wash the precipitate thoroughly with cold water, dissolve with ammonium 

 hydroxid, and determine as magnesium pyrophosphate, as directed under I, 6 and 

 calculate to phosphorus pentoxid (P2O5). 



Volumetric Method. — Tentative. 



1 8 REAGENTS. 



(a) Standard sodium or potassium hydroxid solution. — Strength such that 1 cc. 

 of this solution is equivalent to 0.0005 gram of phosphorus pentoxid (P2O6). 



(b) Standard nitric acid solution. — Strength same as the standard alkali solution 

 described under (a) as determined by titration, using phenolphthalein as indicator. 



19 DETERMINATION. 



Proceed as in 1 7 until all the phosphoric acid is precipitated and then complete 

 the determination in the following manner: 



Allow the solution containing the yellow precipitate to stand for at least 3 hours 

 at a temperature not above 40°C., filter on a small filter paper or on a Gooch crucible 

 and wash with cold water until the filtrate from 2 fillings of the filter yields a slight 

 pink color on the addition of phenolphthalein and 1 drop of the standard alkali. 

 Return the filter and precipitate to the same beaker used for precipitating the 

 phosphomolybdate, dissolve the yellow precipitate in the standard sodium or potas- 

 sium hydroxid solution, add a few drops of phenolphthalein and titrate the excess of 

 alkali with the standard acid. Calculate to phosphorus pentoxid (PaOs). 



20 SULPHURIC ACID.-OFFICIAL. 



Evaporate 100-200 cc. of A, under 11 , nearly to dryness on a water bath to expel 

 the excess of acid, add 50 cc. of water, heat to boiling and add, drop by drop, 10% 

 barium chlorid solution until no further precipitation occurs. Continue the boiling 

 for about 5 minutes and allow to stand for 5 hours or longer in a warm place. Decant 

 the liquid on an ashless filter or tared Gooch, previously heated, treat the precipitate 

 with 15-20 cc. of boiling water, transfer to the filter and wash free from chlorin with 

 boiling water. Dry the precipitate and filter, ignite, weigh as barium sulphate and 

 calculate to sulphur trioxid (SO3). 



POTASSIUM AND SODIUM. 



21 Method I.— Official. 



(1) Treat the filtrate from 20 with ammonium hydroxid exactly as in 13. 

 Evaporate the filtrate and washings to dryness, heat below redness until ammo- 

 nium salts are expelled, dissolve in hot water, add 5 cc.of barium hydroxid solution, 



