306 EXPERIMENT STATION RECORD. 



for h hour, make volume to 500 cc., and filter. In case of substances rich 

 in phosphoric acid, like precipitated phosphate, use 25 cc. of this solu- 

 tion for the subsequent determination ; in case of the poorer phosphates, 

 like superphosphates and slags, use 50 cc. Neutralize the greater part 

 of the free acid with ammonia, precipitate in hot solution w 4 th 100 cc. 

 of a solution of nitro-molybdate of ammonia, boil, and filter while hot. 

 Wash with 100 cc. of 1 per cent nitric acid. Dissolve in as small amount 

 as possible of ammonia of 0.90 sp. gr. Saturate the larger part of the 

 ammonia with hydrochloric acid and precipitate in the cold with 10 cc. 

 of magnesia mixture, adding 2 or 3 drops of the latter at first and stir- 

 ring until precipitation commences, then add the rest of the solution at 

 once. Add 50 cc. of strong ammonia (0.96 sp. gr.), allow to stand for 

 2 hours, filter, wash with 5 per cent ammonia, incinerate, and weigh. 

 Calculate phosphoric acid with the factor 0.64. Substances containing a 

 large amount of organic matter may be dissolved in sulphuric acid, as 

 in the Kjeldahl method. Substances must not be prepared for the 

 determination of phosphoric acid by incineration. 



The citro-mechanical method is carried out with 25 cc. of a solution 

 of phosphate, guano, or ground bone, or 100 cc. of a solution of super- 

 phosphate made as directed above, the acid being almost neutralized 

 with ammonia, aud 30 cc. of Petermann's citrate solution (see below) and 

 10 cc. of ammonia of 0.91 sp. gr. added. The solution is agitated for h, 

 hour, during which time 25 cc. of magnesia mixture is added, drop by 

 drop. The precipitate is collected on a filter, washed, incinerated, aud 

 weighed. 



Phosphoric acid soluble in water and ammonium citrate. — From 1 to 4 

 gm. of phosphate, according to its content of phosphoric acid, is ground 

 up in a mortar, at first in a dry state, afterwards with 20 to 25 cc. of water, 

 until a uniform mixture is obtained. The solution is decanted on to a 

 filter, the filtrate being collected in a 250 cc. flask. This operation is 

 repeated 3 times, when all of the material is thrown on to the filter 

 and washed until the volume of the filtrate amounts to about 200 cc. 

 A few drops of nitric acid is added if the phosphoric acid is to be pre- 

 cipitated by ammonium nitro-molybdate, or hydrochloric acid if the citro- 

 mechanical method is to be used. The filter with the insoluble residue 

 is placed in a 250 cc. flask with 100 cc. of alkaline ammonium citrate. 

 Iu case of precipitated phosphates the previous extraction with water 

 is omitted and the material is treated directly with the citrate. The 

 insoluble residue is digested in the ammonium citrate in the cold for 15 

 hours with stirring, afterwards for 1 hour in a water bath at 40° O. The 

 citrate extract, after cooling, is made up to 250 cc. and filtered. To a 

 mixture of 50 cc. of this solution and 50 cc. of the water solution pre- 

 viously obtained 10 cc. of hydrochloric acid (1.1 sp. gr.) is added and the 

 solution kept at the boiling point for 30 minutes to transform the meta- 

 phosphoric acid into orthophosphoric acid. The phosphoric acid is 

 determined either by the molybdic or citro-mechanical method. If the 

 former is used, previous boiling with hydrochloric acid is unnecessary, 



