METHOD OF HALLE EXPERIMENT STATION 93 



500 cubic centimeter flask with 20 cubic centimeters of nitric 

 acid of 1.42 specific gravity, and 50 cubic centimeters of pure 

 concentrated sulfuric acid, and boiled briskly for half an hour. 

 With substances which contain much organic material, a little 

 paraffin is added to avoid frothing. Such substances also require 

 a larger quantity of nitric acid than that above specified. The 

 flasks are allowed to cool, water added, again allowed to cool, 

 and filled up to the mark at 17. 5. If hydrochloric instead of 

 sulfuric acid be used in making the above solution, when the 

 citrate method is employed, the results are always too high, be- 

 cause the precipitate contains lime and alumina in such quantities 

 as to render any compensation for them inaccurate. In addition 

 to this the sulfuric has this great advantage over the hydro- 

 chloric acid; viz., it does not separate the silicic acid, inasmuch 

 as silicic acid is insoluble in boiling sulfuric acid. 



(3) Citrate-Soluble Acid. As is well known, in acidulated 

 phosphate a part of the phosphoric acid at first soluble in water 

 becomes insoluble, or, as usually expressed, reverted. This por- 

 tion, however, is still soluble in ammonium citrate. By the Halle 

 method it is determined as follows: Two grams of the sample 

 are digested with 100 cubic centimeters of ammonium citrate so- 

 lution, 1.09 specific gravity, for half an hour at 50 in a beaker. 

 Afterwards the soluble matter is separated by filtration with the 

 aid of a filter-pump and the residue washed with a solution of one 

 part water and one part citrate solution until all the dissolved 

 phosphoric acid is removed from the filter. Generally three or 

 four washings are sufficient. The residue on the filter is dried, 

 ignited and dissolved in a mixture of two cubic centimeters of 

 nitric and 20 cubic centimeters of sulfuric acid, the solution 

 made up to a volume of 200 cubic centimeters, filtered, and 100 

 cubic centimeters of the filtrate used for the determination. The 

 .acid in the filtrate is nearly neutralized and 50 cubic centimeters 

 of the citrate solution used in the determination of total acid are 

 added, and afterwards 25 cubic centimeters of magnesia mix- 

 ture and 20 cubic centimeters of 24 per cent, ammonia. After 

 .standing for 48 hours, the precipitate is separated by filtration, 

 ignited, and weighed in the usual way. The difference be- 



