22O AGRICULTURAL ANALYSIS 



which have been kept for a long while. When the slag has ab- 

 sorbed so much of carbon dioxid and water from the air as to 

 be no longer profitable for market, it can be restored to its orig- 

 inal condition by ignition. Any great loss on ignition is a ground 

 for suspicion concerning the purity and utility of a slag. 



(2) Method of Richters-Forster. One of the common adulter- 

 ants of tetrabasic phosphate is aluminum (Rodonda) phosphate 

 The method of detecting this when mixed with the slag is de- 

 scribed by Richters-Forster. 75 The method depends on the fact 

 that soda-lye dissolves the aluminum phosphate, although it does 

 not dissolve any calcium phosphoric acid from the slag. Two 

 grams of the sample to be tested are treated with 10 cubic centi- 

 meters of soda-lye of from 7 to 8 B. in a small vessel, with 

 frequent shaking, for a few hours at room temperature. After 

 filtration the filtrate is made acid with hydrochloric and afterwards, 

 slightly alkaline with ammonia. With pure basic slag there is a 

 small trace of precipitate produced, but this is due to a little silica, 

 which can be dissolved in a slight excess of acetic acid. If, 

 however, the basic slag contains aluminum phosphate, a dense 

 jelly-like precipitate of aluminum phosphate is produced. 



(3) Method of Jensch. Edmund Jensch determines the tera- 

 basic phosphate in slags by solution in organic acids, and pre- 

 fers citric acid for this purpose. 76 This method was also recom- 

 mended by Blum. 77 



It is well known that the tetrabasic phosphate in slags is com- 

 pletely soluble in citric acid, while the tribasic phosphate is only 

 slightly, if at all, attacked. The neutral ammonium salts of or- 

 ganic acids do not at first attack the tribasic phosphate at all, 

 and they do not completely dissolve the tetrabasic phosphate. 

 The solution used by Jensch is made as follows : Fifty grams of 

 crystallized citric acid are dissolved in one liter of water. A 

 weaker acid dissolves the tetrabasic phosphate too slowly and a 

 stronger one attacks the tribasic phosphate present. 



74 Mitteilungen der deutschen l/andwirtschafts-Gesellschaft, 189091, 

 5 : 131. 



Zeitschrift fur angewandte Chetnie, 1890, S : 595. 

 7 ' Zeitschrift fur angewandte Chemie, 1889, 2 : 299. 

 " Zeitschrift fiir analytische Chemie, 1890, 29 : 409. 



