1 66 AGRICULTURAL ANALYSIS 



parative trial with the uranium process, but can not be recom- 

 mended as exact until further approved by experience. 

 The reagents employed have the following composition : 



(1) Disodium phosphate solution containing 10.085 grams per liter 



(2) Sodium acetate " " 50.000 " 



(3) Lead nitrate 40.000 " 



(4) Potassium iodid 50.000 " 

 The titrations should be conducted in the cold. 



147. Water-Soluble Phosphoric Acid. Glaser has modified 

 the volumetric method of Kalmann and Meissels for the volu- 

 metric estimation of water-soluble phosphoric acid in superphos- 

 phates so as to avoid the double titration required by the original 

 method. 24 If methyl orange be used as an indicator in the orig- 

 inal method, the determination does not at once lead to the tri- 

 calcium salt, but the liquid still contains, after neutralization, some 

 monocalcium phosphate, which is determined by a further titra- 

 tion with phenolphthalein as indicator. In the modified method 

 the total phosphoric acid is estimated in one operation as a tri- 

 calcium salt. This is secured by adding, at the proper time, an 

 excess of calcium chlorid. Two grams of the superphosphate are 

 shaken with water several times according to the American official 

 method, and finally, after settling, filtered, and the insoluble resi- 

 due washed on the filter until the total volume of the filtrate is a 

 quarter of a liter. Of this, 50 cubic centimeters are titrated with 

 tenth-normal soda-lye, with addition of two drops of methyl 

 orange, until the acid reaction has entirely disappeared. There 

 is then added neutral calcium chlorid solution in excess. If 

 iron and alumina be present, a slight development of an acid 

 reaction is produced of which no account need be made. 

 Five drops of the phenolphthalein solution are added and 

 the titration continued until the alkaline reaction is noted 

 throughout the whole mass. The alkaline reaction soon dis- 

 appears and to retain it several tenths cubic centimeters of 

 alkali are necessary, and thus an excess is easily used. To get 

 very sharp results it is advisable to place the solutions in a high 

 beaker, which is kept in vigorous rotation until the alkaline reac- 

 14 Chemikcr-Zeitung, 1894, 18 : 1533. 



