OSTERSETZER'S METHOD 189 



pounds of an acid character which react with calcium carbonate 

 to form carbon dioxid. 



When an aqueous solution of CaH 4 (PO 4 ) 2 and free phosphoric 

 acid is titrated with an alkali in the presence of methyl orange, 

 no precipitate is produced ; and, therefore, the precipitate in the 

 above method, ascribed to the formation of CaHPO 4 , is rather to 

 be accredited to the production of a phosphate of iron or alumina. 37 

 The alcohol method of extraction is not, therefore, as Gerhardt 

 has supposed, inapplicable because superphosphate may go into 

 solution, since this does not interfere with the reaction when 

 methyl orange is used as indicator, but it is to be rejected for 

 other reasons. The errors, however, due to iron and alumina may 

 amount to as much as one or two per cent, and are not constant. 



Gerhardt maintains in a later publication that Zockler is mis- 

 taken respecting the non-formation of CaHPO 4 and regards his 

 method as above described as satisfactory, especially when the 

 calcium carbonate is titrated instead of ignited. 38 



161. Ostersetzer's Method. The "free" acid in superphos- 

 phates may consist of several kinds, free phosphoric acid, free, 

 sulfuric acid and acid phosphates which react as free acid. An 

 indicator that may be used in a purely technical way to indicate 

 the proportion of such free acid to the total acid present is aliz- 

 arin sulfonic acid. 39 The determination of total free acidity is 

 made as follows : 



Dissolve 10 grams of superphosphate in 400 cubic centimeters 

 of water in a 500 cubic centimeter flask. Shake for the usual 

 time and add four cubic centimeters of a solution containing two 

 and a half grams of alizarin sodium sulfonate in 500 cubic centi- 

 meters of water. Complete the volume to the mark, filter, titrate 

 50 cubic centimeters of the filtrate, representing one gram of the 

 sample, with half -normal sodium hydroxid solution to transi- 

 tion between yellow and brown, comparing with an equal volume 

 of the original solution to better distinguish the changed color. 

 The free acidity is calculated from the data obtained and com- 

 pared with the total acidity determined in the usual way. 



37 Zockler, Chemiker-Zeitung, 1905, 29 : 226, 338. 



38 Chemiker-Zeitung, 1905, 29 : 276. 



39 Chemical News, 1905, 91 : 215. 



