ESTIMATION OF IRON, ALUMINA AND PHOSPHORIC ACID. 167 



present. It is preferable to work in dilute solution, say 

 decinormal to centinormal strength. 



The estimation is quickly carried out and can also be 

 made use of in the case of iron which is similarly precipi- 

 tated as the orthophosphate Fe 2 (P0 4 ) 2 . The phosphates 

 of iron and alumina are insoluble in neutral solutions con- 

 taining phosphates, and practically insoluble in water. 



Example — 10 cc. of a solution containing '0445 of A1 3 3 

 were put into a 100 cc. flask, 5 cc. of normal monosodic 

 phosphate added, and the whole made up to 100 cc. after 

 neutralising and warming as previously described. 50 cc. 

 were then filtered off through a dry filter. 



50 cc. required 8*0 cc. (corrected for 0O 2 ) — — KOH 



equivalent to *0284 gm. P 2 5 , that is to '0568 gms in the 

 100 cc. 



Total P 2 O. 5 added '1183 gm. calculated 



P 2 5 in filtrate '0568 „ 



P 2 5 combined with A1 2 3 ... '0615 „ 



Al 2 O s corresponding to '0615 P 2 5 *0443 „ '0445 gm. 



The estimation of both iron and aluminium together in 

 the absence of phosphoric acid can also be effected volu- 

 metrically and quickly by means of the same method; the 

 two phosphates are precipitated as previously described for 

 alumiua alone, the phosphate in filtrate estimated, and the 

 difference between this amount and the amount of phosphate 

 solution added gives the phosphoric acid combined with 

 iron and alumina. An estimation of the iron in original 

 solution by means of the iodine method gives the other 

 factor necessary — knowing the amount of iron and the 

 total phosphoric acid in the precipitate, the alumina is 

 calculated. In case of any disturbing elements in original 

 solution, the iron can be simply estimated by dissolving up 

 the precipitate and proceeding as before with the iodine 

 method. 



