96 REMOVAL OF PHOSPHATE IONS FROM THE 



3. The precipitate with NH 4 OH under ordinary conditions 

 would consist only of hydroxides of Fe, Al and Or. If however PO 4 '" 

 ions are present, it may also contain phosphates of the above metals 

 and also of Ca, Ba, Sr and Mg. 



2. Procedure. Dissolve a small portion of the NH 4 OH 

 precipitate in HNO 3 (Sp. g. 1.2) and test for P 4 '" ion* 

 with (NH 4 ) 2 Mo 4 . If a yellow precipitate forms, dissolve 

 the remaining portion of the precipitate in dilute HC1 (Sp. g. 

 1.12). Test a small portion of the solution for Fe with 

 K 4 Fe(CN) 6 . To the remaining solution add Fe C1 3 solution, 

 drop by drop, until (after careful stirring) , a drop of the solu- 

 tion removed by means of a stirring rod gives a brown precipi- 

 tate of Fe(0 H) 3 with N H 4 OH on a porcelain plate. 



Notes. 1. The test for Fe must be made at this point since Fe C! 3 

 is added to the solution later on. 



2. The addition of Fe C1 3 causes a precipitation of Fe P O 4 (white) 

 when the solution is made alkaline by NH 4 OH. When a sufficient 

 amount of Fe ions has been added to combine with aiL 

 PO 4 '" ions, an excess of FeCl 3 causes a precipitation of the brown 

 Fe(OH) 3 



3. Procedure. To the HC1 solution containing an excess- 

 of Fe C1 3 add NH 4 C1 solution and a slight excess of 

 NH 4 OH. Filter. Save the filtrate. 



Notes. 1. After the addition of NH 4 OH, the precipitate will 

 contain, Fe P O 4 . and hydroxides of Fe, Cr, and Al, all the P O 4 '" ions 

 remaining in the precipitate. The filtrate may contain ions of Mn, Ni, 

 Co, Ba, Sr, Ca, and Mg. 



4. Procedure. Dissolve the above precipitate in dilute 

 HC1 (Sp. g. 1.12) and add an excess of NaO H and H,O 2 

 Filter. 



Notes. 1. By the addition of Na O H and H,O 2 the A1(O H) 3 . 

 is converted into the soluble Na 3 AlO 3 , and the Cr(O H), is oxidized 

 to Na 9 CrO 4 , the iron precipitate remaining behind on the filter. 



