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IXDIAXA rXIYERSITY 



3. Studies ox Percbloric Actd: Tpfe Preparation of Perchlo- 

 ric Acid from Sodium Perchlorate. 



By Feank C. Mathers, Assistant Professor of Chemistry. 



Methods of Preparation. The best knoY^n methods for the 

 preparation of i>erehlorie acid are: 



1. Treat a solution of potassimn perchlorate with fliiosilicic 

 acid^. The potassium flnosilicate forms a gelatinous precipitate 

 which is separated by clecantation and filtration from the aqueous 

 solution of perchloric acid. This i)erchloric acid solution contains 

 all of the impurities from the reagents, together ^yith. any excess 

 of iiuosilicic acid that was used in the precipitation, and small 

 amounts of potassium flnosilicate. To obtain a pure solution of 

 perchloric acid, this aqueous solution must be distilled. The great 

 objection to this method is that the fluosilicic acid is as trouble- 

 some to prepare as the perchloric acid itself when other methods 

 are used. 



2. Evaporate a solution of chloric acid until the evolution of 

 white fumes begins. This heating decomposes the chloric acid 

 according to this reaction : 



2HC10, = 2HC10^ - 4C10, ^ 2H2O. 



The gaseous chlorine dioxide is expelled from the solution during 

 the heating. Pure perchloric acid is formed except for the impuri- 

 ties that were present in the original reagents. Chloric acid may 

 be prepared- by a number of metJicds. The methods of making 

 chloric acid are difficult. Tlie yields 01 perchloric acid from the 

 chloric acid are very low. 



3. Treat barium perchlorate with an equivalent amount of sul- 

 phuric acicP. The barnim sulphate thus formed settles rapidly and 

 is easily separated by decantation and nitration from the aqueous 

 perchloric acid. The perchloric acid prepared in this manner is 

 generally impure. It is very difficult to add just the equivalent 

 amount of sulphuric acid, so either Ba or SO^ ions are present. 

 Barium perchlorate is such an expensive starting material that 

 this method cannot have a general application. 



1 Serullas. AnnaJes de chimie ef de phj'sique.^ljy, 210 ; Caspari, Zeitschrift fiir 

 angeicandte Chemle, VI. 6S (JS9o). 



2 Millon, AnnaJes de chimie et de i^hijf'ique, (o) VII. 310: Stadion, GWbert's An- 

 naleii, LII, 197 and 339; Penny. LieVig's Annalen, XXXVII. 203: JouniaJ jiir 

 praktische Cliemie, XXIII, 296; Sernllas. Aiiualcs de chimie ei de physique, XLV. 

 270; Crookes, Select Methods of Chemical Analusis. 7. 



3 Henry, Lietig s Annalenj XXXI, 345. 



