578 AGRICULTURAL ANALYSIS 



THE ESTIMATION OF POTASH AS PERCHLORATE 



.487. General Principles. By reason of the great cost of plat- 

 inum chlorid, analysts have sought for a reagent of a cheaper 

 nature and yet capable of forming an insoluble compound with 

 potash. Phosphomolybdic and perchloric acids are the reagents 

 which have given the most promising results. 55 The principle of 

 the method with the latter salt is based on the insolubility of po- 

 tassium perchlorate in strong alcohol containing a little perchloric 

 acid and the comparative easy solubility of the other bases usually 

 associated with potassium in water. The French chemists have 

 stated that magnesia, when present in considerable quantities, 

 interferes with the accuracy of the results. Since in soil analy- 

 sis considerable quantities of magnesia are often found, this base, 

 according to the French chemists, should previously be removed 

 when present in any considerable quantity, by the process de- 

 scribed in the first volume. Kreider, however, as will be seen 

 further on, working in the presence of magnesia, did not notice 

 any disturbing effects caused thereby. The method is applicable 

 to the common potash salts of the trade and with certain pre- 

 cautions to mixed salts. As will be mentioned later on, sulfuric 

 acid should be previously removed and this is likely to intro- 

 duce an error on account of the tendency of barium sulfate to 

 entangle particles of potash among its molecules and thus re- 

 move them from solution. The barium sulfate should be pre- 

 cipitated slowly and in a strongly acid (nitric or hydrochloric) 

 solution. The loss, which is inevitable, is thus reduced to a 

 minimum and does not seriously affect the value of the numbers 

 found. It is important to have an abundant supply of pure per- 

 chloric acid, and as this is not readily obtainable in the market 

 the best methods of preparing it are given below. The method, 

 while it has not been worked out extensively, is one of merit, and 

 seemingly is worthy of fair trial by analysts. The process is by 

 no means a new one, but was first proposed by Serullas, 50 and 

 prominently called to the attention of analysts by Schloesing, 57 



55 Wiley, Principles and Practice of Agricultural Analysis, 2nd Edition, 

 1906, 1 : 414, 421. 



** Annales de Chimie et de Physique, 1831, [2], 46 : 294. 

 57 Comptes rendus, 1871, 73 : 1269. 



