e&2 AGRICULTURAL ANALYSIS 



salts present carefully removed with sulfuric acid. The mass is 

 allowed to stand for one or two days, and is then filtered through 

 paper and is ready for use. The removal of the barium with 

 sulfuric acid may be omitted, the filtrate being diluted with an 

 equal volume of water and allowed to stand for one or two 

 days to permit the deposition of the last traces of barium silico- 

 fluorid and barium sulfate. The purity of the acid obtained de- 

 pends chiefly on the purity of the hydrofluosilicic acid at first 

 used. Hence to get good results this acid must be free from 

 foreign bodies. If an absolutely pure product be desired, the 

 acid above obtained must be distilled in a vacuum. 



490. Method of Kreider. Kreider has worked out a sim- 

 pler method of preparing perchloric acid which will make it easy 

 for every analyst to make and keep a supply of this admirable 

 yet unappreciated reagent. This method is conducted as fol- 

 lows : 05 



A convenient quantity of sodium chlorate, from 100 to 300 

 grams, is melted in a glass retort or round-bottom flask and 

 gradually raised to a temperature at which oxygen is freely, but 

 not too rapidly evolved, and kept at this temperature till the 

 fused mass thickens throughout, indicating the complete con- 

 version of the chlorate to the chlorid and perchlorate, which re- 

 quires from one and one-half to two hours; or the retort may 

 be connected with a gasometer and the end of the reaction deter- 

 mined by the volume of oxygen expelled, according to the equa- 

 tion 



2NaClO 8 =NaCl+NaClO 4 +O 2 . 



The product thus obtained is washed from the retort to a capa- 

 cious evaporating dish, where it is treated with sufficient hydro- 

 chloric acid to effect the complete reduction of the residual chlo- 

 rate, which, if the ignition has been carefully conducted with 

 well distributed heat, will be present in but small amount. It is 

 then evaporated to dryness on the steam-bath, or more quickly 

 over a direct flame, and with but little attention until a point 

 near to dryness has been reached, when stirring will be found of 

 great advantage in facilitating the volatilization of the remaining 

 65 American Journal of Science, 1895, 149 : 443. 



