KEEPING PROPERTIES OF PERCHLORIC ACID 585 



evaporation of a small portion of the latest nitrate only a small 

 residue is found. The alcoholic solution of the perchlorate is 

 then distilled from a large flask until the perchlorate begins to 

 crystallize, when the heat is removed and the contents quickly 

 emptied into an evaporating dish, the same liquid being used to 

 wash out the remaining portions of the salt. When the distilla- 

 tion is terminated at the point indicated, the distillate will con- 

 tain most of the alcohol employed, but in a somewhat stronger 

 solution, .so that it requires only diluting to 97 per cent, to fit it 

 for use in future preparations. The salt is then evaporated to 

 dryness on the steam-bath and subsequently treated with strong 

 hydrochloric acid for the separation of the perchloric acid. 



One cubic centimeter of the acid prepared in this way on 

 evaporation gave a residue in one case of 0.0369 gram, and in 

 another 0.0307 gram, completely soluble in 97 per cent, alcohol. 

 The residue was then ignited and the chlorin determined by 

 silver from which the equivalent of perchloric acid in the form 

 of salts was calculated as 0.0305 gram. By neutralizing the acid 

 with sodium carbonate, evaporating, igniting in an atmosphere of 

 carbon dioxid till decomposition was complete, collecting the 

 oxygen over caustic potash, allowing it to 'act on hydriodic acid 

 by intervention of nitric oxid, titrating the iodin liberated with 

 standard arsenic and calculating the equivalent of perchloric acid, 

 after subtracting the amount of acid found in the form of salts, 

 the amount of free perchloric acid per cubic centimeter proved 

 to be 0.9831 gram. 



The whole process, even when the separation with alcohol is 

 necessary, can not well require more than two days, and during 

 the greater part of that time the work proceeds without atten- 

 tion. 



491. Keeping Properties of Perchloric Acid. By most author- 

 ities it is asserted that perchloric acid is a very unstable body 

 and is liable to decompose with explosive violence even when 

 kept in the dark. It is probable that this tendency to spontaneous 

 decomposition has been exaggerated. It is not even mentioned by 

 Gmelin. 66 



66 Hand-Book of Chemistry, Translated by Watts, 1859, 2 : 317. 



